# Testable Designs of Toffoli Fredkin Reversible Circuits

H. M. Gaur
Department of Electronics & Communication Engineering
ABES Institute of Technology Ghaziabad
Uttar Pradesh, India
<EMAIL_ADDRESS>
A. K. Singh
Department of Computer Applications
National Institute of Technology Kuruskshetra
Haryana, India
<EMAIL_ADDRESS>
U. Ghanekar
Electronics & Communication Engineering
National Institute of Technology Kuruskshetra
Haryana, India
<EMAIL_ADDRESS>
Dr. Gaur is working as Associate Professor

###### Abstract

Loss of every bit in traditional logic circuits involves dissipation of power
in the form of heat that evolve to the environment. Reversible logic is one of
the alternatives that have capabilities to mitigate this dissipation by
preventing the loss of bits. It also have the potential to broaden the horizon
of futuristic reckon with its applications to quantum computation. Application
of testing strategies to the logic circuits is a necessity that guarantees
their true functioning where the researchers are at par with solutions for the
upcoming challenges and agreements for reversible logic circuits. Novel
methods of designing Toffoli, Fredkin and mixed Toffoli-Fredkin gates based
reversible circuits for testability are put fourth in this article. The
proposed designs are independent of the implementation techniques and can be
brought into real hardware devices after obtaining a stable fabrication
environment. The experimentation for the proposed models are performed on
RCViewer and RevKit tools to verify the functionality and computation of cost
metrics. Fault simulations are carried out using C++ and Java to calculate
fault coverage in respective methodologies. The results confirmed that all the
presented work outperforms existing state-of-art approaches.

_Keywords_ First keyword $\cdot$ Second keyword $\cdot$ More

##  1 Introduction

In the current scenario, electronic industries are facing the problems of
power utilization and overheating of equipment. In the past decades, these
issues were used to be solved by adopting the practice of reducing the size of
traditional transistors which has also been miniaturized up to certain
nanometres. However, if the size of the transistors are further scaled, the
power and overheating problems will increase exponentially Moore (2015); Mack
(2018). Moreover, the well-known Landauer’s theory extricates the limitations
of irreversible computation which also bounds the size of transistors in
conventional logic circuits as they also involve loss of information in the
form of heat Landauer (1961). Meanwhile, the demands of more and more
applications in single SOCs are also leading to a drastic increase in loss of
information. Reversible logic is one of the promising techniques to reduce the
power requirements, as these circuits are theoretically proven for providing
nearly energy free computation by preventing the loss of information and have
the capability of producing ultra high speed and compact electronic devices
Bennett (1973). However, the logic can be applied to traditional logic
circuits, but its applications to quantum computation have been proven for
achieving excellence in terms of power consumption, speed and size Nielsen and
Chuang (2011). The identification and implementation of reversible quantum
circuits have also been achieved using several probabilistic methods and
ideas. Fig. 1 shows some of the dominating technologies where the researchers
are currently exploring the possibilities for employing this logic at physical
foregrounds Gaur et al. (2020a).

Figure 1: Computing technologies

The framework of reversible logic circuits design and synthesis techniques is
based on Toffoli and Fredkin gates, which can be further scaled into $n$-$th$
order gates and libraries, commonly known as Multiple Control Toffoli (MCT)
and Multiple Controlled Fredkin (MCF). Several other gates have also been
proposed in the literature, but the primary components of these gates are MCF
and MCT. Moreover, the final quantum decomposition of the reversible circuits
are based on them Gaur et al. (2019a). The efficiency of the designs are
governed by several performance metrics defining their operating cost. These
metrics are number of wires, gate cost, quantum cost and garbage output.
Testing has also been extensively studied since last decade for the
recognition of several types of fault models in reversible circuits. A number
of novel paradigms have been presented in both the area of online and offline
testing of reversible logic circuits. Online testable environment are provided
over pristine design methodologies and circuit modification principles. Test
data minimization in offline tesing is achieved over new deterministic,
randomized test patter generation algorithms and circuit modification
techniques for respective faults. The reduction of operating cost has been
achieved to some needful extent in all the proposed approaches with respect to
prior ones for narrowing the compensation with overall testing overheads Gaur
et al. (2020b).

A comprehensive and comparative analysis of the existing online and offline
testing methodologies for nearly all fault models in reversible circuits has
been completed in correlation with the problem statement Gaur et al. (2015,
2016a, 2018a, 2020b) in the beginning of the proposed work. An overview of
reversible logic, cost metrics and associated faults models are also explained
for providing background of the work. Overall work in the literature and
deeply analyzed and organized into four set of categories that defining the
plan of action for the novel development in the area. The illustration in Fig.
2 which shows a generalized framework to achieve the quoted objectives. As
overall framework is based on fundamental MCT and MCF gates. At first, an in-
depth and comparative analysis of nearly all reversible gates has been done. A
three level analysis i.e., gate, design and testability level, has been
performed to confirm the efficacy of the fundamental gates Gaur et al. (2018b,
2019a). At last, the proposed testable design methodologies for online testing
are also applied to obtain an efficient set of testable Data Path Elements
(DPE) designs.

Figure 2: Framework for designing testable reversible circuits

The objectives of the work described in this paper from the authors thesis
Gaur (2018) is to development of testable design methodologies at reduced
testing overheads in terms of reversible circuit cost metrics, test data
volume, design complexity and time. There are the following contributions
toward the achievement of stated objectives:

  * •

Novel design methodologies using Multiple Controlled Toffoli (MCT), Multiple
Controlled Fredkin (MCF) and mixed Multiple Controlled Toffoli-Fredkin (MCTF)
gates which shows built-in testability features towards single bit faults.

  * •

New circuit modification methodologies for MCT, MCF and MCTF circuits are
introduced for the detection of single bit faults.

  * •

Efficient circuit modification methodologies along with general test sets are
proposed for MCT, MCF and MCTF circuits for the detection of stuck-at faults
to minimize the volume of test data.

  * •

New testable designs of Full Adder, Ripple Carry Adder, $4$-bit reversible
array based Multiplier and Arithmetic & Logic Unit are proposed using MCT and
MCF gates.

Lining up with the targeted objectives, a detailed overview and analysis of
the proposed work is described in the consecutive sections:

##  2 Circuit Design and Modification Methodologies for Online Testing

Extensive design methodologies are realized for the construction of MCT, MCF
and MCTF circuits. The constructed circuits provide built-in testability
feature for the detection of single bit faults. As the utilization of parity
preserved architecture with arbitrary design methodology ensures the detection
of single bit flip faults in logic circuits. The methodologies are engaged in
the creation of parity preserving circuits using MCT, MCF and mixed MCTF gates
followed by the fault detection process. The design and test flow of the
formulation of these methodologies is depicted in Fig. 3.

Figure 3: Synthesis and test flow of design methodologies for online testing

First, an MCT gates placement technique is proposed for producing parity
preserving circuits Gaur and Singh (2016). Second, the properties of MCF gates
are exploited to showcase the scheme for the detection of faults Singh et al.
(2019). Third, the MCT gate placement method is utilized in combination with
MCF gates to achieve testability in MCTF circuits Gaur et al. (2018c). The
fault detection is achieved by cascading a parity checker in the circuit using
CNOT gates from the inputs and outputs to an additional wire. The circuits
produced using proposed methods are incorporated with testability feature
rather put extra efforts in converting original circuit into their testable
form. A set of benchmark circuits and corresponding testable designs are
implemented to observe the cost of designing and proving the efficacy of
proposed schemes over existing ones.

Modifications for testability in logic circuits accounts a large increment in
operating cost which enhances overall cost of manufacturing. New modification
schemes for MCT, MCF and mixed MCTF circuits are introduced at lower operating
cost. The modification procedures utilize the technique of parity preservation
and generation for providing full coverage of single bit faults. The
modification and test flow of the formulation and obtaining the measures for
these methodologies is depicted in Fig. 4.

