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iTunes Bit Perfect tests in OS X


Bob Stern
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Because Core Audio uses 32-bit float internally, it must convert from 32-bit float to either 16-bit integer or 24-bit integer, depending on the output device setting in Audio MIDI Setup. (The conversion is trivial, but that doesn't preclude a programming bug!)

 

On the Core Audio discussion list, an Apple engineer recently said the conversion from 32-bit float to 16 or 24 bit integer normally is performed by the audio device driver. That implies that "bit perfect-ness" must be tested separately for the built-in Toslink driver, the built-in USB driver, and each non-Apple driver you wish to install, such as the proprietary drivers used by Firewire audio devices.

 

I'm reviving this supposedly dead horse because I remain puzzled by why Amarra software has an audible effect. I'm thinking it circumvents a bug in Core Audio. My understanding is that Amarra uses low level code written for OS 8 and 9, so it bypasses portions of Core Audio.

 

I just re-read the July test results by idiot_savant proving bit perfect output when iTunes plays 16/44.1 with Audio MIDI Setup directing the output to a USB > S/PDIF converter.

http://www.computeraudiophile.com/content/Playback-Engines#comment-21977

 

Question for idiot_savant: With the USB device you used in this test, does Audio MIDI give you a choice between 16 and 24 bit format? If so, do you know which you used? It's possible there is a bug in the 16 bit conversion but the 24 bit conversion is correct, or vice versa.

 

Also, Ray W proved bit perfect output with iTunes playing 24/96 and the output device being a Weiss DAC2 via the Weiss Firewire driver.

http://www.computeraudiophile.com/content/Playback-Engines#comment-22015

 

Mac Mini (2012 i7) > HQPlayer > RME ADI-2 v2 > Benchmark AHB-2 > Thiel 3.7

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that the tests were done using 16 bit source material, but with the Audio Midi settings set to 24 bits ( it seems that with USB devices, you don't get a choice of bit depths - it just selects the maximum the device will accept ).

 

I can confirm that apart from a fade in at the start of the track, in terms of bits, Amarra is the same as iTunes ( well, for the amount of data I examined, which was 20000 points or so )

 

your friendly neighbourhood idiot

 

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