Re: Problem with AAC and AudioFile
Re: Problem with AAC and AudioFile
- Subject: Re: Problem with AAC and AudioFile
- From: William Stewart <email@hidden>
- Date: Wed, 21 Jan 2004 15:24:20 -0800
On 21/01/2004, at 2:17 PM, john wrote:
Hi Bill,
Thanks for the reply! I don't really understand how AAC exists in
mpeg2 and mpeg4, since I thought AAC was mpeg4 based, not that it
matters any since I really know nothing.
AAC was first specified in MPEG-2.
MPEG-4 adding some additional tools to the basic codec (what is called
"Low Complexity AAC") such as PNS (Perceptual Noise Shaping), and
redefined a predictive tool defined in MPEG2 that was computationally
very complex (great english!) (and was specified in the Main Profile
version of AAC), to use LTP (Long Term Prediction) instead (which is
less computationally complex, but achieves a similar result in the
encoding quality)
What I'd really like to know is the details in filling out the
AudioStreamBasicDescription for the AudioConverter AAC type...
set all fields to zero
set the type field to 'aac '
Sorry, its a little complex to explain the complete process... We're
trying to get the SDK out (but I keep thinking of one more thing to
add) that will provide extensive code to show you how to make, read,
play any file type known by the AudioFile API using that,
AudioConverter and the AU output units.
Bill
Thanks!
-- John
The AudioFile API supports the ADTS file - this is an MPEG2 defined
bit stream, but it is not MPEG-4.
QT can make mp4 files, and the iTune's m4a files are essentially
MPEG-4 files that have just audio data in them.
At this stage there's no direct bridge between these two bit streams,
but this will be addressed at some point
Bill
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