[opendtv] Re: 625 video quality is good enough....

At 11:43 AM -0700 10/17/04, Dale Kelly wrote:
>You might recall a recent posting suggesting that HDTV might be of 
>little value in countries using the 625 line system since that 
>system's quality was likely good enough. My position was, that as 
>good as that image might be relative to other analog based 
>standards, it could not compete in quality with true HDTV video 
>displayed on a 720 or 1080 large screen display.
>I'm revisiting this subject only because I saw a very graphic 
>illustration supporting my argument last evening when viewing the 
>BBC production of "Last Night at the Proms", on the Discovery HD 
>channel. This is an excellent program in every way but it was 
>produced in the 625 (580? DTV) 16X9 format* and compared to other 
>HDTV programming on the same network was noticeably softer, 
>particularly on the medium and long shots which are such an integral 
>part of that program. Clearly the viewers in Britain would have 
>noticed and wanted the difference.
>

There is an obvious explanation, one that has significant relevance, 
as the ITU considers whether it should approve 720P for international 
program exchange, and the inclusion of 720@50P into the international 
standards.

I cannot reproduce the submission to the ITU directly, however I can 
paraphrase the key points that the advocates of 720P have made:



- Progressive formats make compression work better (one company 
estimates  a 10%to 30% increase in the required bit rate for MPEG 2 
compressed interlace video as opposed to progressive scan video), 
preserving bandwidth and providing the best quality to viewers.

- Emerging display technologies are progressive friendly and are 
dominated by 1Mpixel types.

- 1280x720 is friendly to modern post-production techniques, which 
often need to de-interlace source for processing, such as spatial 
scaling and rotational manipulations.

- Conversion from 720p to any other format is simpler and provides 
better quality because there is no source de-interlacing involved. 
This is the key to why Dale saw a "soft" picture. In order to present 
the content here in the U.S. the original 1080@50i source (thanks to 
Alan Roberts for this very useful clarification) had to be standards 
converted from 50i to 60i for broadcast by Discovery Networks. This 
requires a de-interlacing step, then frame rate conversion, then 
re-interlacing.

Welcome to the realities of standards conversion. It does not get 
better with HDTV, if we are trying to do frame rate conversions on 
interlaced source. We are trying to create information that was not 
sampled, using samples that have been compromised by interlaced 
acquisition. The net result is that to cover all of the artifacts of 
the standards conversion, we give up significant resolution.

Contrast this with a 50P to 60P conversion or visa versa. We do not 
need to de-interlace the source, and we have excellent spatial detail 
available to do the frame rate conversions. The results are obvious 
on a progressive display.

So bottom line, Dale was seeing the "kinder, gentler, softer" side of 1080i.

It's time to get rid of interlace. PERIOD!

There is NO GOOD reason for this archaic compression technique to be 
concatenated with digital  compression. This is equally true for 
SDTV(525 or 625 line), as better results can be obtained with a high 
quality de-interlacing system, before the source is subjected to 
MPEG-2 compression. Expecting a cheap de-interlacing chip in a 
consumer display to do as good a job as a $75K to 100K deinterlacing 
system is ludicrous. On the other hand, it is dirt simple to convert 
progressive source for interlaced display using noting more than a 
convolution filter to remove the details that would cause offensive 
artifacts on an interlaced display.

If we only put progressive source into the DTV channel, the use of 
interlaced acquisition would
disappear quickly, in favor of progressive HD and EDTV acquisition.

Regards
Craig
 
 
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