USB audio: a feature unit with volume control in the recording path ?

[erdmann]

Hi,

would I would like to do is to control input gain of a USB audio recording device.

For about any USB audio device that offers recording capability, I can see a feature unit in the signal path between microphone input and USB streaming output. It's typically connected to the input terminal (that is: the microphone input) or there is some selector block in between the feature unit and the microphone input terminal.
That feature unit typically offers mute and volume control.

What does "volume control" mean in conjunction with audio recording ? Is that equivalent to gain control ?
I manipulated that feature unit to set "volume" to the minimum and also to the maximum but it seems to make no difference at all, the input level does not seem to change in any way.

Can somebody enlighten me what this is about and if it is possible to control input gain with this control ?

[Jan Axelson]

The USB Audio spec defines both gain control and volume control features as well as automatic gain control. The gain control and volume control features seem quite similar. I don't know if devices are required to support these though I would think most would support volume control.

[erdmann]

Yes, about any device I have come across contains a feature unit that supports volume control for playback and I am using that and it works (I can lower or raise the volume on playback).
The funny thing is that if a device also supports recording, it has a SECOND feature unit in the recording signal path that again claims it can handle volume control. But I have not come across a feature unit that would support gain control. The question was, if a "volume control" in the recording path would be equivalent to a "gain control".

[erdmann]

According to:
https://stackoverflow.com/questions/23519753/usb-audio-descriptor

I am beginning to believe that for recording, the Windows USB audio driver insists on finding a feature unit in between its audio input terminal (microphone) and its output terminal (USB streaming to host). That does not seem to mean it actually does anything (for the recording path ...).

[Jan Axelson]

Yes, that is possible though a functioning volume or gain control would seem to be a necessary feature.

[jb]

According to:
https://stackoverflow.com/questions/23519753/usb-audio-descriptor

I am beginning to believe that for recording, the Windows USB audio driver insists on finding a feature unit in between its audio input terminal (microphone) and its output terminal (USB streaming to host). That does not seem to mean it actually does anything (for the recording path ...).

Yes, the feature unit just tells the operating system which controls to show and how to communicate with the USB device.  Whether the USB device actually does anything with that information is another matter.  It could use that number to control the gain, or control the color of an LED, or do nothing at all, and the OS would have no idea.

[Jan Axelson]

True!