Figure 4: Design and test flow of modification methodologies for online
testing

Gates cascading terminology is used for the modification of MCT circuits at
the start as these gates are widely used for designing reversible circuits.
The method requires only a single wire, around twice number of gates and zero
garbage cost for its formulation Gaur et al. (2016b). Derived gates technique
is used for the modification of MCF circuits. The gates are transformed into
corresponding testable that provide fault detection as well as location
functionality in MCF circuits. It utilized parity preserving gates rather to
than to convert a modify a gate into corresponding parity preserving form Gaur
et al. (2018d). A three stage process is explored for converting MCT into MCTF
circuits which largely decreases the gate cost using a wire by utilizing gates
cascading and parity preserving the property of MCF gates Gaur et al. (2019b).
The method involves the steps of simplification of MCT circuits into MCTF
cascades and modification of the resultant circuit. A large set of circuits
and benchmarks are designed using proposed methodologies and fault simulations
are performed to evaluate the performance and validate their functionality.
The detection of single bit faults are targeted in all the proposed
methodologies. Results prove that the present methods provides excellent
reduction in the operating costs as compared to existing work in this area and
provide full coverage of single bit faults.

##  3 Circuit Modification Methodologies for Offline Testing

Test set generation in reversible circuits is followed by a number of
methodologies for the detection of faults. These methodologies utilize
specific deterministic ATPG, randomized ATPG and modification approaches for
the detection of stuck-at, bridging, missing gate, cross-point and cell
faults. The existence of the trade-off between testability and overheads can
be seen in all the prior methodologies in terms of performance measures like
gate cost and quantum cost, test size and time utilization. New modification
methodologies are introduced for the detection of stuck-at faults in MCT, MCF
and MCTF circuits using test sets of minimal sizes. The flow for the
formulation of these methods and simulations of GTS to obtain the effective
measures is depicted in Fig. 5.

Figure 5: Design and test set application flow of modification methodologies
for offline testing

The MCT circuits are modified in such a manner that the applied test vector
reaches all the levels without any change in values on the wires of the
circuit Gaur et al. (2018e). An ($n+1$) dimensional general test set ($GTS$)
containing only two test vectors is presented, which provides full coverage of
single and multiple stuck-at faults in the circuit. Here $n$ denotes the
number wires contained by the circuit. Deterministic approaches for the
identification and detection of different types of fault models in MCF
circuits are introduced Singh et al. (2019). The conservative property of MCF
gates is utilized for multiple types of fault detection in these circuits by
the three test sets of sizes $2$, $n$ and $2(n-2)$. Moreover, both the schemes
are combined for the detection of stuck-at faults in MCTF circuits Gaur et al.
(a). All the methodologies have experimented on several benchmark circuits,
where an excellent reduction in overall operating costs has been achieved as
compared to prior work experimented in the same platform. Moreover, these
circuits can be tested by general test sets of fixed sizes without spending
the excess time required to formulate specific algorithm under stuck-at fault
detection.

##  4 Testable Designs of Data Path Elements

Modern digital processors are comprised of several data path elements (DPE)
like adders, multipliers, multiplexers, logical shifters, arithmetic logic
unit etc. These elements are the functional units within the microprocessor
which are used to execute computational operations. New testable architectures
of a full adder (FA), ripple carry adder (RCA), multiplier (MUL) and an
arithmetic & logic unit (ALU) using MCT and MCF gates Gaur et al. (b, 2019c).
The design and simulation flow of these elements is shown in Fig. 6.

Figure 6: Design and Implementation flow of DPE

The major role is played by the creation of parity preserving circuits for
incorporating testability in overall circuits realization. Firstly a full
adder is created which is used to develop the architecture of RCA. A $4$-bit
reversible array based multiplier with scalability factor of order $4N$ by
using RCA and Fredkin gates Gaur et al. (b). Design of Arithmetic Logic Unit
(ALU) which can be scalable up to $N$ number of bits is proposed Gaur et al.
(2019c) by combining a novel structure of control unit (CU) using Fredkin
gates and FA. that . The ALU design can be scalable for $N$-bit operations.
These designs can be scalable for $N$-bit operations and incorporates
testability features for the detection of single-bit faults at lower overheads
which shows their exclusive features. The superiority of the designed circuits
is acknowledged by implementing them using reversible circuit analyzer tool
and obtaining corresponding operating costs.

##  5 Results and Assessment

The experiments for evaluating the efficacy of the proposed work in this
thesis were performed on a machine with 64-bit Ubuntu-16.04LTS having Intel
Core I7-4790, 3.60 GHz clock and 4GB memory. The prerequisites which can be
seen in different part of the thesis are listed as follows:

  * •

The benchmark circuits description in the form of pla and tfc are taken from
reversible logic synthesis and benchmark pages D. Maslov, G. and Dueck, N.
Scott (2004); Wille et al. (2008).

  * •

Revkit-A toolkit is used for reversible logic synthesis Soeken et al. (2012).

  * •

The circuits are synthesized using well-known garbage free transformation
based synthesis algorithm.

  * •

RC-viewer and RC-Viewer+ tools for designing reversible circuits is used for
calculating respective measures that define operating costs D. Maslov, G. and
Dueck, N. Scott (2004).

  * •

QCA-designer is used for the implementation proposed designs and finding out
physical reliability at QCA level. Walus et al. (2004).

  * •

Fault coverage is verified using simulations carried out in C++ and Java.

A large set of benchmark circuits are taken from the two platforms which are
experimented on the basis of each design methodologies. Design, synthesis and
implementation of the methodologies are done using RC Viewer and Revkit tools.
QCA structures are also implemented in to obtain the cost measures in some of
the cases. Fault simulation is done by creating programs that realize the
circuits and computing the fault coverage after inducing a type of fault for
which the design method has been developed. The implementation and simulation
of the prior methodologies in the domain has also been executed to compare the
presented work and calculate the efficacy. It has been analyzed the present
work in the domain of design methodologies for online testing (DMOnT) has
achieved the maximum reduction in cost measures by $12\%$ in case of MCT based
designs, $61\%$ in case of MCF based approaches and $51\%$ in case of MCTF
based design methodologies. The proposed work in the domain of modification
methodologies for online testing (MMOnT) has achieved the reduction in cost
measures by $45\%$ in MCT based modification techniques, $49\%$ in case of MCF
based techniques and $75\%$ in case of MCTF based modification methodologies.
The work done in the domain of modification methodologies for offline testing
(MMOffT) has achieved the reduction in cost measures up to $44\%$ in MCT based
techniques, $100\%$ in case of MCF based techniques and $30\%$ in case of MCTF
based methodologies. These analytics also pictured in Fig. 7.

DMOn.TMMOn.TMMOff.T$20$$40$$60$$80$$100$$32$$45$$44$$61$$49$$100$$51$$75$$30$Testable
Design Methodology%age reduction in cost measuresMCTMCFMCTF Figure 7: Result
analysis of presented testing methodologies

The performance of the presented DPE structures is analyzed by implementing
$4$-$64$ bit circuits over reversible circuit analyzer tool and compare the
characteristics with the prior efficient architectures . Reported results
showing the reduction of cost measures are illustrated in Fig. 8. Presented
testable FA circuits achieved an average reduction by $11\%$ in the when all
the considered parameters are combined together. RCA achieved a reduction by
$12\%$ and $44\%$ has been achieved in case of MUL. A reduction up to $60\%$
in the gate cost has been achieved with respect to recently reported
reversible ALU architectures from the literature.

FARCAMULALU$20$$40$$60$$80$$100$$11$$12$$44$$60$DPE Design%age reduction in
cost measuresProposed Designs Figure 8: Result analysis of presented DPE
architectures

The fault coverage is also calculated in nearly all the proposed methodologies
and designs. Full coverage has been achieved in the respective methodologies
for a considered fault model. The calculations show a large reduction in
operating costs when compared to the prior work in all designs and testing
methodologies. The requirements of extra hardware and time to attain
testability can be eliminated by utilizing these methodologies during the
design process. Hence, the methods provide solutions to both the problem of
designing and testability of reversible circuits which can be adopted by any
synthesis algorithm to minimize testing overheads.

