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: Stephen Davis <email@hidden>
- Date: Sun, 04 Mar 2012 18:27:59 -0800
On Mar 4, 2012, at 5:06 PM, Brian Willoughby wrote:
>
> On Mar 4, 2012, at 07:47, SB Tech wrote:
>> 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)*,
>
> You are correct that iTunes modifies the audio data directly when the volume in the GUI is below full. However, the OS volume control does not even have access to the audio data. The OS volume control queries the CoreAudio driver for the audio interface in question, discovers the number of volume controls for the device, and then sends parameter change messages to set the volume. At no time does the "OS volume control" even see audio data, much less have the opportunity to alter the audio data.
Just to be pedantic, this statement by both of you about iTunes is technically false. :-)
iTunes does not modify the output data itself when its volume control is below full. Instead, it tells the CoreAudio default output unit to set its volume accordingly (via the kHALOutputParam_Volume, kAudioUnitScope_Global parameter). How CoreAudio implements that control, either by direct software scaling or by telling the driver to do it, is up to CoreAudio. Splitting hairs, I know. I just wanted to mention it b/c it does impact how the audio data actually flows through the system, which might be useful to know.
stephen
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