<p>They are both right, depending on the context and level of detail. The latter accurately describes what happens within the waveform. If you look further down in your reference, it shows that there is a "front porch" and "back porch" of the horizontal and vertical intervals.</p>

<p>However, the former describes what happens practically, especially in terms of the useful parts of the vertical blanking interval (VBI). Closed captioning, teletext, VITS, VITC, AMOL all use the blanked lines at the top of the field to transmit their data. (You should also note that your reference intentionally chose this layout in order to draw parallels with digital VANC and HANC, which are only at the top and left of the picture.)</p>

CC BY-SA 4.0
2018-11-22T11:40:34.840Z
Thanks, that makes sense. So just to make sure I understood you... the first image just ommits the bottom VBI and right HBI because they are generally not used for ANC data?
Jimbo
CC BY-SA 4.0
2018-11-23T00:06:09.830Z
You are mixing analog and digital. For an analog signal, the first omits or combines the VBI and HBI, either one describes the practical signal. For digital, the VANC and HANC are _only_ at the top and left of the picture.
Michael Liebman
CC BY-SA 4.0
2018-11-23T16:41:50.503Z
So, in the first image, if it were an analog signal the VBI at the top contains the top and bottom intervals from the second image or ommits the bottom interval from the second image... did I understand that right? Thanks again.
Jimbo
CC BY-SA 4.0
2018-11-23T18:08:07.517Z
From a practical perspective, yes that's right.
Michael Liebman
CC BY-SA 4.0
2018-11-21T19:16:45.940Z
2018-11-21T19:16:45.940Z
Michael Liebman
<p>I've been learning about the blanking intervals in video signals and often see diagrams like this:</p>

<p><a href="https://i.sstatic.net/um2oE.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/um2oE.png" alt="Shows vertical and horizontal blanking portions of image"></a>
<a href="http://www.sbe24.org/wba-sbe-shows/archives/Clinic2009/Conrad-Harris-2009.pdf" rel="nofollow noreferrer">(Ref)</a></p>

<p>But, I often also see the blanking regions describes as being on both sides of the image:</p>

<p><a href="https://i.sstatic.net/zDZ8Z.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/zDZ8Z.png" alt="Shows vertical and horizontal blanking portions of image"></a>
<a href="http://csweb.cs.wfu.edu/~torgerse/Kokua/Irix_6.5.21_doc_cd/usr/share/Insight/library/SGI_bookshelves/SGI_Developer/books/VFC_PG/sgi_html/ch03.html" rel="nofollow noreferrer">(Ref)</a></p>

<p>Which is correct? I would have thought the latter as I would have thought for CRTs the electron beam would be blanked at the end of a line so it can return to the start of the next line and then un-blanked. Same for vertical.</p>

<p>Which image/description is correct?</p>

<video>
Do blanking intervals appear on both sides of a frame or just the top and left?
Jimbo
# Do blanking intervals appear on both sides of a frame or just the top and left?

(Asked by: username_0 on 2018-11-21)

I've been learning about the blanking intervals in video signals and often see diagrams like this:

[![Shows vertical and horizontal blanking portions of image](https://i.sstatic.net/um2oE.png)](https://i.sstatic.net/um2oE.png)
[(Ref)](http://www.sbe24.org/wba-sbe-shows/archives/Clinic2009/Conrad-Harris-2009.pdf)

But, I often also see the blanking regions describes as being on both sides of the image:

[![Shows vertical and horizontal blanking portions of image](https://i.sstatic.net/zDZ8Z.png)](https://i.sstatic.net/zDZ8Z.png)
[(Ref)](http://csweb.cs.wfu.edu/~torgerse/Kokua/Irix_6.5.21_doc_cd/usr/share/Insight/library/SGI_bookshelves/SGI_Developer/books/VFC_PG/sgi_html/ch03.html)

Which is correct? I would have thought the latter as I would have thought for CRTs the electron beam would be blanked at the end of a line so it can return to the start of the next line and then un-blanked. Same for vertical.

Which image/description is correct?

**Tags:** video


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## 1 answers

### Answer by username_1 on 2018-11-21. Score: 1 | Accepted answer

They are both right, depending on the context and level of detail. The latter accurately describes what happens within the waveform. If you look further down in your reference, it shows that there is a "front porch" and "back porch" of the horizontal and vertical intervals.

However, the former describes what happens practically, especially in terms of the useful parts of the vertical blanking interval (VBI). Closed captioning, teletext, VITS, VITC, AMOL all use the blanked lines at the top of the field to transmit their data. (You should also note that your reference intentionally chose this layout in order to draw parallels with digital VANC and HANC, which are only at the top and left of the picture.)

> **Comments:**
> **username_0**: Thanks, that makes sense. So just to make sure I understood you... the first image just ommits the bottom VBI and right HBI because they are generally not used for ANC data?  
> **username_1**: You are mixing analog and digital. For an analog signal, the first omits or combines the VBI and HBI, either one describes the practical signal. For digital, the VANC and HANC are _only_ at the top and left of the picture.  
> **username_0**: So, in the first image, if it were an analog signal the VBI at the top contains the top and bottom intervals from the second image or ommits the bottom interval from the second image... did I understand that right? Thanks again.  
> **username_1**: From a practical perspective, yes that's right.  

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