From: jesup@cbmvax.commodore.com (Randell Jesup)
Newsgroups: rec.video,rec.video.satellite,rec.video.production
Subject: Re: Compression artifacts in NBC Olympic coverage
Date: 27 Jul 92 19:11:35 GMT
Reply-To: jesup@cbmvax.commodore.com (Randell Jesup)
Organization: Commodore, West Chester, PA

brown@vidiot.UUCP (Vidiot) writes:
><There's no question about it:  there are block artifacts in the
><regions where a fast-moving object occludes or reveals the background.
><For example, NBC showed the compulsory floor exercise of Maria
><Neculita (Romania) at around 5:30pm EDT.  Look at the first tumbling
><run, in which Neculita's legs flash past the audience.

>You are forgetting something very important.  All of the video images are shot
>using PAL equipment.
>
>You are seeing the problem with the newer motion-artifact PAL -> NTSC standards
>converter.  These are the same standards converters that were used during the
>winter Olympics.  I noticed problems back then.

	Sorry, wrong, it is NOT caused by PAL-NTSC conversion (that's done
merely by repeating every 5th frame or so to change 50Hz->60Hz, and resampling
the lines vertically to go from ~625 to 525 (per two fields - or maybe the
drop every 5th line).

	What did cause it (here and at the winter olympics) is the fact that
they're being recording in HD-MAC (HDTV).  The compression artifacts are 
European HDTV motion artifacts.  They are there - I caught a few on my SHVS
VCR during hte volleyball game last night.  I saw them (rarely) in realtime,
and tracked a few down in single-frame mode.  They occur usually when you have
a fast pan following a moving object with a highly-detailed background,
especially when there's a lot of contrast between the object and the background
(i.e. following someone with a crowd behind them).  The moving objects/
background have to cover most of the screen before this happens.

	For example, when Samuelson (sp) was walking in front of the crowd
after the volleyball match, in a close-up of his upper body, you could
occasionally see a "shadow" of his bald head in front of him as the motion-
vector handling screwed up (or got overloaded because of all the background
detail movement).  Easily visible in realtime if you know what to look for,
very visible in single-frame, but not objectionable.  Note, however, that
volleyball normally wouldn't cause artifacts (few close-ups), and that was
just the first one I found - I know there are worse ones elsewhere.

	Hopefully the US standards, having longer to evolve, will have less
artifacts than HD-MAC does.  In any case, it's not objectionable even so.

>What is really strange is that during one portion of the gymnastics, I stepped
>through a portion whereby the white vertical bar was scanned from left to
>right (to change images).  The vertical white bar broke up alot.  The standards
>converter had a real tough time with it.

	Yeah, a high-energy contrast change with lots of background motion 
past it can cause overload of the bandwidth, resulting in artifacts.

	In any case, this is not NBC's fault, so don't blame them for that.
Instead, blame them for giving 0 coverage of about 1/3 of the sports 
(including Archery, Fencing, Handball, yachting, TKD, shooting, badminton,
pentathalon, etc).  I would have bought the TripleCast if there was even
a couple of hours of fencing coverage.

-- 
"Rev on the redline, you're on your own; seems like a lifetime, but soon it's
 gone..."  Foreigner
-
Randell Jesup, Jack-of-quite-a-few-trades, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com  BIX: rjesup  
Disclaimer: Nothing I say is anything other than my personal opinion.