Why does NTSC video usually get captured at a resolution of 720x480?

[Videogamer555]

Logically it would make sense to capture it directly at 640x480 (or better yet, 648x486). There's no actual horizontal pixel count specified in the NTSC baseband video specification, because analog video is transmitted as a sequence of 486 continuous horizontal lines. The horizontal resolution of the video is limited only by the FCC TV broadcast specification, which imposes a 6MHz bandwidth for the entire channel (both audio and video). Because an actual physical SD TV screen has an actual physical aspect ratio (as measured by a ruler) of 4:3, it makes most sense to have video capture cards capture directly to 640x480 pixels. Actually it would make even more sense to capture to a resolution of 648x486, to keep the 4:3 aspect ratio while also keeping the original number of vertical lines (there are actually 3 lines above and 3 lines below the normal 480 found in video captures, and the top line and bottom line are actually only half-lines in that the top line starts about half way into the image and the bottom line ends about halfway into the image). The horizontal resolution of a displayed image on an analog TV with CRT is limited only by the resolution connected to the composite video input of the TV (how many pixels wide is the CCD chip of the video camera), so you could theoretically have a video source that is 1000x486 and it should display properly on an analog SD TV with just exceptionally high horizontal resolution (objects looking very sharp in the horizontal direction, such as very sharp vertical edges on objects).

If you are talking about the limits imposed by the NTSC and FCC for broadcast TV https://antiqueradio.org/art/NTSC%20Signal%20Specifications.pdf then then you need to limit the video baseband bandwidth to 4.5MHz. A 4.5MHz bandwidth could be captured by an analog to digital converter sampling at a rate of 9MHz. The line rate of video in color NTSC video is approximately 15734.26 lines per second. With a sample rate of exactly 9MHz, you have approximately 572 pixels per line (including both image content and H-blanking period). On each line, 83.5% of the line is image (the remainder being the H-blanking period). This gives you an image content line width of 477.62 pixels. As pixels are discrete, the final image will need to be 478 pixels wide. So the proper resolution for capturing broadcast TV, would be 478x486 (which is almost a perfectly square image, and would need to be stretched horizontally on playback to fit the 4:3 aspect ratio).

If you are talking about digitized VHS tape (which has a reduced bandwidth, even when compared to broadcast TV), then the screen width is only 320 pixels, so it's actually quite a bit taller than it is wide, so would need to be highly stretched in the horizontal direction to fit the 4:3 aspect ratio. My source of info on the width of a VHS tape frame is from https://www.mediacollege.com/video/format/vhs/ and it gave it in TV Lines resolution, a resolution measurement which is described here https://en.wikipedia.org/wiki/Television_lines on Wikipedia. So basically a VHS tape has a horizontal resolution of 240 TVL, which means that in a portion of the screen that's as wide as the screen is tall, how many individual alternating 240 alternating black and white vertical lines (120 of each) can be seen. This is equivalent (on a display using pixels) to 240 pixels wide in a portion of the screen as physically wide (as measured by a ruler) as it is tall. Since the aspect ratio of the screen is 4:3, that means you have 240/3*4=320, for a width of 320 pixels across the full screen. Of course that doesn't take into account the overscan portion of the image. In the broadcast standard I was talking about, 478x486 pixels is for the full image, and that includes pixels in the overscan region. There's more than 320 pixels wide in a VHS tape stored image, as 320 is only the visible screen portion. According to https://build.com.au/what-overscan the overscan on a TV can cut off up to 5% of width and height of an image. If the on screen portion of the image is 320 pixels wide, then the actual full width of the image could be as large as approximately 336.84 pixels. So to properly digitize a video on VHS tape, you would need to capture it with a width of 337 pixels, for a full image size of 337x486. Again, this would need to be drastically stretched horizontally on playback to fit the correct aspect ratio of 4:3.