##  6 Conclusion and Future Scope

The change in technology will give rise to new challenges where the
manufacturers would have to commence certain proofs for their truthful
functionality to be deliverable in the huge market containing highly ambitious
consumers. Testing is the only way out for them to rescue from these
situations. However, it is a necessary exercise but it deals with a large
increment in operating costs. Moreover, a huge amount of power consumption is
governed by the testing methodologies used by the manufacturers. Numerous
approaches for constructing built-in testable MCT, MCF and mixed MCTF circuits
over novel design methodologies and circuit modification techniques are
presented for the detection of single bit faults. The performance of all the
approaches is analyzed by experimenting on a set of benchmark circuits. As the
logic circuits are very much prone to the occurrence of stuck-at faults, new
circuit modification techniques for minimization of test data in MCT, MCF and
mixed MCTF reversible circuits are introduced for their detection. In
addition, this work introduces new testable designs of scalable adders,
multiplier and arithmetic logic unit for future microprocessors. MCT and MCF
gates are taken into account for the formulation of all the proposed
approaches as they are proven universal as well as superior for designing and
testing of reversible circuits. The efficacy of all the modules is justified
by providing the implementation on reliable tools for reversible circuits. The
fault tolerance designing model has been identified that utilized the proposed
methods in this paper Gaur et al. (2020c), however, some of the possible
limitations and scope still exists:

  * •

This work is merely a start to the research needed in determining the
feasibility of reversible circuits as a replacement to present CMOS
technology.

  * •

Development of a comprehensive tool for synthesizing reversible circuits based
on the proposed framework.

  * •

Efficient ATPG Algorithms can be explored to minimize the test-data volume for
the detection of multiple types of fault models in reversible circuits.

  * •

MCF gates based synthesis algorithm can be developed, as it has not gained
significant attention by the researchers working in this area.

## Acknowledgment

Sincerest thanks to all the reviewers for their extensive and insightful
comments and suggestions on the thesis and supported manuscripts. Each and
every comment on the manuscript motivated the authors to perform better.
Special thanks to Dr. Jimson Mathew, Associate Professor and Head, Department
of Computer Science & Engineering, IIT Patna, for his excellencies during the
final defense of work.

.

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# Magnetic and dielectric properties of $A_{2}$CoSi2O7 ($A$=Ca, Sr, Ba)
crystals

M. Akaki  J. Tozawa  D. Akahoshi  and H. Kuwahara  Department of Physics,
Sophia University, Tokyo 102-8554, Japan<EMAIL_ADDRESS>
###### Abstract

We have investigated the magnetic and dielectric properties of $A_{2}$CoSi2O7
($A$=Ca, Sr, and Ba) crystals with a two-dimensional network of CoO4 and SiO4
tetrahedra connected with each other through the corners. In Ca2CoSi2O7, a
weak ferromagnetic transition occurs at 5.7 K, where the dielectric constant
parallel to the $c$ axis shows a concomitant anomaly. The large
magnetocapacitance effect is observed below 5.7 K;
$\Delta\varepsilon(H)/\varepsilon(0)\equiv[\varepsilon(H)-\varepsilon(0)]/\varepsilon(0)$
reaches 13 % at 5.1 K. These results indicate a strong coupling between the
magnetism and dielectricity in Ca2CoSi2O7. Sr2CoSi2O7 shows a similar
magnetoelectric behavior to that of Ca2CoSi2O7. In contrast, in Ba2CoSi2O7,
which has the different arrangement of SiO4 and CoO4 tetrahedra from that of
Ca2CoSi2O7, the magnetocapacitance is hardly observed. The key for the
magnetocapacitance effect of $A_{2}$CoSi2O7 lies in the quasi-two-dimensional
crystal structure.

##  1 Introduction

Since the discovery of the giant magnetoelectric effect in TbMnO3 [1],
multiferroic materials that are both magnetic and dielectric have been
attracting much attention. The mechanism of the magnetic ferroelectricity is
well explained in terms of a spin-current model proposed by Katsura et al [2].
According to this model, the ferroelectricity of multiferroic materials
originates in a spiral spin structure. Therefore, materials with spin
frustration and/or nontrivial spin structures have attracted renewed interest
as promising candidates for new magnetoelectrics.

In this context, we expect $A_{2}$CoSi2O7 ($A$=Ca, Sr, and Ba) as one of such
candidates because $A_{2}$CoSi2O7 is a derivative of Ba2CuGe2O7, which has a
spiral spin structure below 3.26 K [3]. However, the magnetic transition
temperature of Ba2CuGe2O7 is rather low probably because of a spin fluctuation
of Cu2+ ($S$=1/2), which makes detailed measurements of the magnetoelectric
properties difficult. We thus select Co2+ ($S$=3/2) -based compound instead of
Cu-based one in order to raise the magnetic transition temperature. The
crystal structures of Ca2CoSi2O7 (Sr2CoSi2O7) and Ba2CoSi2O7 are shown in
Figs. 1(a) and (b), respectively [4, 5]. As seen from Figs. 1(a) and (b), both
of them have a two-dimensional structure in which SiO4 and CoO4 tetrahedra are
connected through the corners, but the arrangement of SiO4 and CoO4 tetrahedra
is different from each other. The crystal structure consists of an alternate
stack of the two-dimensional CoSi2O7 and Ca2+ (Ba2+) layers along the $c$
($b$) axis. However, little is known about the physical properties of
Ca2CoSi2O7 and Ba2CoSi2O7, and Sr2CoSi2O7 has not been investigated so far. In
this work, we have investigated the magnetic and dielectric properties of
$A_{2}$CoSi2O7.

##  2 Experiment

Single crystalline samples were grown by the floating zone method. Sample
characterization was performed by powder X-ray diffraction measurements at
room temperature. We confirmed that the obtained crystals are of single phase
and that Sr2CoSi2O7 has the same crystal structure as Ca2CoSi2O7. All the
specimens used in this study were cut along the crystallographic principal
axes into a rectangular shape by means of X-ray back-reflection Laue
technique. The magnetic properties were measured using a commercial apparatus
(Quantum Design, Physical Property Measurement System (PPMS)). The dielectric
constant was measured at 100 kHz using an LCR meter (Agilent, 4284A).

##  3 Results and Discussion

Figure 1: The crystal structures of Ca2CoSi2O7 (Sr2CoSi2O7) (tetragonal,
space-group $P\overline{4}2_{1}m$) (a) and Ba2CoSi2O7 (monoclinic, space-group
$C2/c$) (b). Temperature dependence of dielectric constant (c), (e), (g) and
magnetization (d), (f), (h) of $A_{2}$CoSi2O7 crystals. The magnetization
after zero-field-cooling and dielectric constant were measured in warming
scan. Figure 2: Magnetic field dependence of magnetocapacitance of Ca2CoSi2O7
(a), (b) and Sr2CoSi2O7 (c), (d) with different measurement configurations at
several fixed temperatures.

