[MPlayer-users] Mac OS X Audio 5.1 Channel Output Problems

[galenz at zinkconsulting.com]

On Jan 17, 2008, at 2:07 AM, Reimar Döffinger wrote:

> Hello,
> On Thu, Jan 17, 2008 at 01:45:40AM -0800, galenz at zinkconsulting.com  
> wrote:
> [...]
>> However, this is extremely un-Mac like and un-intuitive, given the
>> available data. The Apple audio API automatically detects the number
>> of channels physically available. The macosx audio out module should
>> pass this value along to mplayer and set the number of channels
>> accordingly.
>
> I am not aware that there are any sound cards that can detect  
> whether 2,
> 4, 6 or 8 speakers are connected to them. Does Apple have some special
> hardware there?

Apple only does stereo audio out with their analog hardware. They have  
never shipped a Mac with more than one analog audio out port. This  
allows them to avoid the scenario where it is uncertain how many  
speakers are connected. Read below for more details.

>> This is what I mean about mplayer not detecting the multple channels.
>> Various other applications - such as QuickTime Player and VLC - rely
>> upon detecting available channels in this manner and make audio
>> conversion/output settings accordingly.
>
> I'm almost certain that they behave exactly like MPlayer with  
> channels=6
> at the expense of giving wrong output for those who only connect two
> speakers then.

Using Apple hardware, it is only possible to output >2 channels using  
a digital (optical) link. All x86-based Macs (and many newer PowerPCs)  
- including notebooks - have a hidden optical audio out inside their  
stereo analog jack (via mini TOS link connector.) This digital link is  
all auto-detected and everything is handled with extraordinary elegance.

Third party hardware can provide other configurations involving analog  
out (which cannot be reliably detected), but by default, adding new  
analog hardware only uses 2 channel audio out. Normally, 5.1 and 7.1  
configurations are setup using Audio MIDI Setup. Then you can simply  
choose between various outputs as the "primary" in the Sound system  
preference - built-in stereo, third party stereo, 5.1, 7.1, etc.

Given that using Apple hardware, the system handles auto-detection  
perfectly, and the system has excellent system-level configuration for  
third party hardware that cannot auto-detect, applications are  
normally expected to utilize all available channels, should they have  
content suitable for said channels.

Given my experience with every other Mac application behaving like  
this, my natural assumption was that mplayer failed to properly detect  
the channels, hence my (in retrospect) rather stupid question. In  
fact, it was just un-intuitive behavior. It would be worth changing  
this in mplayer, as there are basically no scenarios where it will  
cause problems, but many scenarios where it will confuse users who are  
accustomed to the behavior of all other applications on their system.

> Greetings,
> Reimar Döffinger
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