View Full Version : 2 channel and 6 channel ac3 quandry
01-25-2003, 06:11 PM
Has anyone else noticed that if you extract a Dolby Digital broadcast the ac3 file is 2 channel if you extract the program with commercials before it starts?
I've found that if I use tytool to skip the chunks before the program starts (begin processing at the beginning of the program - without anything before it) that the resultant ac3 file is then a 6 channel audio stream.
One would think that when the stream changes from 2 channel to 6 channel that the dvd player would decode accordingly but it doesn't - the whole dvd plays as 2 channel.
This is why I would like to see an option in jdiner's gopeditor use the cut list to process the ty file into m2a & m2v files.
01-25-2003, 09:29 PM
When you extract a tystream using tytool, the audio is set for the first type it sees. If you know the stream has an ac3 soundtrack (it's called 5.1, not 6 channel), try extracting a small portion of the stream but set an initial jump point such that the actual extraction doesn't start until the beginning of the ac3 stream. I've attached the audiocheck utility developed by jdiner that shows exactly when the 5.1 soundtrack begins in an extracted tystream. I forget what the exact command line is but a quick search for audiocheck should find a post that describes its use.
As an alternative, you might want to give olaf's tydemux splitting utility a try. It scans the entire tystream to see what the primary audio stream is that is contained in the stream and automatically begins extraction at the point where the primary stream begins. You can find the latest version of tydemux in olaf's extraction and splitting tool thread.
01-26-2003, 06:48 AM
Though a little clumsy, you can use -j and -l options in vsplit or use the Jump and Length options in TyTool to jump to the AC2/AC6 segments you want after you use audiocheck to find which GOP chunk you need to start at. There's more info on the threads, especially the older threads, on how to do this and how it works. Olaf has some helpful background info on his thread too.
Run audiocheck on a number of your streams. You might be surprised at some of the results.
01-26-2003, 12:09 PM
I realize that it is called Dolby Digital 5.1, but the 2 channel and 6 channel reference comes from Maestro. If it is a 2 channel stream then it is basically just Dolby stereo, but if it is a 6 channel stream then it is Dolby surround .
Also, I've tried the audio check program and haven't had much success. This program usually will simply find the beginning of the first type of ac3 stream it sees, usually 2 channel.
Thanks for the tip on olaf's program, I'll give it a try.
What your explaining is exactly how I've been processing my ty streams (after I realized what was going on).
But as you said "A LITTLE CLUMSY". This is why if the cut file in jdiner's program could be used to process the ty file into m2a & m2v then editting to the point of gaining 6 channel ac3 audio would be a snap, as would editting out commercials and authoring/burning dvds as we already do.
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