Figure 1 shows the temperature dependence of the dielectric constant and
magnetization of $A_{2}$CoSi2O7. The magnetization ($M_{\perp}$) of Ca2CoSi2O7
shows a jump at 5.7 K when applying magnetic fields perpendicular to the $c$
axis (Fig. 1(d)), indicating that a weak ferromagnetic (WF) transition occurs
at the temperature. On the other hand, no anomaly is found in the
magnetization ($M_{\parallel}$) measured with external magnetic fields
parallel to the $c$ axis. The dielectric constant perpendicular to the $c$
axis ($\varepsilon_{\perp}$) does not show any anomaly, while the dielectric
constant parallel to the $c$ axis ($\varepsilon_{\parallel}$) does a slight
increase below the WF transition temperature ($T_{\rm WF}$) (Fig. 1(c)). The
simultaneous change of the $M_{\perp}$ and $\varepsilon_{\parallel}$ at 5.7 K
implies that the magnetism is coupled with the dielectricity in Ca2CoSi2O7. As
seen from Figs. 1 (e) and (f), the magnetic and dielectric properties of
Sr2CoSi2O7 are similar to those of Ca2CoSi2O7. In Sr2CoSi2O7, the $T_{\rm WF}$
shifts to slightly higher temperature of 7 K, below which the
$\varepsilon_{\parallel}$ shows an abrupt increase compared with that of
Ca2CoSi2O7. On the other hand, the magnetic and dielectric properties of
Ba2CoSi2O7 are quite different from those of Ca2CoSi2O7 and Sr2CoSi2O7. The
magnetization of Ba2CoSi2O7 shows an anomaly at 5 K (Fig. 1(h)), suggesting
that some magnetic transition occurs, while the $\varepsilon_{\perp}$ shows
little change at the temperature (Fig. 1(g)). These results suggest that a
coupling between the magnetization and dielectric constant is very weak in
Ba2CoSi2O7.

In Fig. 2, we show the magnetic field dependence of the magnetocapacitance
($\Delta\varepsilon(H)/\varepsilon(0)\equiv[\varepsilon(H)-\varepsilon(0)]/\varepsilon(0)$)
at several fixed temperatures. Below the $T_{\rm WF}$, the
$\varepsilon_{\perp}$ of Ca2CoSi2O7 strongly depends on magnetic fields
parallel to the $c$ axis, and the large positive magnetocapacitance is
observed (Fig. 2(b)). With decreasing temperature, the peak position of the
magnetocapacitance curves shifts to higher magnetic fields of 5 T (5.5 K) and
8 T (5.1 K), and the magnetocapacitance effect is further enhanced;
$\Delta\varepsilon(H)/\varepsilon(0)$ reaches 13 % at 5.1 K. The
magnetocapacitance of Ca2CoSi2O7 is relatively large compared with those of
other recently discovered multiferroic materials (TbMnO3: 10 % [1], MnWO4: 4 %
[6], LiCu2O2: 0.4 % [7]). The observed large magnetocapacitance effect
provides clear evidence for a strong coupling between the magnetism and
dielectricity in Ca2CoSi2O7. Although Ca2CoSi2O7 does not show spontaneous
ferroelectric polarization in the absence of magnetic fields (not shown),
applying magnetic fields induces electric polarization below the $T_{\rm WF}$.
This is so-called ”magnetic-field-induced pyroelectricity” [8], which has not
been reported so far in other multiferroic materials to our knowledge. The
$\varepsilon_{\parallel}$ of Ca2CoSi2O7 is slightly suppressed by applying
magnetic fields perpendicular to the $c$ axis (Fig. 2(a)), i.e., the negative
magnetocapacitance is found.

In Sr2CoSi2O7, the $\varepsilon_{\parallel}$ depends on magnetic fields
perpendicular to the $c$ axis. The relatively large negative
magnetocapacitance is observed below the $T_{\rm WF}$ (Fig. 2(c));
$\mid\Delta\varepsilon(H)/\varepsilon(0)\mid$ reaches 3.5 % at 2 K. Compared
with the other multiferroic materials without magnetic-field-induced
polarization, the magnetocapacitance of Sr2CoSi2O7 is relatively large
(BiMnO3: 0.5 % [9], TeCuO3: 1.0 % [10], BaCo2Si2O7: 0.2 % [11]). The negative
magnetocapacitance is ascribed to suppression of the $\varepsilon_{\parallel}$
by applying magnetic fields perpendicular to the $c$ axis below the $T_{\rm
WF}$. In contrast to the case of Ca2CoSi2O7, the $\varepsilon_{\perp}$ of
Sr2CoSi2O7 is independent of magnetic fields parallel to the $c$ axis, and
magnetic-field-induced pyroelectricity does not show up.

In Ba2CoSi2O7, the dielectric constant is insensitive to magnetic fields (not
shown), and electric polarization does not appear. This means that the
correlation between the magnetism and dielectricity is almost negligible in
Ba2CoSi2O7. The difference among the magnetoelectric behaviors of Ca2CoSi2O7,
Sr2CoSi2O7 and Ba2CoSi2O7 is probably due to the difference in their two-
dimensional networks of CoO4 and SiO4 tetrahedra. Therefore, further
information on their crystal and magnetic structures are required for a full
understanding of the mechanism of the large magnetocapacitance effect of
$A_{2}$CoSi2O7. Synchrotron X-ray and neutron diffraction measurements are now
in progress.

##  4 Conclusion

In summary, we have investigated the magnetic and dielectric properties of
$A_{2}$CoSi2O7 ($A$=Ca, Sr, and Ba) and have observed the large
magnetocapacitance effect in Ca2CoSi2O7 and Sr2CoSi2O7 crystals. The large
magnetocapacitance effect indicates a strong coupling between the magnetism
and dielectricity in Ca2CoSi2O7 and Sr2CoSi2O7. In contrast, Ba2CoSi2O7 hardly
shows the magnetocapacitance, indicating that a coupling between the magnetism
and dielectricity is almost negligible. The arrangement of CoO4 and SiO4
tetrahedra is significant for the large magnetocapacitance of $A_{2}$CoSi2O7.

## Acknowledgment

This work was supported by Grant-in-Aid for scientific research (C) from the
Japan Society for Promotion of Science.

## References

  * [1] Kimura T, Goto T, Shintani H, Ishizaka K, Arima T, and Tokura Y 2003 Nature 426 55
  * [2] Katsura H, Nagaosa N, and Balatsky A V 2005 Phys. Rev. Lett. 95 057205
  * [3] Zheludev A, Shirane G, Sasago Y, Kiode N, and Uchinokura K 1996 Phys. Rev. B 54 15163
  * [4] Hagiya K, Ohmasa M, and Iishi K 1993 Acta Cryst. B 49 172
  * [5] Adams R D, Layland R, Payen C, and Datta T 1996 Inorg. Chem. 35 3492
  * [6] Taniguchi K, Abe N, Takenobu T, Iwasa Y, and Arima T 2006 Phys. Rev. Lett. 97 097203
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  * [8] The detailed results will be reported elsewhere.
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  * [11] Akaki M, Nakamura F, Akahoshi D, and Kuwahara H 2008 Physica B 403 1505


# Uplifting Interviews in Social Science with Individual Data Visualization:
the case of Music Listening

Robin Cura<EMAIL_ADDRESS>0000-0001-5926-1828 CNRS, UMR
Géographie-cités, ParisFrance ,  Amélie Beaumont  Université Paris 1 Panthéon-
Sorbonne, UMR Cresppa-CSU, ParisFrance ,  Jean-Samuel Beuscart  ,  Samuel
Coavoux  0000-0001-7991-3555 Orange Labs, ParisFrance ,  Noé Latreille de
Fozières  CNRS, UMR Géographie-cités, ParisFrance ,  Brenda Le Bigot
Université de Poitiers, UMR MIGRINTER, PoitiersFrance ,  Yann Renisio
0000-0001-9415-6193 CNRS, UMR Observatoire Sociologique du Changement,
ParisFrance ,  Manuel Moussallam  0000-0003-0886-5423 Deezer ResearchFrance
<EMAIL_ADDRESS>and  Thomas Louail  0000-0001-8563-6881
<EMAIL_ADDRESS>CNRS, UMR Géographie-cités, ParisFrance

(2021)