As you have noticed, none of these assessments I've done on various digitized video sources, have shown that there should ever be a width of 720, or that the correct aspect ratio of the video should be 3:2 (which is the aspect ratio of 720x480). I also don't know why 480 is the standard height for capturing NTSC video, as 480 is not the correct number of lines for a video frame, and the difference of 6 lines high is only 1.23% of the video height (the height being 486 pixels). 1.23% is less than any amount of overscan that might exist (which is 2% to 5%), so I don't know why the standard is to crop off the upper 3 and lower 3 lines of the image during video capture, as overscan would crop off a lot more than that.

Can someone here explain how the resolution of 720x480 became the standard for digitizing NTSC video?

[davexnet]

See this discussion
https://forum.videohelp.com/threads/405627-The-myth-of-native-720-samples-per-line-whe...eo#post2655653

[pandy]

not sure what is your goal, also not sure if you intentionally (or not) ignore some signal processing aspects and/or technology background associated with video signal processing. Anyway i strongly recommend to use worldwide standard reference https://www.itu.int/rec/R-REC-BT.601/ .
Btw this perhaps could be good topic for discussion somewhere before year 2000 but now overall world is focused on UHD, 8k, and i think most interesting for future TV standards https://www.bbc.co.uk/rd/projects/high-frame-rate-tv - very good lecture for today https://www.bbc.co.uk/rd/publications/whitepaper282


Forgot to add link to paper that answers your questions. https://tech.ebu.ch/docs/techreview/trev_304-rec601_wood.pdf

[Videogamer555]

not sure what is your goal, also not sure if you intentionally (or not) ignore some signal processing aspects and/or technology background associated with video signal processing. Anyway i strongly recommend to use worldwide standard reference https://www.itu.int/rec/R-REC-BT.601/ .
Btw this perhaps could be good topic for discussion somewhere before year 2000 but now overall world is focused on UHD, 8k, and i think most interesting for future TV standards https://www.bbc.co.uk/rd/projects/high-frame-rate-tv - very good lecture for today https://www.bbc.co.uk/rd/publications/whitepaper282

Yes, old media and signal types. But these need to be archived, before the source material (especially magnetic material) becomes degraded beyond usability. It's exactly BECAUSE they are old, that I'm trying to find the best digitizing technique to capture these in as close as possible the digital equivalent of their native format. This means not always capturing at 720x480 (and in fact not even always capturing in the same format between sources, for example SVHS has a much higher bandwidth than VHS, and broadcast standard has an even wider bandwidth than that, while baseband video signal sources like cameras have no official bandwidth specification). Capturing always in 720x480 will miss some data in the vertical direction, while capturing more data than needed in the horizontal direction. While oversampling is initially needed by the analog-to-digital converter for precise alignment of the sync signals, once the image is formed it can be downsampled back to its native size. 9MHz would be the minimum sample rate to get all the data from an NTSC broadcast, and typically several times that sample rate would be needed to get sub-pixel accuracy in aligning the sync signals. But once aligned it should be digitally downsampled again by the onboard DSP chip or driver or the control software for the digitizer, to match the original format for video source (for example a width of 648 for digitized directly received NTSC broadcast video, and a width of 478 for digitized VHS tape playback). And all this would help to keep the digitized copy of the source material as close to accurate as the actual original analog source material as possible.

[ConsumerDV]

This means not always capturing at 720x480 (and in fact not even always capturing in the same format between sources, for example SVHS has a much higher bandwidth than VHS, and broadcast standard has an even wider bandwidth than that, while baseband video signal sources like cameras have no official bandwidth specification). Capturing always in 720x480 will miss some data in the vertical direction, while capturing more data than needed in the horizontal direction.

720x486 (later modified to 720x480 because 480 divides by 16) and 720x576 have been decided upon based on broadcast standards of that time and on interoperation between them. These numbers are as good as it gets. Number of scanlines is fixed across the board, number of unique samples in a scanline is fewer than 720, in case of VHS or Hi8 much fewer.

You cannot use 640x480 instead of 720x480 because:*

• you video may have proportions other than 4:3, for example 16:9 or 14:9
• 720x480 usually includes overscan, so it does not represent a 4:3 or 16:9 frame anyway, it is slightly wider

Some combinations of hardware and software offer 640x480 frame size with correctly cropped overscan, like Dazzle DVC 100 + VirtualDub. Nothing wrong about that, IMHO, if you agree that your video is 4:3 and that it has symmetric overscan and that you trust the software to crop it and that you don't plan to make a DVD. In fact, I think this should be the default "easy" option in all software packages. But then there should be a custom mode as well.

