MP Digital Optical Audio Problem

Discussion in 'Mac Pro' started by strausd, Aug 21, 2010.

  1. strausd macrumors 68030

    Joined:
    Jul 11, 2008
    Location:
    Texas
    #1
    So I am using Logitech Z-5500s with my MP. Under the sound settings with digital optical selected, it says: "The selected device has no output controls." And at the top of the screen where it shows the volume, it is grayed out. And when I hit the volume up/down/mute button on my keyboard, it has a circle with a line through it. If I play a song on iTunes, sound comes out and I can control the volume through the Logitech controller like normal. But even when something is playing, it still won't let me adjust the volume through the computer. Does anyone know what is going wrong?
     

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  2. trankdart macrumors member

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    Jul 28, 2010
    Location:
    Los Angeles, CA, USA
    #2
    Does the volume slider near the top of the iTunes window work?
     
  3. strausd thread starter macrumors 68030

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    Jul 11, 2008
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    Texas
    #3
    Ya, but thats application specific, the other three are for everything.
     
  4. trankdart macrumors member

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    Jul 28, 2010
    Location:
    Los Angeles, CA, USA
    #4
    Not everything. I play sound through an external firewire interface. It behaves the same way that your SPDIF connector does. I'm not positive about this, but I think digital audio outputs default to a hardware passthrough mode that doesn't modify the bitstream, and your player and/or transducers have to control the volume. I think you might be experiencing default behavior.

    In any case, my MP 1,1 has always worked that way, albeit not with the built-in SPDIF. I have to change my volume either with the app or with my speakers.
     
  5. Executor macrumors member

    Joined:
    Mar 1, 2008
    #5
    Optical audio is meant to be controlled by an external audio receiver. Software control of the audio is not available as a global setting. That's always been the case and I am on my third MacPro.
     
  6. strausd thread starter macrumors 68030

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    Jul 11, 2008
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    Texas
    #6
    Well thank you very much for the clarification!
     
  7. skiffx macrumors 6502a

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    Feb 5, 2008
    #7
    Yep, although I dont have it myself, thats how my friend's setup works as well.
     
  8. trankdart macrumors member

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    Jul 28, 2010
    Location:
    Los Angeles, CA, USA
    #8
    Apple COULD build in an independent volume control that didn't care what the app was doing. But the different apps that put out different kinds of bitstreams (i.e. the DVD player puts out 5.1 and iTunes puts out stereo and whatever else) probably wouldn't appreciate it. And neither would you when all that came out of your computer after you adjusted the optical volume was...a hiss.

    The standard audio output always puts out straightforward analog stereo. It's not hard to control it with a digital emulation of an old fashioned pot; you don't have to know the app's intent.
     
  9. CaptainChunk macrumors 68020

    CaptainChunk

    Joined:
    Apr 16, 2008
    Location:
    Phoenix, AZ
    #9
    This has always been the case with digital audio, even on AV receivers. Adjustment of volume happens at amp level, after the audio signal has been converted by the DAC. You can't physically adjust the output level of a PCM or bitstream signal. Quite simply, they're just streams of data.

    I think someone would have done this by now, if it were possible. But it isn't. You can't preamp a digital signal. You can only do that to analog signals.
     
  10. studiox macrumors regular

    Joined:
    Aug 3, 2004
    Location:
    Stockholm / Sweden
    #10
    That is so true, also if your playing 5.1 sound from a DVD player or from VLC the sound can't be processed anywhere else than in the amplifier as its digital and is a stream that OSX has no clue about (Its not processed by coreaudio at all)

    If you where to control a sound like that you have to decode it, change the amplitude of the various channels and encode it once again.

    The only reason you can change the sound in Itunes is that it's still analog. But as the same time you send it to a digital output CoreAudio is actually encoding it to PCM and after that you can't touch the sound.

    Edit: In Windows it does not however work the same way, you can change the sound before the drivers take care of the PCM encoding.
     
  11. dknightd macrumors 6502

    Joined:
    Mar 7, 2004
    #11
    If you are using the itunes volume control, I suggest you use audio midi set application to output 24 bit. This gives itunes more bits to play with.
     

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