###### Abstract.

Collecting accurate and fine-grain information about the music people like,
dislike and actually listen to has long been a challenge for sociologists. As
millions of people now use online music streaming services, research can build
upon the individual listening history data that are collected by these
platforms. Individual interviews in particular can benefit from such data, by
allowing the interviewers to immerse themselves in the musical universe of
consenting respondents, and thus ask them contextualized questions and get
more precise answers. Designing a visual exploration tool allowing such an
immersion is however difficult, because of the volume and heterogeneity of the
listening data, the unequal ”visual literacy” of the prospective users, or the
interviewers’ potential lack of knowledge of the music listened to by the
respondents. In this case study we discuss the design and evaluation of such a
tool. Designed with social scientists, its purpose is to help them in
preparing and conducting semi-structured interviews that address various
aspects of the listening experience. It was evaluated during thirty interviews
with consenting users of a streaming platform in France.

personal data visualization, individual interviews, music streaming data,
computational social science

††copyright: acmcopyright††journalyear: 2022††conference: CHI 2022: ACM CHI
Conference on Human Factors in Computing Systems; April 30 – May 6, 2022; New
Orleans, LA††booktitle: CHI 2022: ACM CHI Conference on Human Factors in
Computing Systems, April 30 – May 6, 2022, New Orleans, LA††ccs: Human-
centered computing Human computer interaction (HCI)††ccs: Human-centered
computing Information visualization††ccs: Applied computing Sociology

Figure 1. An example of AMPLI’s exploration part. Here, there is both a
graphical selection brushed in France Normandy region, and selecting only the
year 2020 of the data. We can see that the streams originating from this area
comprised more Jazz music than usual for this interviewee (as illustrated by
the ”Tags” bar-plot in the bottom-right). The song the interviewee played the
most in this area and during that time-frame was ”Smile” by Jimmy Durante (as
seen on the music player widget on the left menu).

This image shows the exploratory part of AMPLI graphical interface. It
displays a song being played (Jimmy Durante’s ”Smile”), and various
interactive bar-plots showing the temporal and spatial dimensions of a given
Deezer user’s streams. Both a temporal selection (streams from 2020 only) and
a spatial selection (only streams located in Normandy) are made, and the
visual elements of the window reflect the differences in musical genres
listened to by the interviewee when these filters apply.

##  1\. Introduction

This case study is part of an interdisciplinary project (www.records.huma-
num.fr) in which social scientists and computer scientists, in collaboration
with the research department of a music streaming platform, are investigating
the social stratification of music listening practices in France. As part of
this research, individual interviews are conducted with users of the platform.
These are sampled ($n\approx 100$) among the respondents of a questionnaire
survey ($n\approx 15000$) who gave their written consent (i) to participate in
a recorded interviewed and (ii) for their several-years listening history data
on the platform to be linked with their answers to the questionnaire survey.
The interviews browse a number of subjects related to music listening and
music taste. These include the interviewee’s actual use of the streaming
service ; the spatial and temporal contexts in which he/she listens to music ;
his/her music taste and distaste, preferences for certain genres and artists,
etc. ; and finally his/her use and appreciation of the platform’s recommender
system. To assist the social scientists in preparing their interviews – that
is to develop some first insights of the streaming practices and the music
taste of the persons they will interview – we wished to design an exploratory
data analytics tool. This tool, named AMPLI – for Amplifying Music listening
Practices studies with Logs-augmented Interviews – should allow interviewers
to visualize the answers of the respondents to the survey, and it should also
allow to query and visualize the several-year listening history data of the
respondents collected by the streaming platform. These data contain many bits
of information for each single time-stamped stream, including the
song/artist/album listened to, if the song was liked/banned/skipped or not,
the estimated location of the user, the type of device used (was it a mobile
or a tablet or a web browser?), whether the stream was ’organic’ (e.g.
playlist, search) or recommended (algorithmic recommendation or editorially
edited platform playlist), etc. We intended the primary users of this tool to
be the social scientists conducting the qualitative interviews, and in the
following we will refer to them as _the/our users_.

In the next section, we give a general overview of the relevant research
literature. We then present our users’ needs and the constraints we
identified. Then we present the design and interactivity choices we made to
provide the users with a tool suited for the anticipated use cases. Finally,
we discuss the feedback we received from the users after a first round of
thirty individual interviews, focusing on the unexpected uses that were made
of AMPLI.

##  2\. A synthetic review of music data visualization

The design of AMPLI took place at the crossroads of different research fields,
including Computer-Human Interaction and Data visualization. Our goal was to
build a tool that would allow to visualize and to interact with musical data.
From the typology introduced in Khulusi et al. (2020)’s recent literature
review, we identified two particular tasks of the ”visualization of musical
collections” category : the ”Explorative analysis” and the ”analysis of
listening statistics” tasks. For both tasks, Dominikus Baur and his team made
several proposals (Baur and Butz, 2009; Baur, 2011; Baur et al., 2012). In
particular, Baur et al. (2012) designed arcs.fm, an application to visually
analyze Last.fm users’ listening history data. Its purpose is to support a
conversation between two Last.fm users who can visually compare their
listening activity on the service. They can thus easily identify the music
taste and distaste they have in common, and talk about it. While several
visual proposals in arcs.fm are undoubtedly inspirational, its use requires at
least a basic knowledge of the songs and artists that were listened to. This
makes it unsuitable for our own use case because our users visualize someone
else’s listening history data, not their own. Indeed, one of the main
requirements of the social scientists was to gain insights and develop a
general intuition of what the interviewees listen to, that may be very
different from what themselves listen to.

The existing literature on how to visualize music streaming data can be
roughly divided in two parts, on one side visualizations and interaction
schemes that are specific to musical data, and on the other ”generic”
visualizations and interactions. While the former (Torrens et al., 2004; Zhang
and Liu, 2017; Ono et al., 2015) results in visual synthesis of music data
that are more expressive, we had to consider the unequal ”visual literacy” of
our users who may be not used to interacting with state-of-the-art
visualization methods. Consequently, we decided to rely on simpler, standard
data visualization components such as histograms, bar-plots, line-plots,
geographic maps and interactive graphs. Each of these plots alone carries less
information, but their assembly and cross-interactivity allow for complex
visual queries and information retrieval. This approach was followed in (Baur
et al., 2012; Chen et al., 2010; Dias et al., 2012) and also by Wirfs-Brock et
al. (2020) in an experiment where a few interviewees were exposed to ”silent
data” extracted from their Spotify listening history in order to help them
defining how they would command a voice assistant to choose music. Although
our users are different – namely social scientists conducting interviews –
this study is inspirational in using simple plots that are easy to understand
for any user, even the less ”visually literate”.

Another body of work that proved useful relates to the use of personal data
and visualization interfaces during the interview process. Up to our knowledge
social science research on this issue has been limited, although the
increasing availability of individual digital traces should soon support new
methodological proposals, and hopefully the emergence of standards and good
practices. Dubois and Ford (2015)’s ”Trace Interviews” may be one of the very
first attempts to frame the various problems encountered when using data
visualization as a companion tool for social research interviews. The authors
collected publicly available digital footprints that were produced by
political representatives as part of their activity on the internet (including
tweets and wikipedia edits), and then invited these persons for an interview.
During the interview, people were presented with different visualizations of
their data and were asked to discuss both the content and the form. As the
authors phrase it:

> ”Trace interviewing provides opportunities to (1) enhance recall and
> generate an understanding of an actor’s decision-making process, (2)
> validate trace data-generated results, (3) address the data joining problem
> as interviewees explain traces on multiple platforms, and (4) respond to
> ethical problems relating to the use of personal data without consent or
> involvement by users.”

A major difference between this experiment and ours is that the interviewees
of (Dubois and Ford, 2015) were selected according to some content they
deliberately wrote on the internet. More precisely, they were selected because
what they wrote was interesting enough, both in quantity and quality, to
identify topics that could be discussed and enriched through a qualitative
interview. Furthermore, internet data collected in this study – and the
associated visualizations – were mono-dimensional: the two case studies
mentioned in this paper rely upon a couple of static plots, illustrating a
well-defined issue that does not require interactivity or data drill-down to
be discussed. In our case the constraints are different, and to the opposite
our goal was to assemble dozens of plots to embrace the multiple aspects of
the experience of navigating and listening to music on a streaming platform –
as illustrated by Figure 1 in the introduction.