Well-designed user-friendly software usually has a wizard-style UI that guides you through the process: "Is it a standard or widescreen video? -> Move the wireframe on this preview to select the part of the video you want to capture -> Do you want to make a DVD? -> Do you want to upload to YouTube? -> Do you want to retain the original image rate? Etc." Bad software takes the most common and least objectionable path. And then there is software that puts a user in charge, this the kind of software that members of this forum use.

YouTube accepts video with non-square pixels if PAR is correctly specified in the header. Too bad that many people either forget to do it, or don't know about non-square pixels, or they use containers that do not clearly define how to set PAR, so they end up with videos that are stretched. But since you are here, I am sure you are not one of these people

*You can, but then you may need to re-sample again.

--

I think I wrote too much as usual. To address the original quote: no, you will not lose anything by capturing into 720x480.

[pandy]

Capturing always in 720x480 will miss some data in the vertical direction, while capturing more data than needed in the horizontal direction. While oversampling is initially needed by the analog-to-digital converter for precise alignment of the sync signals, once the image is formed it can be downsampled back to its native size. 9MHz would be the minimum sample rate to get all the data from an NTSC broadcast, and typically several times that sample rate would be needed to get sub-pixel accuracy in aligning the sync signals. But once aligned it should be digitally downsampled again by the onboard DSP chip or driver or the control software for the digitizer, to match the original format for video source (for example a width of 648 for digitized directly received NTSC broadcast video, and a width of 478 for digitized VHS tape playback). And all this would help to keep the digitized copy of the source material as close to accurate as the actual original analog source material as possible.

I will address some of your comment briefly.

720x480 is broadcast standard - you can't capture less than BT.601 standard offer per se - 486 lines in NTSC are present only in pre 601 tmes - then you may loose some information if you trying to capture it with 601.

Oversampling has nothing or not much with synchronization - using ADC to acquire whole video seem to be waste (as 8 bit ADC usually offer sub 8 bit accuracy - try familiarize with for example ENOB .

Oversampling allow to deal with some signal processing limitations - there is many befits for using oversampling - extreme case are so called 1 bit converters (99% or more for nowadays audio ADC/DAC) - with massive oversampling such as 64x times or more with help of 1 bit you can reach 16 bit resolution and sometimes also accuracy. Nevertheless oversampling is your friend.

If capture sample rate and playout sample rate are equal then you have 1:1 mapping there is no distortions in terms of aspect ratio.

Also aspect ratio before directly addressable pixels type of displays (i.e. LCD, PDP, OLED etc) i.e. in CRT times was less stable - there is many factors affecting display aspect ratio on CRT's and usually error around 10% was common - also DAR in CRT times was heavily dependable on scene brightness.
CRT was never perfect display from videophile purist perspective - many factors involved - you can't ignore limitations of the technology in those times - cameras using tubes https://en.wikipedia.org/wiki/Video_camera_tube - and shared some limitations commonly with CRT's.

601 allowed to content exchange between Europe and USA - it was beneficial for both sides - and this is key for 601. It tried to address problems encountered earlier and thanks to 601 we have modern digital video technology.

[dellsam34]

This means not always capturing at 720x480 (and in fact not even always capturing in the same format between sources, for example SVHS has a much higher bandwidth than VHS, and broadcast standard has an even wider bandwidth than that, while baseband video signal sources like cameras have no official bandwidth specification). Capturing always in 720x480 will miss some data in the vertical direction, while capturing more data than needed in the horizontal direction.

Sampling luminance at 720 (704 without black bands) is already more than enough for S-VHS which theatrically could achieve 400 samples per scan lines before it gets recorded on tape, but when recorded on tape it does not exceed 300 samples or steps per line, So 704 is double sampling, and triple sampling for VHS (about 220 luma samples per scan line), Broadcast tape formats are 300-340 luma samples per scan line depends on what format.

