9 Replies Latest reply: Dec 13, 2012 4:39 AM by sprksld
sprksld Level 1 (0 points)

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:

  1. original left
  2. monofied (left)
  3. monofied (right)


On my soundcard I can disable the extra outputs, and than I hear:

  1. original left + maybe monofied right
  2. 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.



Mac OS X (10.6.8)
  • fane_j Level 4 (3,660 points)



    Here's a quick-and-dirty test I ran on my MBP.


    1. Launched Audacity.
    2. Inserted a new audio track, filled it with clicks (Generate > Click Track…, used default values).
    3. Inserted a new audio track, filled it with a sine tone (440Hz).
    4. Designated one track as left channel, the other as right channel.
    5. Played file. Result with headphones as expected: L channel, clicks; R channel, tone.
    6. System Preferences > Universal Access > Hearing > Play stereo audio as mono
    7. Played file. Result with headphones: L channel, clicks + tone; R channel, clicks + tone.


    So it looks like your hypothesis is invalidated—insofar as the MacBook Pro, at least.

  • sprksld Level 1 (0 points)

    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.

  • fane_j Level 4 (3,660 points)

    sprksld wrote:


    It would be more interesting though to not use one's ears for proof.

    Maybe; but, for this purpose, it's good enough for me.

    I have reason to believe that this result is valid for any two channel audio interface.

    Whatever your reason, the result not valid for my machine.


    What I did is the equivalent of the binaural stereo technique (see Self, 2010, "Audio Engineering Explained" ISBN 9780240812731, pp 56–57). When the "Play stereo audio as mono" is disabled, I get a different signal in each channel. When it is enabled, I get the same signal in both channels.


    Case closed, AFAIC.

  • sprksld Level 1 (0 points)

    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.

  • fane_j Level 4 (3,660 points)

    sprksld wrote:


    Did you see the third audio channel?

    If there is a third audio channel in the final output, then the output is not stereo. If you start with more than two channels, you have to downmix them somehow to 2 channels to get stereo (and we should not confuse audio tracks—which can be, even when multiple, mono or stereo—with channels). Moreover, the physical analogue interface does not support more than 2 channels.


    I don't know what your problem is, but, as I said, AFAICT on my machine, the setting does exactly what it's supposed to do.

  • sprksld Level 1 (0 points)

    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.

  • sprksld Level 1 (0 points)

    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?

  • flyfish7873 Level 1 (0 points)



    You are correct.  I am having the same issue.  Didyou figure out anything to fix it?

  • sprksld Level 1 (0 points)

    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.