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

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.

If a listening test is good enough for you and you don't experience the same problem as I do, than you simply don't have a problem that needs to be solved, and your case is indeed closed.

My case is not closed, because I do have a problem that I like solved.

I have performed your quick-and-dirty test, with Audacity.

What results did you have performing my test? Did you see the third audio channel?

Or is anyone else willing to perform this test?

Thanks for pointing out that very interesting book to me!

And let me re-phrase that second quote, because it was meant to support your findings, but the sentence can easily be misread.

"I have reason to believe that the result you got with your test is valid for any two channel audio interface."

It's the multiple outputs audio interfaces that I believe have this problem.

I'm talking about audio interfaces that do support more than two channels of output.

E.g. http://www.focusrite.com/products/audio_interfaces/saffire_pro_40/

or http://www.apogeedigital.com/products/ensemble.php

Discussing your MBP is not the issue here; neither is my MBP, or someone else's MBP.

I'm also not talking about multitracking, multichannel audio files or mixdown stuff, but a simple stereo audio file played from iTunes.

When using an interface with more than two outputs to playback a regular stereo track from iTunes (in stereo), there is signal on just the first two outputs; as should be.

When selecting mono, there should still be just two channels of audio, but strangely a third one appears.

If a third audio channel appears, then the resulting signal could be anything, not necessarily stereo or mono.

The volumes should be exactly the same on both channels left and right, but the first channel is about two times louder than the second and the third channels. In mono both channels should be the same volume.

There should not be a third channel at all, when it wasn't there before enabling mono.

I guess you did not do the test I descibed and therefore you probably haven't seen the third channel.

Using Garageband for my test was only to prove my case to people who do not own a multi channel interface without, or if they do but don't have a control panel with level meters on the outputs.

I mean, if even a virtual interface (or loop back device) shows a thirds channel. How wrong can I be?

You seem to have a hard time believing me.

If you really want to believe me and see for yourself: do the test! Use an interface that has more than two outputs or try it with Soundflower.

( and test it on 10.6 this is a 10.6 discussion )

A friend of mine reported today that the instant he selects mono with his Presonus FirePod, iTunes crashes.

I will do some more tests with interfaces of other friends, owning RME Fireface and Motu HD192, and report back here.

I still hope other people will join this discussion.

The following audio interfaces all have this issue:

• M-Audio Projectmix i/o ( visible on front panel of the unit )
• M-Audio Profire Lightbridge ( visible in the level meters of the control panel )
• Presonus Firepod FP10 ( iTunes crashes )
• RME Fireface 400 ( visible in the level meters of the control panel )

This virtual audio interface (used as loopback device) also has this issue:

Soundflower 16ch ( visible via the input meters when test recording with Garageband )

All tests were done on 10.6.8 by simply playing back a song from iTunes or Finder,

and then enabling Universal Access > Hearing > Play stereo audio as mono

I will soon test the Apogee Ensemble, Motu HD192, Lynx Aurora 16 and M-Audio Profire 610, but after that I run out of friends that have these kind of audio interfaces and a Mac.

Is this proof enough to contact Apple? How do I contact Apple?

Finally someone who agrees with me!

I found more and more interfaces, even high-end, with the same problem.

Apple seems to have fixed it in 10.7, but did not fix it for previous versions.

After discovering this and seeing other interfaces with the same problem pile up, I gave up on this thread, hence my late reply.

I'm using a plugin now, though it's not the best solution available.