[Videogamer555]

This means not always capturing at 720x480 (and in fact not even always capturing in the same format between sources, for example SVHS has a much higher bandwidth than VHS, and broadcast standard has an even wider bandwidth than that, while baseband video signal sources like cameras have no official bandwidth specification). Capturing always in 720x480 will miss some data in the vertical direction, while capturing more data than needed in the horizontal direction.

Sampling luminance at 720 (704 without black bands) is already more than enough for S-VHS which theatrically could achieve 400 samples per scan lines before it gets recorded on tape, but when recorded on tape it does not exceed 300 samples or steps per line, So 704 is double sampling, and triple sampling for VHS (about 220 luma samples per scan line), Broadcast tape formats are 300-340 luma samples per scan line depends on what format.

720 seems arbitrary. And I'm not sure whether it covers overscan region or not. It very well could just be covering the portion of the image that's guarantied to be in on the screen of a normal TV, and excluding the overscan region. 720 pixels wide seems arbitrary.

[ConsumerDV]

720 seems arbitrary.

720 may include overscan. It does not include blanking.
https://tech.ebu.ch/docs/techreview/trev_304-rec601_wood.pdf

[Sharc]

Never mind, but it seems we are going to repeat what has recently been addressed and argued here, and discussed in many similar threads over the past decade(s).
Everyone is free to re-invent the wheel and develop his private digitization standards wrt sampling rates and "resolutions" though, one each tailored for NTSC, PAL, SECAM, Video8, Hi8, VHS, SVHS, laserdisc ... and so on, and eventually try to find the support of the industry .
The Rec.601 was a breakthrough in finding a "common denominator" with the 13.5MHz sampling rate which has subsequently been accepted for analog SD video digitization, even though it is some overkill for certain low resolution sources with their "approximate" resolutions or spectral bandwidths.

[lollo]

Everyone is free to re-invent the wheel and develop his private digitization standards

Yes. For the rest of us, we are lucky that a standard called ITU-R BT.601-4 (formerly "CCIR-601" or "Rec.601") exists, and it perfectly fits all our needs

[Pinto007]

This topic is poor trolling IMHO. Such topics should be closed as soon as possible without any discussion.

[dellsam34]

720 seems arbitrary. And I'm not sure whether it covers overscan region or not. It very well could just be covering the portion of the image that's guarantied to be in on the screen of a normal TV, and excluding the overscan region. 720 pixels wide seems arbitrary.

It's not arbitrary, it's based on mathematical calculations and weeks of debating and drafting before reaching a finalized standard. Just read up white papers online, there is nothing we can add here not available freely online.

[dvd3500]

720 seems arbitrary. And I'm not sure whether it covers overscan region or not. It very well could just be covering the portion of the image that's guarantied to be in on the screen of a normal TV, and excluding the overscan region. 720 pixels wide seems arbitrary.

It's not arbitrary, it's based on mathematical calculations and weeks of debating and drafting before reaching a finalized standard. Just read up white papers online, there is nothing we can add here not available freely online.

It was early Internet days so it likely was months if not years!

[Skiller]

720 may include overscan. It does not include blanking.

Don't agree with that terminology.
Overscan is something a display (TV) does. It's not related to the signal itself.
Blanking is what's left of the signal if we subtract active image, sync pulses and color burst. In other words, the front and back porches as well as most of the vertical sync area are blanking (altough the latter can be used to carry data such as closed captioning in 60 Hz systems and teletext in 50 Hz systems).

According to ITU-R BT.601, 720 covers the duration of 53.333 µs of the analog signal, which means it carries some blanking (part of the front and back porch) for both 50 and 60 Hz systems. This is the commonly observed ~16 pixels in total at the left and right sides of a 720 capture.

The math is 53.333 µs x 13.5 MHz = 720 px

[ConsumerDV]

Skiller, I guess this is what I meant. It is interesting that on the second picture there is a 40T area (16+24 or 8+32) that corresponds both to the analog blanking and to video data (but is excluded from digital blanking).

