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zabs
01-29-2003, 11:04 PM
I recorded a special on the tivo the other day.
It was 2hours and 45 minute long and the ty stream was 5.3 gigs in size. For some reason the recording cut the program both early and late. I was missing a few seonds from the beginning and ending of the show.
I caught a replay of it, and had tivo record start 1 minute early and 1 minute late. The resulting ty stream (now 2 minutes longer then the original) was only 4.11 gigs ????

What gives? I can see a change of a few megs occuring, maybe even a few hundred megs, but more than a full gig difference? The second ty is almost 25% smaller than the first.

I have skimmed the second recording and have seen no problems, but haven't sat down to watch all of it in full. I have also split the 2nd stream and gone ahead with the authoring process and there have been no unusual errors. It seems fine.
Anybody have any idea what is going on here?

captain_video
01-30-2003, 09:36 AM
Welcome to the world of DTV compression! DTV varies the amount of compression used on broadcasts all the time. I've seen the same episode of Seinfeld broadcast on two different channels on the same day vary from 450MB to 850MB in size for the same show. I record PPV movies off DTV all the time and the file sizes for the same movie on the same channel but at different times can vary widely.

zabs
01-31-2003, 02:58 PM
Captain,
I was wondering if you had anymore info about this?
The first stream that was larger, was it then better quality? (higher bit rate?)

I would think that if it were "compression" (as I understand compression) and dtv was in need of more bandwidth for whatever reason, they would compress the stream, without affecting quality, and the signal would be interpretted (decompressed) by the receiver as normal and output to the tv (or in this case to the tivo's hard drive) would be the same quality.

Of course I don't know how the dtv stream works, so maybe they do affect quality when they compress. In which case it's not so much compression as it is, just dropping some of the data.

Just rambling....

captain_video
01-31-2003, 03:33 PM
I can't say that I've noticed any differences in the video quality between a highly compressed video (i.e. smaller file size) or one that uses less compression. There seems to be a trend of using higher compression on a lot of the locals vs. the major channels but even that does not appear to be consistent. I've been archiving the entire Seinfeld series and I get up to four different episodes a day on various channels. The nationally syndicated series appears on two of my local stations as well as the east coast national Fox network feed. TBS broadcasts the series in a different sequence than the nationally syndicated series.

Each channel broadcasts two different episodes a day. I can get 3 different recordings of the same two episodes of the nationally syndicated series and one each of the two TBS daily broadcasts. The three syndicated showings are always consistantly different in size. The DC local Fox station appears to have the highest compression (file size typically <500MB for 1/2-hour show); the Baltimore local comes in second at about 650MB and the national Fox feed is generally about 750MB for the same exact episode. TBS usually hovers just below 700MB per episode.

The variations in file sizes could result from the different commercials they broadcast based on their content. I would tend to think that the locals use lower resolution (i.e. lower quality) videos in their commercials to keep the costs down whereas the national feeds would be somewhat better. I really don't have any other explanation as to why the files vary so much in size and everything I've stated is purely speculation on my part. I'm sure there are experts here that can do a better job of explaining the technical aspects than I can.