Re: Changing the way the OS responds to a USB DAC
Re: Changing the way the OS responds to a USB DAC
- Subject: Re: Changing the way the OS responds to a USB DAC
- From: SB Tech <email@hidden>
- Date: Sun, 04 Mar 2012 15:47:25 +0000
On 3 March 2012 23:31, Brian Willoughby
<email@hidden> wrote:
On Mar 3, 2012, at 09:46, SB Tech wrote:
perhaps it's easier to simply outline my understanding of the digital audio output process, indicate what I perceive as an undesired response by the OS to my digital audio setup, and go from there. I'm fully willing to accept that my misunderstanding of the DAC process is causing confusion and misapprehensions.
In brief: ALAC file is unpacked to LPCM by iTunes, which sends the resultant data via the USB port to the attached DAC, which converts it to analogue audio and sends it to the amp, and thence to the output (headphones or speakers).
You're missing a link. iTunes does not send data via USB. Instead, iTunes sends data to CoreAudio, which then sends data to USB or non-USB hardware as selected by the Default Output Audio Device.
Ok, that makes sense.
It's at this link where the OS volume setting takes effect. If a given CoreAudio device advertises a volume control, then the OS will use it if it is in the right configuration. Otherwise, if a CoreAudio device has no appropriate volume control, then the OS volume setting is defeated.
Hmm. I wasn't aware that the OS was capable of directly controlling certain USB Audio Devices' volume directly (ie. presumably by "talking" (in)directly to their amp(s)), but that makes sense. However, in my case neither of my devices can have their output volumes directly controlled by the OS, so that would surely mean they are reporting their capabilities incorrectly to the OS. If this is the case, then please tell me, because the designer of the Beresford is a frequenter of my audiophile forum and I could directly request a feature change there (he's on the verge of releasing a new model as we speak). As for the Fiio: perhaps a firmware update could be requested.
For my part, I had assumed that the OS volume control modified the volume by modifying the audio data directly (the same way that adjusting the volume in iTunes does)*, and for that reason found it odd that the OS volume controls remained when the DACs were chosen, yet didn't modify the volume of the playing track when adjusted.
Either way, the availability of the OS volume controls when my DACs are selected is confusing and a red herring, and really ought to be disabled, by whichever method is the most appropriate.
S.
* To clarify: I don't know exactly what happens, all I know is that, if you have 16-bit output chosen in Audio Midi.app and slowly reduce the volume in iTunes, the actually quality of the sound deteriorates until it's just a crackly mess.
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