[pandy]

Skiller, I guess this is what I meant. It is interesting that on the second picture there is a 40T area (16+24 or 8+32) that corresponds both to the analog blanking and to video data (but is excluded from digital blanking).

This diagram describe relation between digital data (that may flow on for example BT.656 compliant interface) and analogue signal timing - analogue video standard provide some tolerance as in physical implementation analogue timing is used (and there are some tolerances in analogue electronic components) - in digital data tolerance is specified only by timing accuracy (usually way more restricted than analogue timing) - clearly digital data organization is way simpler - 1440 video samples (720 of Y and twice 360 for chroma - Cb, Cr) - start and end of active video are signaled by SAV and EAV strobes (they use forbidden from video signal perspective values 0 and 255) - blanking area is used for digital data.
As professional video equipment usually use 10 bit video then all video samples are sent in 10 bit (8.2) format - 8 bit equipment ignore (truncate) fractional part of 8.2 format) - raw video speed is 270 Mbits due of this.

Overscan is unrelated to analogue and digital video standards - it is not covered either by BT.470/BT.1700 or BT.601.

[Videogamer555]

720 seems arbitrary. And I'm not sure whether it covers overscan region or not. It very well could just be covering the portion of the image that's guarantied to be in on the screen of a normal TV, and excluding the overscan region. 720 pixels wide seems arbitrary.

It's not arbitrary, it's based on mathematical calculations and weeks of debating and drafting before reaching a finalized standard. Just read up white papers online, there is nothing we can add here not available freely online.

The 480 height seems arbitrary though. I mean there are 486 total lines that include at least some image signal (the top and bottom only have half of a line of image, with the other half on each of them being blank). If you assume between 2% and 5% of the image is lost to overscan (I read those percentages on another website), then that leaves you with 476 to 462 lines that would be visible on a real CRT TV. So even if your capture specs were intended to simulate overscan, by capturing only the non-overscan part of the video (cropping out the overscan part), then the image height should be between 462 and 476 lines. So the numbers I'm looking at here are 486, 476, and 462. None of these are 480. I don't see any reason for a height of 480, other than the fact that very old CRT computer monitors (which did not suffer from overscan like CRT TVs) tended to actually be exactly 480 pixels heigh (there were physically 480 rows of pixels on those old CRT computer monitors).

480 seems to be less about the specification of the analog signal, and far more about keeping the digitized copy of the NTSC signal compatible with the CRT computer monitors that existed at the time that the NTSC digitization technique was standardized (making it so that on playback, the video would exactly fill the height of the computer monitor). We have monitors with a lot more vertical resolution than 480 now though, so there's no reason for any modern analog video capture device to not capture all 486 lines.

[ConsumerDV]

Overscan is unrelated to analogue and digital video standards - it is not covered either by BT.470/BT.1700 or BT.601.

Right, overscan is not part of the standards, but it should be [or should have been] taken into account when creating a TV/video program.

The 480 height seems arbitrary though. I mean there are 486 total lines that include at least some image signal (the top and bottom only have half of a line of image, with the other half on each of them being blank).

486 has been reduced to 480 because 480 mod 16 = 0. Why 16? Because early digital video encoders used 16x16 blocks. I think I already mentioned this above. Why do you care about 6/486 = 1.2% of vertical resolution? You would not see these lines if you were watching a TV/video program on a CRT TV set.

If you assume between 2% and 5% of the image is lost to overscan

Traditionally it is considered that 10% (5% each side) can be lost to overscan. Remaining 90% is action-safe area. Title-safe area is usually 70%. Some think that these recommendations should be adjusted or completely rescinded for digital TV, but for analog TV they are as I wrote above.

[pandy]

Right, overscan is not part of the standards, but it should be [or should have been] taken into account when creating a TV/video program.

And usually it is taken into consideration during content production - overscan was used mostly as practical way to deal with analog and early digital TV technology limitations - there is still available video content where on overscan area, visible are different video layers and some mixing/muxing garbage - many young videophiles forget that at the beginning of 80's RAM technology barely allow to store single field (and amount of circuitry to do so was enormous), similar with other technologies used for video processing...
Today i see commonly that even reputable TV stations don't care about overscan (some news tickers are partially cropped on TV's incapable to do 'just scan') - this is common trend, every generation of engineers (except single cases) seem to be stupider than preceding one. I seriously doubt if today something like NTSC or PAL could be created from scratch - seem monochromatic TV is maximum for modern engineers... Sad but true...