Our case study bears more similarity with a recent study (Moore et al., 2021b,
a) where personal data availability was not the initial trigger but came as a
supplementary material for enriching otherwise scheduled interviews. Here the
authors developed a ”middle ground approach” of personal data visualization,
lying between what they call ”lightweights methods – designing sketches or
building data physicalization (Jansen et al., 2015) of a user personal data –
and ”heavyweight methods” – where tools are designed to let users answer
specific and predefined questions about their own data. By showing
interviewees the computational notebook they used to analyze their data, the
authors collect insights on how interviewees engage with their own logged
behavior when visualizing the resulting data.

##  3\. Designing an application for ”augmented” interviews

To design AMPLI we followed three steps identified by Oppermann and Munzner
(2020) in their ”data-first design studies” proposal. The first step consisted
in demonstrating to our intended users – social scientists who were to conduct
the interviews – the potential of listening history data to retrieve
information that would prove useful to prepare the interviews, and that would
get the interviewees to clarify some points that otherwise would likely not be
raised. We did so by showing them many examples of possible visualizations.
This is the elicitation phase. In the second step, we gathered their needs,
constraints and ideas in order to quickly elaborate a first prototype of the
tool. This is the winnowing phase. The third step consisted first in observing
the actual uses of the software during interviews – different users made a
different use of it –, and then in focusing on how to gather relevant feedback
that would answer the following questions: what was missing, what features
could be added, and maybe more importantly how exactly the application was
used in ”real-life” conditions – that is during the interviews. This is the
deployment phase.

  * •

Elicitation : we developed a visual analytics prototype to make our users
aware of the diversity and amount of information that are associated to each
individual stream. This prototype allowed them to explore the several-year
listening history data from a single random user of the streaming platform
(see figure 2). It focused on the spatial and temporal dimensions of the
streaming data, and on the music genres that were listened to. The main
purpose was to illustrate how interactive cross-filtering of the space-time
variables – approximate geographical location retrieved from the IP address of
time-stamped streams – could help in uncovering interesting patterns from an
individual listening activity. For example, by filtering appropriate months
and days, one could find out the approximate location where the interviewee
spent his holiday season, duration of the trip, and so on. Cross-filtering
also highlighted the music genres that were listened to in this peculiar time-
space frame, possibly different from the person usual behavior – and
understanding the reason for this discrepancy could be addressed in the
interview, e.g. listening during family meeting. This prototype was conceived
as a proof of concept, and proved useful in encouraging our users to exchange
ideas about the types of behaviors and the individual’s evolution of taste
that could be visualized analyzed thanks to the streaming data. It also helped
the researchers in evaluating the potentials, limits and bias of the streaming
history data.

  * •

Winnowing : The second step aimed at helping AMPLI’s prospective users to
express their visualization needs. To make them comparable, all interviews are
led according to a reference ”interview guide” that was conceived
collaboratively. The guide consists in a series of topics (and questions
within) to address with each interviewee, not necessarily in a fixed order.
Interviews are conducted by six different social scientists, each of them
being specifically interested in a particular topic. Those include music
tastes and distastes; the biographical approach to taste formation; the use of
music recommender systems; how people organize their music collections on
platforms (playlists, etc.); or the different social contexts associated to
different genres of music. To gather more qualitative material, social
scientists collectively elaborated a ”modular interview guide” that includes a
common set of questions for each topic/module, and more detailed questions
that will be addressed or not depending on who conducts the interview. During
the making of this guide, interviewers were asked to directly suggest
visualizations – or alternatively questions that visualization could answer –
for each module. This resulted in a list of 3 to 5 visualization requests per
module. The interviewers later refined these lists into actual visualization
proposals.

  * •

Deployment : This consists first in developing AMPLI, and then in improving it
continuously thanks to the feedback provided by its users. Social scientists
first used AMPLI during a wave of thirty individual interviews that were
conducted in Spring 2021. Before the wave started, each individual
visualization item was reviewed to ensure that it fulfilled the requirements
of the interviewers who had asked for it. Individual interviews were then
prepared and conducted with AMPLI. We then asked our users for their feedback
on how exactly they used the tool, at which moments of the interview they used
it, which parts of the app revealed the most useful and which were skipped,
etc.

Figure 2. Early prototype of AMPLI. Its purpose was to convince the
prospective users – social scientists who conduct individual interviews – that
the visual exploration of people’s listening history data had great potential
to suggest hot topics for the forthcoming interview, and to make people talk
about their music listening practice, provide details and nuances, etc. and
finally improve the quality of the information collected during the interview.

This figure displays an early prototype of the AMPLI application. It shows the
streams of a Deezer user, and using bar-plots, their temporal occurrence in
terms of hourly, daily, monthly and yearly distribution. On the second row of
the application, this interface displays a geographical heat-map of where the
streams were made, and a tree-map of the musical genres that those streams
corresponds to.

For the implementation, we chose to rely upon a programming language and
selected visualization libraries, rather than using an interactive, all-in-one
software like Tableau. This is mainly because of the strong coupling between
the pre-processing of the respondents’ streaming data, the queries performed
for each visualization, and the graphical rendering engine. We chose to use
the R software (R Core Team, 2021), both for its advanced analytical and
visualization features, and for its ecosystem of libraries, including the
shiny (Chang et al., 2017) framework AMPLI is built upon. Using a versatile
programming language, we can integrate all the tasks into a single
”visualization pipeline” (Keim et al., 2010), from data storage to graphic
rendering on the interactive user interface111Visualizations are designed
using the packages ggplot2 (Wickham, 2016) and echarts4r (Coene, 2021)..

##  4\. An overview of AMPLI

We named the application AMPLI – for ”Amplifying Music listening Practices
studies with Logs-augmented Interviews”. It is a web application that runs on
a distant server and is accessed inside the browser. It allows to explore the
data of a single individual that gave his/her full written consent to
participate in the RECORDS research program. More precisely, the individuals
whose data can be queried through the app gave their written consent for (i)
their web survey responses data to be linked with their several-year listening
history data collected by the streaming platform (ii) agreed to be contacted
for a recorded interview. Once logged in, the user can select the consenting
respondent whose data will be explored. The application window is divided in
two parts: the data visualization area in the center, and a control panel on
the left from which the user can select among three available tabs (see Figure
3). The left panel also includes a music player from which the user can
play/pause the songs that can be selected in the tabs. The three tabs
correspond to different use cases.

  * •

Report : the main tab displays a series of plots and tables that allow the
user to get a general overview of the respondent’s profile, thanks to his/her
responses to the web survey – including self-declared sociodemographics
information – and his/her listening behavior on the streaming platform.
Content organization in this tab mirrors the structure of the interview guide,
and consists in independent ”modules” (e.g. see the ”sociodemographics” module
in Fig. 3 and the ”listened genres” module in Fig. 5). The order of appearance
of the modules in the window was decided collectively. We conceived this part
as a static report to be used when preparing an interview and to keep notes
during it, so interactivity is limited on purpose. Following users requests,
more interactivity was introduced in the ”tastes” and ”distastes” modules, to
allow interviewers to listen to songs that are typical of the respondent’s
listening activity (as seen in Fig. 4, with a-ha being the most listened
artist of this respondent). This feature ensures an immediate immersion within
the music of the interviewee, whose taste may be very different from the
interviewer’s. Indeed, being able to listen to the songs and artists the
respondent played the most and the ones he/she explicitly ’like’, but also the
songs he/she systematically skipped or even banned provides the user a much
deeper understanding of the respondent’s musical ”universe”, especially when
compared to self-declared questionnaire data.

  * •

Survey: This tab presents the interviewee’s responses to the survey he/she
previously filled in a user-friendly way. We did not include this tab in the
initial design, but added it after numerous requests from the users. The main
reason for it is that the web survey is long (approximately 50 questions), and
several questions accept multiple combinations of possible answers. In this
tab, surveys answers are organized within tables that are dynamically
generated and highlighted (see fig. 4). This lets the interviewer get a quick
grasp of their interviewee’s answers.

