Bob Stern Posted December 2, 2009 Share Posted December 2, 2009 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 HQPlayer (on 3.8 GHz 8-core i7 iMac 2020) > NAA (on 2012 Mac Mini i7) > RME ADI-2 v2 > Benchmark AHB-2 > Thiel 3.7 Link to comment
idiot_savant Posted December 2, 2009 Share Posted December 2, 2009 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 Link to comment
wgscott Posted February 21, 2011 Share Posted February 21, 2011 So audio midi setup now allows 32 bit out ... at least with my new mini and I think the latest Powerbooks. Link to comment
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