The 480 height seems arbitrary though. I mean there are 486 total lines that include at least some image signal (the top and bottom only have half of a line of image, with the other half on each of them being blank). If you assume between 2% and 5% of the image is lost to overscan (I read those percentages on another website), then that leaves you with 476 to 462 lines that would be visible on a real CRT TV. So even if your capture specs were intended to simulate overscan, by capturing only the non-overscan part of the video (cropping out the overscan part), then the image height should be between 462 and 476 lines. So the numbers I'm looking at here are 486, 476, and 462. None of these are 480. I don't see any reason for a height of 480, other than the fact that very old CRT computer monitors (which did not suffer from overscan like CRT TVs) tended to actually be exactly 480 pixels heigh (there were physically 480 rows of pixels on those old CRT computer monitors).

720/480=1.5 - there is nothing arbitrary - this is outcome of many practical aspects - for today you can easily buy IC capable to perform 10 GFLOP's and this IC cost perhaps 10$ - in those times digital multi IC video processors offered barely 20..100MIPS and less than 1MFLOP.
Using neat numbers simplify design, make it possible (feasible from technology perspective) - fractions are difficult to deal, require additional error processing and there is no benefits from it - so if you can loose 6 lines and cut complexity by half then it is obvious decision - it cost less, has no practical impact on quality - imagine using 4164 DRAM circuits with 250ns R/W cycle - you need 8 of such IC's to create 64KiB memory where you can store array 256x256 8 bit pixels so not even single field of your composite video, 250ns R/W cycle means that you can maximally transfer 4MB per second i.e. you need introduce memory interleaving to get faster transfers and memory interleaving means you need more memories so practically you need to use not 8 IC's but 32 of them to create 512x512 array capable to transfer at least 16MBps - 32 DRAM's means you need large PCB and additional electronics it means it will be power hungry and definitely not for consumer use.
At the beginning of 90's DCT require dedicated, specialized IC capable only to perform 8x8 DCT on 8 bit integer with speeds like 15MPixel per second... you need few of such IC's to build video JPEG realtime codec...
Every engineering decision made in 70's/80's/90's there is strong foundation of math and technology limitations from those times... Imagine that DCT as foundation for every modern video codec doesn't existed before 1972 (and it took almost 10 years to adopt mathematical theory behind DCT to practical implementations).
Those 6 lines are meaningless especially that in 99% cases there is no useful video there.

[Brad]

All of your questions are answered in a book I recommended to you in October.

there are 486 total lines that include at least some image signal (the top and bottom only have half of a line of image, with the other half on each of them being blank).
[...]
We have monitors with a lot more vertical resolution than 480 now though, so there's no reason for any modern analog video capture device to not capture all 486 lines.

If you'd ever actually captured at 720x486, you would know that the analog NTSC line count wasn't simply "484, plus 2 half-lines". It varies with each source. I have TV recordings where the number of active lines, and placement of the top half-line, varies from shot-to-shot.

It does make sense that consumer capture devices stick with the consumer 480 resolution, while the professionals who won't be troubled by the "garbage" at the top of the signal can use professional 486 capture devices. The fact that 720x480 captures reveal VHS head-switching noise is already confusing enough for consumers.

there are actually 3 lines above and 3 lines below the normal 480 found in video captures, and the top line and bottom line are actually only half-lines in that the top line starts about half way into the image and the bottom line ends about halfway into the image

Had you read the book, you would also know that this is wrong. There's only a half-line below a typical 480-line capture. All of the other extra lines in a 486-line capture are from above.

