Design flaw in 'Play stereo audio as mono' of Universal Access Hearing-tab

Question

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[1] + channel[2] instead of channel[0] + channel[1]
I don't know what happens to sound that is sent to channel[2] (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.
Thanks!

Answer 1

Thanks for the quick-and-dirty test.
Your findings are not a surprise to me.
It would be more interesting though to not use one's ears for proof.
Also, I have reason to believe that this result is valid for any two channel audio interface.
With the test below I can proof that audio is sent to three channels instead of just two, when enabling "Play stereo audio as mono" on an(y) audio interface with more than two outputs (enabled).
The test takes a little longer than the quick-and-dirty test.
If you don't have an audio interface with more than two channels of output, you can still perform the test by installing Soundflower first.
Soundflower is a virtual audio interface with multiple inputs and outputs, and can be used as an audio router or audio patchbay.
Get Soundflower from http://code.google.com/p/soundflower/ - it's free.
Here's the test …
Open System Preferences, choose the Sound prefpane and select the Output-tab
Select any audio interface with more than two channels of audio outputs as your sound output device ( Or use "Soundflower (16ch)" )
Keep the System Preferences open
Start Garageband
Create a new project
From Preferences select for Audio Input your interface-with-more-than-two-channels-of-audio ( Or use "Soundflower (16ch)" )
( selecting an Audio Output is not necessary, because we're not actually going to use our ears for proof )
Enable multitrack recording from the menu (Track / Enable multitrack recording )
Create four tracks as "Real instrument"; each one with an increasing Input Source: Mono 1, Mono 2, Mono 3, Mono 4
( the last one is just for verification )
Enable recording on these four tracks (if you can't select more than one, try enabling multitrack recording)
Open System Preferences, choose Universal Access (probably click 'Show All' first) and select the Hearing-tab
Make sure audio is played in stereo (unchecking the checkbox)
Keep the System Preferences open
Switch to the Finder or iTunes
Playback any song
Switch back to Garageband and check the meters on the individual tracks
( remember this is stereo, notice the number of channels with signal )
Switch back to System Preferences to enable mono (on the Universal Access prefpane)
Switch back to Garageband and check the meters on the individual tracks ( this is mono ! )
End of test ...
You will see that in mono a third audio channel is addressed, while there should have been only two in both mono and stereo.
The signal strength of channels 2 + 3 seem equal, and channel 1 looks much louder.
This difference in signal strength is very noticable when listening to channels 1 and 2 on separate speakers or headphone.
What is that playing on channel 3?
I must admit (against my original post) that this test does not proof that the signal on channel 1 is or is not mono or anything else; that requires another test.
One can of course record these channels and do some comparisons and subtractions or some other clever tricks.
Hope anyone can second this, preferably with an actual audio interface like Apogee Ensemble, Motu, RME, Presonus, Focusrite, etc.