22 Mar
2006
22 Mar
'06
10:51 p.m.
I have a large AC3 file:
$ file chicken.ac3
chicken.ac3: ATSC A/52 aka AC-3 aka Dolby Digital stream, 48 kHz,,
complete main (CM) 3 front/2 rear, LFE on,, 448 kbit/s reserved Dolby
Surround mode
I want to create a much smaller file with the same parameters, ideally
silent, with a duration of about 2 sec, then concatenate the small file
at the beginning of the existing large file.
If I'm not mistaken, an AC3 file is made out of independent frames of
fixed size. Is that correct? If yes, what is the size of a frame?
If the frames are fixed-size, I could simply use dd to generate a small
fragment out of the existing file, then use cat to glue the fragment and
the original file.
--
Florin Andrei
http://florin.myip.org/
23 Mar
23 Mar
7:11 p.m.
On Wed, Mar 22, 2006 at 06:51:01PM -0800, Florin Andrei wrote:
I have a large AC3 file:
$ file chicken.ac3
chicken.ac3: ATSC A/52 aka AC-3 aka Dolby Digital stream, 48 kHz,,
complete main (CM) 3 front/2 rear, LFE on,, 448 kbit/s reserved Dolby
Surround mode
I want to create a much smaller file with the same parameters, ideally
silent, with a duration of about 2 sec, then concatenate the small file
at the beginning of the existing large file.
If I'm not mistaken, an AC3 file is made out of independent frames of
fixed size. Is that correct? If yes, what is the size of a frame?
If the frames are fixed-size, I could simply use dd to
generate a small
fragment out of the existing file, then use cat to glue the fragment and
the original file.
Frames vary in size based on the parameters chosen. In a stream, the
frames should stay the same size. If they don't, then that probably
means that there was a cut from one stream to another, like if you were
recording AC3 over spdif from a satalite reciever while changing
channels.
Using dd to generate a fragment will be non-trivial if you aren't
intimately familiar with AC3. Or at least thoroughly aquainted with
it. The ATSC spec is available online. I wouldn't call it easily
readable, but you can work your way through it.
A better technique might be to make a silent wav file, then use ffmpeg
to convert it to AC3, using the parameters of the original file. If
the parameters match (or are at least close enough, which is a little
bit decoder dependant), you can just cat them together. I don't know
that ffmpeg will allow you to actually tinker with all the parameters
though. For safest results (which is technically degraded, but is how
the professionals would usually do it), it might be best to just convert
from the AC3 to .wav or .aiff, then place your silences, then convert
back.
--
Joshua D. Boyd
jdboyd(a)jdboyd.net
http://www.jdboyd.net/
http://www.joshuaboyd.org/
28 Mar
28 Mar
9:33 p.m.
I've found this document:
http://www.rmworkshop.com/dvd_info/related_info/ac3hdr.html
According to it, 0x0B77 is the beginning of an AC3 frame. I opened up my
AC3 file in hexedit and, lo and behold, there were 0x0B77 strings at
regular intervals in the file.
Using dd, I cut a chunk of the file from the beginning until right
before the second occurrence of 0x0B77. Apparently, that chunk is a
single AC3 frame and a valid AC3 stream on its own.
Using cat and a bit of shell scripting, I concatenated this chunk 69
times at the beginning of the existing file. The result, according to
a52dec, mplayer and mplex, was a valid AC3 file with just the right
amount of delay at the beginning (69 times the duration of one frame was
what I determined experimentally to be needed).
--
Florin Andrei
http://florin.myip.org/
6816
days inactive
6822
days old
linux-audio-user@lists.linuxaudio.org
2 comments
2 participants
participants (2)
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Florin Andrei -
Joshua Boyd