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AAC file header
Jfaul
post Aug 20 2002, 11:22
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I saw a description for the AAC file header here:
http://www.audiocoding.com/wiki/index.php?page=AAC , but I could not find how can I get the bitrate/duration from the ADTS header? Does anybody know a better description?

And another question: what encoder uses the ADIF header?

Please help...
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menno
post Aug 20 2002, 11:47
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Count the number of frames, then you'll know the filesize, samplerate and number of frames, with that you can calculate the bitrate.
There's no easier way I believe, unless someone here has a good suggestion?

Menno
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menno
post Aug 20 2002, 11:56
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BTW: ADTS is the same as the headers in MP3, so you should be able to use about the same code (mind the emphasis bits for MPEG-2).

ADIF is supported by PsyTEL and very old versions of FAAC too (before it had ADTS support). Also, the FhG command line encoders use it by default and all the ISO reference AAC files have an ADIF header.

Menno
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Jfaul
post Aug 20 2002, 12:04
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> menno: Count the number of frames

Should I read the whole file trough? There must be a better solution, I think...
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Jfaul
post Aug 20 2002, 12:06
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How much samples does an AAC frame contain?
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menno
post Aug 20 2002, 12:13
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AAC files always have 1024 samples per frame.
In MP4 files it can be 960 too, but I guess you're not working on that yet?

Menno
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menno
post Aug 20 2002, 12:15
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What solution are you thinking of? There is no way of telling the number of frames in a VBR file without going through the whole file. There's also no extra header available for that.

Menno
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Jfaul
post Aug 20 2002, 13:04
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How the ACC-Plugin for Winamp get the duration? It doesn't scan the whole file for calculating the number of frames.
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menno
post Aug 20 2002, 13:13
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Yes it does, check aacinfo.c.
It can also be set to check 30 seconds for example and take the average bitrate of that.

Menno
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Jfaul
post Aug 20 2002, 13:36
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Where can I find the source of the AAC-Plugin for Winamp?
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menno
post Aug 20 2002, 13:44
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http://www.audiocoding.com/download.php
Or get the latest code directly from the CVS.

Menno
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DSPguru
post Aug 20 2002, 13:54
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QUOTE
Originally posted by menno
AAC files always have 1024 samples per frame.
1024 samples per frame, per channel wink.gif


--------------------
DSPguru's Webpage - home of BeSweet & The OggMachine.
BeSplit - 7kb of A loseless AC3/AAC/MP3/MP2/WAV Cutter :)
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menno
post Aug 20 2002, 16:32
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Right! :-)
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menno
post Aug 20 2002, 16:35
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BTW: Upto channel_configuration == 6 the number of channels is the same as the channel_configuration.
I'll try to add some more info about the headers on the wiki soon.

Menno
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Jfaul
post Aug 20 2002, 16:40
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Where can I download AAC examples with ADIF header?
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menno
post Aug 20 2002, 16:44
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Here:
http://www.research.att.com/projects/mpegaudio/

Menno
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Jfaul
post Aug 20 2002, 16:49
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I would like to implement support for AAC files in my project MPEG Audio Collection , therefore I asked for the info.

1) What kind of AAC files are there?
2) Where can I find other ACC encoders (like FhG etc.)?
3) I need ACC examples (various encoders, modes etc.) - for in-depth tests.
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menno
post Aug 20 2002, 17:00
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The ISO reference files found at that at&t website should cover everything. You will not find the FhG encoder that creates AAC files.
There are three different AAC formats possible (4 actually but let's not get into that):
RAW
ADIF
ADTS

You will not be able to extract any info from RAW files unless you decode them (which would make binary distribution of your program illegal). ADIF and ADTS I told about, you can find more info on http://www.audiocoding.com/wiki.

Menno
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Jfaul
post Aug 21 2002, 13:57
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How can I get the sampling rate from the ADIF header?
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menno
post Aug 21 2002, 14:23
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Sampling rate is taken from the sampling_frequency_index in the program_config_element.
I updated the ADIF page to describe this program_config_element:
http://www.audiocoding.com/wiki/index.php?page=ADIF

Menno
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Jfaul
post Aug 21 2002, 14:38
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Thank you!
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Mac
post Aug 21 2002, 17:57
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Aah its cool to see you're adding AAC support to your MAC program smile.gif Thanks JF!
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Jfaul
post Aug 22 2002, 13:07
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I would like to get a sample encoded with the FhG ACC coder. Where can I download one?
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menno
post Aug 22 2002, 13:15
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If you send me an email, I can send you some.

Menno
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Jfaul
post Aug 22 2002, 13:39
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> menno: If you send me an email, I can send you some.

jfaul at gmx dot de
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