[dellsam34]

I mean there are 486 total lines that include at least some image signal (the top and bottom only have half of a line of image, with the other half on each of them being blank). If you assume between 2% and 5% of the image is lost to overscan (I read those percentages on another website), then that leaves you with 476 to 462 lines that would be visible on a real CRT TV. So even if your capture specs were intended to simulate overscan, by capturing only the non-overscan part of the video (cropping out the overscan part), then the image height should be between 462 and 476 lines. So the numbers I'm looking at here are 486, 476, and 462. None of these are 480. I don't see any reason for a height of 480, other than the fact that very old CRT computer monitors (which did not suffer from overscan like CRT TVs) tended to actually be exactly 480 pixels heigh (there were physically 480 rows of pixels on those old CRT computer monitors).

I have yet to come across a tape that contains any useful video at the top extra 6 lines that is reserved for signaling and other proprietary barcoding. I have devices that allow me to tweak the sampling parameters as seen in the screenshots below:

[Brad]

I have yet to come across a tape that contains any useful video at the top extra 6 lines that is reserved for signaling and other proprietary barcoding.

Really? What about that cartoon trailer we both have (Grandma Got Run Over by a Reindeer)?

Been meaning to add some examples to my old thread, so here you go: https://forum.videohelp.com/threads/359690-Geek-fun-with-VBI-Macrovision-DVD-recorders...es#post2656567

[Videogamer555]

Right, overscan is not part of the standards, but it should be [or should have been] taken into account when creating a TV/video program.

Those 6 lines are meaningless especially that in 99% cases there is no useful video there.

I'm thinking in terms of archival. Not like making a quick digital video to post on YouTube, but rather a way of archiving the video as a digital master copy, that loses NONE of the original information that would have been present in the image portion of the signal (this means all 486 lines must be recorded). Yes, there can be useful informatino there. There are ways though you can store digital data on those 6 missing lines, in particular closed caption text data. Other types of digital data from various sources may also use these 6 extra lines for their own source-dependent metadata (maybe some professionally made VHS tapes would store metadata describing the content of the tape, or broadcast sources might have used it to store the call letters of the TV station). For the purpose of archiving tapes, 486 lines recording is essential, as it can be more than just image signal stored there.

[Videogamer555]

All of your questions are answered in a book [URL="https://forum.videohelp.com/threads/403503-What-exactly-is-the-purpose-for-the-16-to-235-luma-range-in-YUV-encoding#post2634938]
Had you read the book, you would also know that this is wrong. There's only a half-line below a typical 480-line capture. All of the other extra lines in a 486-line capture are from above.

Thanks for that info. I wasn't aware of that. Also I don't know what book you are talking about. Did you post a link I had missed?

[Cornucopia]

Right, overscan is not part of the standards, but it should be [or should have been] taken into account when creating a TV/video program.

Those 6 lines are meaningless especially that in 99% cases there is no useful video there.

I'm thinking in terms of archival. Not like making a quick digital video to post on YouTube, but rather a way of archiving the video as a digital master copy, that loses NONE of the original information that would have been present in the image portion of the signal (this means all 486 lines must be recorded). Yes, there can be useful informatino there. There are ways though you can store digital data on those 6 missing lines, in particular closed caption text data. Other types of digital data from various sources may also use these 6 extra lines for their own source-dependent metadata (maybe some professionally made VHS tapes would store metadata describing the content of the tape, or broadcast sources might have used it to store the call letters of the TV station). For the purpose of archiving tapes, 486 lines recording is essential, as it can be more than just image signal stored there.

Professionals follow the professional rules, and those rules include:

Don't use blanking lines for anything except the reserved values (e.g. CC, VITC, WS, program guide). So not just anything else added there.
Don't put "essential" image info anywhere except safe area. So, nothing important around the edges.
Use the most common pro standards for universality of interchange. So 486 is not expected to be as common as 480.

Above all else: anything you/we are capturing off of consumer formats (vhs, etc) would almost never be considered "masters", unless that is the only format on which it exists (e.g. camera raw footage from home video).


