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In Reply to: RE: Sampling Rate to Choose in Audio Midi Settings in Mac. posted by Crimson on November 01, 2009 at 04:34:35
What do you mean by "it"? What else other than Core Audio is doing the sample rate conversion? Does iTunes have its own SRC or does it use Core Audio?
Another question is although the SR in Core Audio is changed and iTunes isn't relaunched but the DAC is restarted (this happens automatically when I change the SR in Audio Midi or the Prism Interface - both change the other automatically) does the SR change?
My understanding is that you are using core audio in both cases -- but iTunes sets up different parameters compared to Audio Midi. I think I learned that on the Apple forums... but I can seem to locate the discussion now. Sigh.
The DAC will look at the settings from Audio Midi, regardless of what iTunes is playing. This is easy to check visually if your DAC as an indicator for incoming sample rate.
If you start iTunes when audio midi is set to 44.1, iTunes will convert everything it plays to the target value of 44.1 kHz. Suppose your next track is 96 kHz. If, without closing and restarting iTunes, you switch audio midi to 96 kHz then iTunes will take the 96kHz source file and convert it to 44.1 kHz to match what it thinks audio midi is set for. Audio Midi will then receive 44.1 kHz input but, as it is set now to 96 kHz, it will convert the 44.1 kHz input into the 96 kHz output that it is now set for. The Dac will be happily receiving 96 kHz signal -- but which as been converted twice.
Whether these conversions make a difference in what you hear may depend on a number of factors, not least is the quality of the original source file.
Flatmap,
That is what I was thinking. I need to test if I can hear it. Thank you. My DAC will show the sample rate as whatever Audio Midi is set for. But,,I assume I tunes needs to be restarted after each AM change in SR. I wonder if this is true in the latest release of Snow Leopard or the new iTunes updates (one just last night).
Not to sound distrustful but can anyone confirm if this is true 100%? I will try to confirm it audibly tonight.
In a 9/12/09 post on Apple's "Core Audio API" discussion list, an Apple programmer on the iTunes team confirmed that the need to quit and relaunch iTunes when you change the sample rate is a bug in iTunes itself and has nothing to do with which version of OS X you use. He said that iTunes 9.0 did not address this bug, and that he was not permitted to disclose when they expected to get around to fixing it.
Bob,
Thanks. I just found an old post on AA referring to a Stereophile Article that explains it very well. Now I am clear. It is a PITA.
Yep, it's a PITA.
It's interesting from an engineering point of view as their perspective is something like: "Why should any application running on the computer have special access to the hardware settings? All applications can access to this hardware and, what do you want, for one application to lock out all other apps?"
Of course this "no application is special" perspective is logical for a general purpose computing device. Equally natural is that those of us who use our computers as the source for music playback really do want our music player to have special rights to the hardware settings.
So I think the right kind of solution here is to have another setting in iTunes -- something like "Let me takeover Audio Midi" and other applications (like the new email message sound in your email client) would just have to be satisfied with whatever iTunes does.
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