HDTV Forum

Under NTSC, we have a defined frame of 525 lines in two interlaced fields. Not all lines contain video information- some are defined for CC and other matters, some are undefined. In practice, a perfect NTSC signal "could" contain 480 lines of resolution, displayed. But the source matters. I have read where DVDs produce 400 lines, or elsewhere, 450 to 480 lines. OTA signals might actually be transmitted at about 350 lines, and VHS might be only 200-250 lines. What is the correct story?

And for ATSC signals, if the transmitted resolution in nominally 720 lines, how many are video data? Or are they all, at least theoretically, video data and the EPG, CC, data, etc are transmitted outside/above this 720 number? And could the station cheat a bit and transmit only, say 702 lines? Can they fudge the vertical and horizontal resolution a bit? Why would they? How would this work for 1080i and 480 SD & ED (widescreen)?

Answers, or a link to a good location for answers, would be appreciated. My searching has not yielded clear answers.

The NTSC horizontal oscillator runs at 525 lines per 60th of a second. About 483 of those lines carry image. The rest occur during vertical retrace and carry some digital information such as closed caption. Unlike computers, which under-scan (leave a black border), TVs over-scan, wasting about 5% of the image, depending on how the set is adjusted. So most people see about 460 lines. DVDs have 480 lines of true image (I think). VHS tapes smear this to the equivalent of about 250 lines, but S-VHS gets close to 480.

For 1080i, the horizontal oscillator runs at 1125 lines per 60th of a second. 1080 of those lines contain image. The story for 720p is similar. About 5% of those 1080 or 720 image lines are lost to over-scan on a CRT monitor, but usually all 1080 or 720 are visible on a digital display such as plasma.

I am not sure how often

The ATSC data stream has blocks of data that don

In practice, a perfect NTSC signal "could" contain 480 lines of resolution, displayed. But the source matters. I have read where DVDs produce 400 lines, or elsewhere, 450 to 480 lines. OTA signals might actually be transmitted at about 350 lines, and VHS might be only 200-250 lines. What is the correct story?

NTSC is 480 horizontal lines called vertical resolution and scan rates are defined by the vertical resolution of the image provided with a P for progressive or I for interlaced following them. Check the video dictionary in the library for a list of scan rates.

Yes, NTSC and the rest have more lines beyond the 480, 720, 1080 but they do not contain image information so the whole number is dropped and we use the image line count instead. That is where many end up confused as you are told 480P is actually 525 lines. Just stick with the number most are using and disregard these other versions based in technicalities.

I have read where DVDs produce 400 lines, or elsewhere, 450 to 480 lines. OTA signals might actually be transmitted at about 350 lines, and VHS might be only 200-250 lines. What is the correct story?

Lines of resolution are in the vertical and horizontal plane. Vertical resolution are the horizontal lines and horizontal resolution are the vertical lines. Confused? The numbers you are now citing are the numbers we concern ourselves over. For NTSC composite video the horizontal resolution is about 350 lines max and when you record that on a VCR it drops to about 250 unless it is an S-VHS VCR. DVD component video is expected to have at least 480 and could potentially go as high as 640 to 720 depending on which camp you are listening to. For HD this is the 1280 of 720P or the 1920 of 1080I.

Regardless of the SD or HD source your equipment must still be checked to see if it actually passes these signals correctly. DVD players are doing a much better job of meeting this spec than our HD STBs. Explaining and showing this with scope waveforms will be an upcoming feature of HD Library.