Audio MIDI Setup settings for proper CD output

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Level 3

606 points

Audio MIDI Setup settings for proper CD output

Hi,

I came across the following statement in a forum:

"There is an aspect of the conversion from the internal "32 bit floating-point" math […] of Core Audio to the "fixed point" output that means if you do not set the output word length to "24 bit," a 16 bit (CD) file will not be reproduced with "bit accuracy." This means that the lower level bits of the signal may be changed, so the quality may suffer as a result."
From, http://www.pinkfishmedia.net/forum/archive/index.php/t-56493.html

Is this actually the case? The thread doesn't cite a source for the information. I know that Audio MIDI Setup comes pre-set as 24-bit, 44.1kHz.

S.

MacBook Pro (Unibody) 15" 2.4GHz C2D, 4GB RAM w/, Mac OS X (10.6), ; Aluminium iMac 20" Mid-2007 w/ Mac OS X 10.5.7; iPod touch 2G 8GB, OS 3.0

Posted on Sep 17, 2009 6:12 PM

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Answer 1

SiRGadaBout Author

Level 3

606 points

Nov 2, 2009 6:43 PM in response to SiRGadaBout

bump

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Answer 2

Nov 2, 2009 9:30 PM in response to SiRGadaBout

When playing 16-bit audio, if the application and system volume are at 100%, then 16-bit digital output should not reduce the sound quality. If the volume is reduced, though, there will be a reduction of quality.

A 16-bit signal can represent 65,536 different levels. If you reduce the volume to 10% the signal is restricted to 6,553 level, so the smallest step size is now a lot larger percentage of the maximum signal.

If the output is set to 24-bits, a 16-bit audio can have its volume decreased by up to a factor of 256 and still have 65,536 levels available on the 24-bit output.

To avoid quality lose with digital audio, don't reduce the volume before the DAC.

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Answer 3

SiRGadaBout Author

Level 3

606 points

Nov 3, 2009 1:53 PM in response to Malcolm Rayfield

Very interesting. This adds another layer of complexity to the process. Some would say that it is best to avoid over-sampling (ie. setting 24-bit output in Audio Midi Setup), others disagree. However, from what you're saying, avoiding oversampling in OS X means slightly reducing flexibility in volume output. Thanks for the clarification on this.

Still, this is a different issue. Even at 100% volume in, eg. iTunes, it's possible at 16-bit output that OS X isn't sending bit-perfect audio anyway. Anyone able to confirm that?

Thanks.

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