[Alsa-user] Trying to add an softvol control to all USB devices

[Bakman L. <lau...@dk...>]

Hi

I am building a small form factor PC communication device for a communication system. The PC is running Ubuntu 12.04 (using a small set of installed packages). We use a number of USB audio devices attached to the PC to perform the actual audio input and output. In this case a PC might have 3 devices connected simultaneously: a handset, a headset and a speaker. For each of these devices I would like to be able to set a reference volume (a volume that we set from the system to a configured value) and provide a volume control that the user is able to control - but control relative to the reference value.

For some devices we are able to do this using the Master volume control (as reference) and the PCM volume control for the user. We control the UI for changing the volume so the name of the control is not important.

My problem is that not all of the devices we have provide a Master volume control, so my goal is to provide this "Master" control as a softvol control either for devices that does not have them, or create an entirely different control, perhaps "Reference" and use that.

I have followed http://alsa.opensrc.org/How_to_use_softvol_to_control_the_master_volume and configured individual devices to add this control and doing it this way works quite well. The only problem is that I have to do this for each device individually and I would rather if I could define this once and have it created for all USB audio devices.

I have tried to create an /etc/asound.conf to see if I could do this but unfortunately I am not very strong in writing these configuration files so I have been unable to write something that actually works.

I there anyone who can help me shed some light on whether it is possible to do it generally and if so, how to do it? 

The audio configuration is based on the scripts that are part of the Ubuntu 12.04 (LTS) distribution and has not been modified.

One of my tries has looked like the following, but I am definitely missing something:

pcm.reference {
       @args [ CARD ]
       @args.CARD {
               type string
       }

       type softvol
       slave.pcm {
               type hw
               card $CARD
               subdevice 0
       }
       control {
               name "Reference"
               card $CARD
       }
}

pcm.!default {
       type plug
       slave.pcm "reference"
}

Med venlig hilsen/ Best Regards

Lau Bakman
Systems Engineer
Business Area Security & Defence Solutions

Direct:  + 45 36 38 30 23
Mobile: + 45 41 87 60 23
E-mail: lau...@dk...

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