Re: Converting multichannel file to stereo with down-mix.
Re: Converting multichannel file to stereo with down-mix.
- Subject: Re: Converting multichannel file to stereo with down-mix.
- From: Brian Willoughby <email@hidden>
- Date: Thu, 07 Jun 2012 13:43:59 -0700
Yes. You can do this with CoreAudio using Apple's MatrixMixer.
Since ExtAudioFile includes an AudioConverter, but AudioConverter
will not mix channels, you may need to create an AUGraph if you don't
already have one in your application. Then, in the AUGraph, you need
to link the output of the ExtAudioFile to the input of the
MatrixMixer, such that the input bus has 6 channels and the output
bus has 2 channels. You will also need to set individual mixer node
volume levels according to which channels you want in Left, and which
you want in Right, along with their relative levels. You might also
need some kind of dynamics processing, such as Apple's PeakLimiter,
to prevent the mixed channels from overloading the full scale volume
limit.
There should be examples of using the MatrixMixer, and the header
files for AudioUnits has some degree of documentation of how the
MatrixMixer node volume parameters work.
Brian Willoughby
Sound Consulting
On Jun 7, 2012, at 18:45, email@hidden wrote:
I'm trying to figure out how I can convert a multichannel
(surround) audio file to stereo using the ExtAudioFile and Audio
Converter APIs. I would like the stereo output file to be a fold-
down mix of all the channels, similar to what quicktime does. No
matter what I try, the resulting stereo file only has the first two
channels of the source file.
For example, I have a 6-channel AIF file whose channel layout is
kAudioChannelLayoutTag_MPEG_5_1_A (L R C LFE Ls Rs). I've tried
setting the ExtAudioFile's client and file channel layout... I've
tried setting the Audio Converter's input and output channel
layout. I always get only the L and R channels on the output. Is
there any possible way to do a fold-down mix using Core Audio?
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