  * •

Exploration : The exploration tab is the most interactive part of AMPLI. It
was included after the users expressed their regret the report tab was no
longer interactive, and wished to retrieve some of the features of the
prototype (see fig. 2). This tab is then largely based on the elicitation-
phase prototype. It contains visual analysis tools like multidimensional
cross-selection, drill-down or interactive graphs (see Fig. 1) that refine the
numerous possibilities of analysis of the respondent listening history data.
Such interactivity helps in identifying temporal and spatial patterns within
the respondents’ listening habits. It can sustain more precise, contextualized
questions about the contexts in which the respondent listens to music, or
about his/her musical taste. For example, the dynamic graph exploration lets
the interviewer finds artists that the interviewee did not listen on the
platform, but that are musically very close to the ones often listened to.
This can constitute a good entry to break the ice when asking the respondents
about their distastes. Another use case of the exploration tab is ”fact-
checking”, as it is easy with visual selections to isolate patterns mentioned
by the respondent during the interview (for example, a respondent talking
about the local music she listened when traveling abroad). By querying the
respondents’ data in many possible directions, visual exploration tab helps
the interviewer in checking the correspondence between what the respondent
said in the interview, and what he/she did according to his/her activity data.

Figure 3. The ”static report” tab of AMPLI. This page provides a synthetic
report on a surveyed individual. It includes both its responses to the RECORDS
survey questionnaire and a summary of its listening activity on the streaming
service. In this screenshot, all the ”modules” of the interview guide can be
seen. Using an accordion design, the numerous tables and plots of each module
can be hidden, as it is shown here (only the module ”Sociodemographic
informations” is visible).

This figure displays one of the three tabs of the AMPLI application, that is
the static report. It displays a table containing sociodemographic information
about a survey respondent : his/her age, gender, home location, marital status
and other additional information.

Figure 4. An example of AMPLI’s survey part - The respondant’s answers to this
question (”How would you like Deezer to help you discover new songs ?”) are
highlighted to speed up the interviewee’s answers analysis.

This figure displays a screenshot of the survey part of the AMPLI application.
It shows a survey respondant answers to the question ”How would you like
Deezer to help you discover new songs ?”. Positive answers (by proposing
”other tracks of already liked artists”, ”new versions/remixes of already
known songs” and ”songs with similarities of already liked ones”) are
highlighted in white (on a black background) while negative ones are grayed
out.

Figure 5. AMPLI survey part - We show here the time evolution of the musical
genres listened to by a given interviewee on the streaming platform. It shows
some volubility within the listened genres over time, the most noticeable
being that while pop music is always the main and dominant genre (top plot) of
the interviewee, the relative position of the other genres vary a lot over the
years (middle plot), like electronic music varying between 2nd and 6th
position.

This figure displays the ”Listened musical genres” module of the static report
part of AMPLI. Two plots are displayed. The first one is a bar-plot of this
user’s most listened genres (pop, hip-hop, r&b, etc.) and sub-genres (refined
by decade, music language, etc.). The second plot, commented in the caption,
is a line-plot of the relative ranks of each musical genre for each year. It
shows, for example, that the relative importance of electronic music in the
user’s listening has changed a lot over the years (the second most listened
genre of the user in 2016 and 2018, but only the 6th in 2019 and 2020).

##  5\. Discussion and future work

The first thirty interviews that were conducted with AMPLI highlighted some
unanticipated use cases that will guide the future developments. Firstly, some
of the interviewers shared some selected parts of AMPLI’s graphical interface
with the people they interviewed, in order to elicit their feelings and
reactions on topics that are known to be hard-to-grasp during traditional,
unequipped sociological interviews – including distaste in music. This
spontaneous use case resembles the ”personal data” visualization approach we
mentioned in section 2. We are now considering the development of a lighter
companion, interviewee-side app that would show selected screens from the
interviewer’s app. The interviewer could decide which screens he/she wants to
share, and could hide some other screens to prevent ”flooding” the interviewee
with too much information, and limit the risks of a negative reaction to such
an amount of personal data.

Another remarkable and unanticipated use-case that was reported by one of the
users consisted in using AMPLI as a ”fact-checker”, immediately after the
interview was completed, to confirm surprising assertions that were made by
the respondent about his/her use of the platform. Some AMPLI users also got
back to it several months after the interviews, when transcribing and tagging
the recorded audio, to retrieve some details provided by the interviewee, but
that were inaudible or misunderstood, including names of songs or bands.
Meanwhile, providing a general picture of an interviewee’s activity and
zooming on a very specific aspect of his/her activity history are two
different tasks, and we are currently working on the exploration part of the
application to make it suited to include both features.

These unanticipated use cases prove that a mixed methodology can greatly
improve our understanding of many ordinary practices that are increasingly
interfaced by digital technology. However, we are also convinced that such a
methodology is not straightforward to implement, especially when the digital
tracks – user logs – are both massive, highly multidimensional and depend on
the service. How to bind together people’s survey data with their digital
activity data in a single interface raises many questions, some ethical and
some technical. Those are fundamental questions social scientists will face in
the digital age, and further interdisciplinary work and other case studies are
needed for standards and good practices to emerge.

## Author contributions

RC, TL and MM designed the study. RC designed and implemented the application,
and drafted the manuscript. RC, TL and MM revised and edited the manuscript.
AB, JSB, SC, NL, BLB and YR conducted the interviews, used the application and
provided feedback. JSB coordinated the preparation of the interviews. All
authors approved the final version of the manuscript.

###### Acknowledgements.

This work is a part of the RECORDS (https://records.huma-num.fr) research
project. RECORDS is funded by the French National Research Agency (ANR) under
the reference ANR-19-CE38-0013.

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##  Appendix D Supplemental section

## Stellar environment and radiation process

In the paragraphs below we outline the essentials of emission mechanism of
compact objects common to most of the models developed for that purpose. There
are various models those have been successful to some extent to describe the
emission mechanism from compact objects, like neutron stars or white dwarfs.
Some of the most used ones are polar-cap[109], slotgap [110], outergap [111]
models etc..
We will consider here a simple picture of the enegetic-emission-physics from a
typical compact object. The specific details can be found in [112, 113] and
the references provided there in. The basic picture according to these models
is, the electric field $\rm{\bf{E}_{||}}$ is produced due to the rotating
dipolar magnetic field $\rm{B}$ of the compact object and is directed parallel
to the ambient dipolar field. Due to the action of electric field (
$E_{\parallel}$), charged particles ($e{{}^{+}}$ or $e^{-}$) are pulled out of
the surface of the compact star. Their number density is $n_{GJ}$ (where
$n_{GJ}=\frac{\Omega B}{2\pi ce}$) [120]. The radiation is emitted from the
charged particles, those are accelerated by the electric field
$\rm{\bf{E}_{||}}$.

