Author Topic: How many lines of analog NTSC composite video are actually considered image?  (Read 379 times)

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Offline Ben321

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I know the total line count in a frame is 525, and that that the number of scanlines that that contain actual image are 486. I've verified that there are 486 lines that contain at least some image signal, and 484 full image lines (the other 2 are half image lines). If you add the 2 half lines together that gives you 485 lines worth of image present in a frame. However, I recently read elsewhere on the internet that there are supposedly actually 488 image lines in an NTSC image. This begs the question, where does this 488 lines come from? I can't find it in any official specification.
 

Offline BrianHG

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Not counting the potential video inside the vertical back-porch and the occasional half-line just before the vertical front-porch, the industry settled on 480 being the de-facto standard.

There used to be a little more until closed captioning text has been added plus in the all-analog days, there used to be parts of color bars inside the vertical back-porch so that studios could track and correct color errors along multiple broadcast pipelines and recordings.  There also used to be frame counter time codes there, even stock information encoded there within old analog C-band broadcasts on some new channels.  This has all become obsolete with all digital broadcasts beginning in the early 2000s.

See the Analog Video Standards R-REC-BT.470-6 and SMPTE standards here: https://www.eevblog.com/forum/beginners/ntsc-problem-with-atmega328/msg3764348/#msg3764348 if you want the official active picture lines of video.
« Last Edit: October 28, 2021, 07:38:13 pm by BrianHG »
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Offline Benta

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However, I recently read elsewhere on the internet that there are supposedly actually 488 image lines in an NTSC image. This begs the question, where does this 488 lines come from? I can't find it in any official specification.

You just answered your own question. The internet is not "the ultimate answer to everything". Someone posted something. So?

It reminds me of the assertion that a horizontal sync pulse is 4.77 us. It's not. It's 4.7 us (PAL).
But someone made a typo in a 1970s book, and it's been around ever since.



« Last Edit: October 28, 2021, 09:04:11 pm by Benta »
 

Offline BrianHG

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The absolute maximum is 507, ie 525 - 2x(3 line stable pulses,  3 line vsync, 3 line stable pulses).
Amiga home computers were able to do the 485 and they could software emulate the closed captioning codes via software if you really needed to.

This does not means your monitor will show them (vertical yoke needs time to deflect and the monitor may mute the video during this time) or that any video capture hardware will capture these extended lines, but, many composite video sampling ICs can be programmed to grab these lines of video inside the vertical back porch.
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Offline EPAIII

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The NTSC video signal, IN THE US, was 525 lines that were divided into two fields of 265.5 lines each. Each field had 20 lines that were reserved for the vertical interval for a total of 40 lines. 525 - 40 = 485. That was what the official, US FCC regulations specified and that is what was sent from all the TV studios that I worked at, maintained, and built and what was transmitted from all the TV transmitters that I worked at.

Due to the half lines that existed at the top of one field and at the bottom of the other and due to the color SC being out of phase in alternate frames (two fields = one frame) there were four different waveforms that were possible for the vertical interval. This drawing shows all four of them.

This does not say that there were not any variations. Video that was not intended for broadcast could vary widely and even with broadcast videos, problems could and did crop up. But this was the standard.
 


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