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 (Ref)

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 (Ref)

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?


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.)

  • 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 Nov 22 '18 at 11:40
  • 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 Nov 23 '18 at 0:06
  • 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 Nov 23 '18 at 16:41
  • From a practical perspective, yes that's right. – Michael Liebman Nov 23 '18 at 18:08

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