For a typical pulsar of period $0.5$ sec., and magnetic field $\rm{B}\sim
10^{12}$ Gauss, the Goldreich Juliean number density $n_{GJ}$ turns out to be
$3.6\times 10^{11}/$cm3. These charged particles on their way along the curved
magnetic field, radiates EM radiation that is called curvature radiation. The
total energy released in the process is given by,

$\displaystyle\dot{E}_{rot}=\frac{2}{3}\frac{\Omega{{}^{4}}\bar{R}^{6}\rm{B}^{2}_{s}}{e^{3}}.$
(D.1)

In the expression (D.1) $\bar{R}$ is the radius, $\rm{B}_{s}$ is the surface
magnetic field and $\Omega$ is the rotational frequency of the compact star.
The total observed energy or total luminosity coming from these objects have
been observed to lie between $10^{25}-10^{35}$ erg/sec. That emitted energy
can be as high as $10^{7}$ MeV for a pulsar having magnetic field $\rm{B}\sim
10^{12}$ Gauss and rotational period $P=1sec$ [115]. The radiated photons of
this energy may create secondary plasma of $e^{+}$ and $e^{-}$ pairs through
the process, $\gamma+\gamma\to e^{+}+e^{-}$. These secondary particles alter
the number density of charge particles in the pulsar magnetosphere.
The energy of the emitted photons due to curvature radiations of the charge
particles in the magnetosphere of the compact object can be expressed in terms
of the instantaneous Lorentz factor ($\Gamma$) of the radiating charged
particles. It is given by $\omega=\frac{3}{2}\frac{\Gamma^{3}}{R_{c}}$, when
$R_{c}$ is the radius of curvature of the dipolar magnetic field lines.
Evolution of $\Gamma$, taking radiation reaction into account and energy gain
due to the electric field is described by [112],

$\displaystyle
m\,\frac{d\Gamma}{dx}=e\rm{\bf{E}_{||}}-\frac{2e^{2}\Gamma^{4}}{3R^{2}_{c}}.$
(D.2)

When the distance from the centre of the star is given by: $r=x+\bar{R}$ (
$\bar{R}$ is the radius of the compact object and x is the distance from the
surface of the star). This equation can be derived from the Lorentz-Abhram-
Diraction equation discussed in [147]. When the energy-gain becomes equal to
energy loss in Eqn.( D.2), a quasi-steady state is reached, that gives the
estimate of $\Gamma$ in terms of electric field. This is given by,

$\displaystyle\Gamma=\left(\frac{3\rm{\bf{E}_{||}}R^{2}_{c}}{2e}\right)^{\frac{1}{4}}.$
(D.3)

The electric field, $\rm{\bf{E}_{||}}$ at a position r, ($r>\bar{R}$) from the
centre of the  compact object is given by [115] and [146] in the space charge
limited flow in pulsar emission model:

$\displaystyle\rm{\bf{E}_{||}}\sim\frac{1}{8\sqrt{3}}\left(\Omega\bar{R}\right)^{\frac{5}{2}}\rm{B}\sqrt{\frac{2\bar{R}}{r}}.$
(D.4)

In Eqn. (D.4), ${\rm{B}}$ is the surface magnetic field of the compact object.
Since $\rm{\bf{E}_{||}}$ is position dependent, it introduce position
dependence to $\Gamma$, there by to the emitted radiation, $\omega$. Once the
emission region for the radiation of energy $\omega$ is determined, the
distance it would travel in the magnetosphere of the object can be estimated,
from the knowledge of $R_{LC}$ (when $R_{LC}=1/\Omega$, is light cylinder
radius).
Though in principle one can solve Eqn. (D.2), to find out position dependence
of the Lorentz factor, but solving it analytically is difficult. However it is
possible to solve Eqn. (D.2) numerically. A numerical solution is provided in
fig.[4]. The upward turning point ($P$) corresponds to the quasi static limit
in Eqn. (D.2). Similar behaviour for the primary particles has also been
reported in [145].

Figure 4: Evolution of $\Gamma$ (Lorentz factor) of primary charge particle in
the pulsar atmosphere with radiation reaction. The lowest point ($P$) in the
curve represents the situation when energy gain by the charge particle is
equal to energy loss due to radiation reaction i.e., when (D.2) equal to zero.

Since the peak emitted energy $\omega_{c}$, of the curvature photon is give
by:

$\displaystyle\omega_{c}=\frac{3}{2}\frac{\Gamma^{3}}{R_{c}},$ (D.5)

The equations (D.3),(D.4) and (D.5) can be used to estimate the emission point
of photons of a particular energy 888Here we have considered
$R_{c}=\sqrt{r/\Omega}$, which is a good approximation for dipole magnetic
fields, close to the light cylinder.. The relation between, angular speed
$\Omega$, the point of emission$\frac{r}{{\rm R}_{LC}}$, radius
${\rm\bar{R}}$, magnetic field ${\rm B}$ and energy of the emitted photon
$\omega$, can be expressed in dimension-less quantities as :

$\displaystyle\Omega=\frac{1}{{\bar{R}}}\left[\frac{\left({\bar{K}}\frac{{\rm
e\rm{B}}}{m^{2}_{e}}\right)^{3}}{(m_{e}{\bar{R}})^{2}}\sqrt{\frac{r}{R_{LC}}}\left(\frac{\omega}{m_{e}}\right)^{4}\right]^{\frac{1}{7}}.$
(D.6)

In equation (D.6), we have used a numerical parameter $\bar{K}$, defined as
$\bar{K}=4\pi\alpha\frac{160}{\sqrt{6}}\left(\frac{2}{3}\right)^{4/3}$. One
can estimate the path-length $z=(R_{LC}-r)$ for photons produced at point $r$
with energy $\omega$ from Eqn. (D.6) for various values of neutron star
parameters like radius, magnetic field and rotational time period. The
estimation of the density of the plasma is provided below.
The net number density of plasma produced in the magnetosphere can be
estimated by dividing the luminosity of star by their estimated energy [112].
The combined effect of pair produced plasma and the plasma ejected from the
surface of star can create a final plasma density, more than $n_{GJ}$.
Following this point of view, we have taken the plasma frequency $\omega_{p}$
to be of the order $10^{-2}$ eV. We have further considered the photon path
length to be $z=1.2$Km [121]. For these numbers of $\omega_{p}$ and $z$, we
have estimated various oscillation probabilities in KeV energy range
(20-100)KeV as shown in Fig: [1] and Fig: [2].
The basic motivation for this choice of parameters, was to find out the
imprints of $\phi F^{\mu\nu}F_{\mu\nu}$ interaction on the non-thermal
spectro-polarimetric signals from the star magnetosphere is realised in
nature. In presence of such interaction, some of these energetic photons may
eventually oscillate into a scalars and go out of the magnetosphere of the
compact object, and get detected by oscillating back into photon. Although the
same could be pointed out, with the results available in the literature [10]
and the references there in; however, in this work, we have considered the
effect of magnetized medium to the oscillation probability. Our numerical
estimates show that, emitted photon energy $\omega>>\omega_{p}$, therefore no
self absorption in the medium.
However the main process of obstruction, to the propagation of primary
photons, comes in the form of absorption. There are few processes those
contribute to this phenomenon. They are, (i)$\gamma+{\rm B}\to e^{+}e^{-}$,
(ii) $\gamma+\gamma\to e^{+}e^{-}$, (iii) scattering with ambient electrons in
the medium, (iv) synchrotron self absorption etc. .
For sub-threshold photon energies $(i.e.,\omega<2m_{e})$ (as is the kinematics
considered here) the process (i) is forbidden. In order to have the second
process viable, for photons with $\sim 100$ KeV energy, the minimum energy
required for the second photon $\omega_{th}\geq m^{2}_{e}\omega$ which is also
above pair production threshold. It is unlikely that at $r>0.99\rm{R}_{LC}$
the primary electrons will generate photons with energy $\omega=10m_{e}$ or
more. Therefore the second process also seems unlikely.
For stellar, objects of mass $M_{s}$, temperature T, the scale hight ( defined
as $h=\frac{k_{B}T{\bar{R}}^{2}}{m_{p}GM_{s}}$) is the quantity that
determines the density of the atmosphere of the steller object. Although,
proton mass $m_{p}$ is used in estimating the same, however if one invokes
local charge neutrality the scale hight of electrons would also be similar.
For a compact object with about one solar mass and temperature around $10^{5}$
Kelvin the ambient atmosphere close to the light cylinder would be extremely
thin to contribute to degradation of photon energy through compton scattering.
Lastly, the synchrotron self-absorption coefficient, follows a power law
behaviour $\alpha_{\omega}\propto\frac{1}{\omega^{s}}$, where the index $s>2$.
Hence the same may also be neglected for considering towards absorption.
Therefore for the kind of physical situation we have considered so far, the
main mechanism towards photon depletion would essentially be due to conversion
to scalars.