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

1342 Views 9 Replies Latest reply: Dec 13, 2012 4:39 AM by sprksld RSS
sprksld Calculating status...
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Apr 7, 2012 5:18 AM

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 Level 4 (3,655 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.

  • fane_j Level 4 Level 4 (3,655 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.

  • fane_j Level 4 Level 4 (3,655 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.

  • flyfish7873 Calculating status...



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


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