[vlc-devel] Re: Multichannel output on OS X.

[Jean-Alexis Montignies]

On 20 avr. 04, at 03:41, Derk-Jan Hartman wrote:

> I finally found some real information on how to do multichannel output 
> on Mac OS X i think.
> I think this should be implemented with a AudioUnit.

Yes I knew that ;), I'm lurking on the core audio list for some years 
now.

In fact there is an AudioConverter that can do (beside decoding and 
downmixing) anything we need, including mapping to different channels 
as you can specify in the AudioMidiSetUp applications (on every MacOSX 
in utilities, you can define when you have a multi-channel interface, 
you can define mapping for which outputs to use for each channel (front 
left, center...) ).

The drawback is that it's not available (I think) on all MacOS X 
versions.

As for non pcm digital output, if I'm not mistaken, there's no 
audiounit that can handle that format yet, so it's still required to 
use the HAL directly.
By the way how does VLC does for synchronizing video and audio in this 
case?

As for the point of view of the interface, I think the main requirement 
is to choose which type of output the user wants (stereo, dolby 
surround: the one encoded in a stereo signal, quad, 5.1, 7.1, digital). 
The stream to use would be another choice but there less need to expose 
directly I think. The channels to use can be chosen in the AudioMidi 
Setup application.

For example, in my setup, I have only 4 speakers, so I would like to 
downmix the 5.1 to quad.
My father has a dolby surround setup (stereo inputs, 4 speakers).

I can start looking at the code now I've compiled it (well I don't have 
AC3, but I think DTS should work though I have only a few DTS DVDs).

Jean-Alexis Montignies
http://www.sente.ch/

> Information is available on the mailinglist 
> coreaudio-api at lists.apple.com
> Look for the posts title: HALOutputUnit and all followups (91 !!!)
> posted on January 7th and beyond
> Original posting by James Coker
>
> Real useful information by William Stewert on the 10th of January. 
> He's the one to contact if we want help from apple (bills (at) 
> apple.com)
>
> However, it seems most information would be available in these 
> threads. Maybe a question on digital audio needs to be asked...
>
> I still don't have time for this, but at least a big hurdle has been 
> tackled. At least we now know how it should be done.
>
> DJ
>
> ---
> Videolan - VLC media player
> Derk-Jan Hartman (hartman at videolan dot org)
> http://www.videolan.org/vlc
>
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