The option to 'play stereo audio as mono' that comes with OSX 10.6 does not work as I expected.
When I enable this option it sounds like only one of the two channels becomes mono, while the other one does not sound mono to me.
It's hard to notice this, because the total sound does change to 'less' stereo.
I've done some testing with different soundcards and testing signals, and found out that when the option is enabled, the mono signals are routed incorrect.
While the right channel becomes mono, the left channel seems to be something else; I'm not sure if it's the original left channel of the stereo signal, though.
I have reason to believe that the resulting mono sound on the right channel correctly is the 'monofied original sound', but that from the left channel.
The resulting mono sound on the left channel sounds like the original left channel of the stereo sound, maybe added with the mono sound of the right channel.
Why do I think so? When using a sound card with more than two outputs, I can clearly see that sound is being sent to the second and the THIRD output, instead of the first and the second output, immediately after I enable mono. Resulting in output on THREE channels:
- original left
- monofied (left)
- monofied (right)
On my soundcard I can disable the extra outputs, and than I hear:
- original left + maybe monofied right
- monofied (left)
I think the programmer of this great feature made an 'off-by-one error', consequently sending the monified output to the wrong channels;
to channel + channel instead of channel + channel
I don't know what happens to sound that is sent to channel (the third channel) on a two channel soundcard.
It could be mapped to the the first channel, but that of course is speculation. Could be tested though, with the right audio signal.
I hope someone can confirm this. If more people experience this problem, maybe Apple can fix this so I can enjoy listening in mono.
I have tested this on the internal soundcard and three external firewire audio interfaces, of which 2 have 8+ output channels.