The challenge of legalizing file-based video
Jul 1, 2007 12:00 PM, BY THOMAS DOVE
Testing
In order to do the testing of the baseband as required for video legalization, the compressed video file must be fully decoded to baseband. If there is then a gamut/legality problem and the video is then legalized, the following may be needed:- The file must be recompressed to the same video standard — MPEG-2, MPEG-4/AVC, VC-1, etc.
- It must keep the same parameters, which are sometimes set manually over a range of frames to get the optimum appearance.
- The compressed video will need to be remultiplexed with the audio and metadata.
- The metadata might need to be updated as well.
This is not easy to do, and there is a great chance that this process will introduce errors. As a result, rather than fixing a minor video legality problem, more serious errors have been introduced.
To legalize file-based video, it must be decoded and re-encoded. This can destroy the video quality, which is evident in the image on the right.
Click image to see difference.
In addition, typically a content provider or broadcaster would have carefully assessed and chosen specific encoders to be optimal for their requirements. With automatic legalization, this will likely use whatever encoder the legalizer has — whether it is good, bad or indifferent. Plus, the encoder in the legalizer would have to be able to deal well with all the different video standards and be able to remultiplex these seamlessly.
The content provider will have all the correct tools and setup to encode correctly and check the video. It is, therefore, far better for a broadcaster to do a comprehensive check at ingest and go back to the content provider in the event of problems. This then means that the content is resupplied with the visual quality that the content provider intended. Also, reporting the problems back to the content provider may mean that future content is perfectly OK.
Crushing
In SD digital video terms, black is assigned a value of 16, and white is assigned a value of 235 (in 8-bit systems like DVD and DV). Legalizers will clip the video signal at those levels. There will never be a sub-black or over-white signal on a DVD, though the format is capable of carrying the entire 0-255 range. (The dynamic range is limited to these values, but it's not relevant to this point.) The legalizer controls can be driven to ensure that the video signal coming off tape and being color-corrected lies between 16 and 235 and is not crushing. Of course, there's always a margin of error in any kind of process that is controlled by a human operator, but it wouldn't be expected that this crushing would exceed 1 percent, which is negligible.
So, if there was a sequence of video bytes, say a luminance ramp from black to white, which was coming in as 16, 17, 18, 19, 20, 21, 22, 23 … 233, 234, 235, and the lift control was turned down so these values became 13, 14, 15, 16, 17, 18, 19, 20 … 230, 231, 232, then at the output of the legalizer, the signal would be 16, 16, 16, 16, 17, 18, 19, 20 … 230, 231, 232.
Thus, some original detail has been clipped off or crushed out and could never subsequently be recovered. If the lift control is later turned back up on this modified signal, the sequence would be 19, 19, 19, 19, 20, 21, 22, 23 … 233, 234, 235. Most of the picture would be returned to its original value, but the blacks would now be raised up, and black would be a dark gray; the original near-black detail is gone forever.
Conclusion
Video legalization can have a role in quality checking of file-based video, but this method only deals with a small subset of the errors that can occur with the content. There are two key points about video legalization of file-based video:
- Although the color gamut can be corrected, legalizing the video can degrade video quality badly and can result in a file that has been re-encoded in a way that was not intended.
- The video can be legally compliant. It can have the correct gamut but still have incorrect syntax, which can cause the set-top box to crash.
The most effective way to check the health of file-based content prior to transmission is by checking that the syntax of the file is correct. It is useless to check gamut if the syntax is not correct, so syntax must be the first check.
File-based video generally comprises one or more complex digital files with many elements, all of which must be correctly decoded for the file to play. A large proportion of file-based video has some syntax errors, so it's important to look for tools that can automatically check for correct syntax, enabling you to find the errors before you get complaints that the end-consumer's set-top box has crashed.
Thomas Dove is senior manager, compressed video, Tektronix.
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