Scott

[pandy]

I'm thinking in terms of archival. Not like making a quick digital video to post on YouTube, but rather a way of archiving the video as a digital master copy, that loses NONE of the original information that would have been present in the image portion of the signal (this means all 486 lines must be recorded). Yes, there can be useful informatino there. There are ways though you can store digital data on those 6 missing lines, in particular closed caption text data. Other types of digital data from various sources may also use these 6 extra lines for their own source-dependent metadata (maybe some professionally made VHS tapes would store metadata describing the content of the tape, or broadcast sources might have used it to store the call letters of the TV station). For the purpose of archiving tapes, 486 lines recording is essential, as it can be more than just image signal stored there.

Even for archival you need to separate VBI from Video - unless you are using non standard (i.e. data on video lines).
VBI is not video so to parse VBI use dedicated HW.
Going with your way of thinking then for archival copying you need to store also VBI area i.e. stop recording video as video but record samples with sync pulses and remaining structure - for example Macrovision - it should be archived or exactly opposite - Macrovision shall be stripped from source?
Where is border in accurate copy? At sync level, video samples? content?

[Videogamer555]

Don't use blanking lines for anything except the reserved values (e.g. CC, VITC, WS, program guide). So not just anything else added there.

You are right about not using blanking for anything other than reserved values. However, those 6 lines I was describing are NOT blanking lines. Those are technically image lines, though they aren't safe from overscan. And guess what, many professional sources will embed digital data on one or more of those video lines. And consumer grade 480 height capture devices miss those, which means you can't say that your consumer grade device has truly archived the signal, as there's some missing signal on those 6 lines. Most sources won't include anything on those 6 lines (particularly consumer grade camcorders or other consumer grade video sources), but studio VHS tape production equipment certainly can (as can TV broadcast equipment), and many companies do use such equipment to include their own proprietary data on those video lines for home video releases on VHS (or even on TV broadcast, which may have been recorded by some people's VCR recordings to VHS). To archive and analyze recordings for such data embedded in lines of video normally not captured by 480 line capture cards, it's essential to make sure you are capturing all 486 lines.

[Videogamer555]

I'm thinking in terms of archival. Not like making a quick digital video to post on YouTube, but rather a way of archiving the video as a digital master copy, that loses NONE of the original information that would have been present in the image portion of the signal (this means all 486 lines must be recorded). Yes, there can be useful informatino there. There are ways though you can store digital data on those 6 missing lines, in particular closed caption text data. Other types of digital data from various sources may also use these 6 extra lines for their own source-dependent metadata (maybe some professionally made VHS tapes would store metadata describing the content of the tape, or broadcast sources might have used it to store the call letters of the TV station). For the purpose of archiving tapes, 486 lines recording is essential, as it can be more than just image signal stored there.

Even for archival you need to separate VBI from Video - unless you are using non standard (i.e. data on video lines).
VBI is not video so to parse VBI use dedicated HW.
Going with your way of thinking then for archival copying you need to store also VBI area i.e. stop recording video as video but record samples with sync pulses and remaining structure - for example Macrovision - it should be archived or exactly opposite - Macrovision shall be stripped from source?
Where is border in accurate copy? At sync level, video samples? content?

Nonstandard isn't completely impossible. Sometimes copyprotection data is embedded in blanking areas of the video. In fact, if I remember right, not only does VBI and HBI sometimes contain copyprotection data, as people got more sophisticated in breaking copyprotection, some of the final versions of the Macrovision copyprotection actually altered the sync signals as well. They were altered just enough that a TV could still properly sync, but VCRs wouldn't be able to sync properly (due to VCR sync detection circuits usually being stricter in only detecting sync that stays on spec, while TV sync detectors are intended to pick up even poor quality signals so they can still detect the intentionally out-of-spec sync pulses). For the purpose of archiving a video source, ideally the entire source signal would be archived (all image, blanking, sync, absolutely everything) so that people could study the structure of such video signals (both in-spec signals, and signals with proprietary stuff added). It could make for an interesting study, but to make such a study possible, the equipment to capture the signals is needed first. Obviously that equipment would be likely expensive, but I wouldn't think that it would be too expensive to make a cheap capture card that could capture all 486 lines of video (even if it captured nothing else), and that itself would be an interesting study. See how many different companies sources embedded stuff in the video lines just outside of the normal 480 lines.

[Sharc]

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