salsa

Simple ALSA. This repository is a set of tools that ease the configuration of ALSA

Usage

Main script:

sh get_usage.sh

or

watch -n1 "sh get_usage.sh" (will show the output of the script continuously with a refresh every second)

ALSA

I. Concepts:

What is the difference between pcm.!default and ctl.!default ?

The answer is pretty simple yet not easy to find online...

pcm.!default will define the default card used for playback and capture. It's refered to as the PCM card.

ctl.!default will define the default card that is affected when changing the sound level or tuning any settings via for example: alsamixer. It's refered to as the Control card.

II. Useful tools:

Multiple read microphone (DSNOOP)

When several processes need to access the same microphone.

Single read speaker (Loopback device)

The loopback device lets you play some audio that can then be captured by another devices as if it was a microphone.

⚠️ Your loopback device can be loaded as any card index. (card 0/card 1/...)

Correspondance:

Given that the loopback device is loaded on card 1 (hw:1)

Any sound that will be played to hw:1,0,1 can be recorded here hw:1,1,1.

hw:1,0,0 —> hw:1,1,0

hw:1,0,1 —> hw:1,1,1

hw:1,0,2 —> hw:1,1,2

...

When you want to access the output of a speaker as if it was a microphone.

Multiple write speaker (DMIX)

When multiple processes need to send audio to the same speaker.

III. Example configs:

Use different cards for capture and playback

Inside pcm.!default you can use the module asym and set a different card for playback and capture.

    pcm.!default {
        type asym
        playback.pcm "plughw:0"
        capture.pcm  "plughw:1"
    }

⚠️ This is not possible to do something like this for the control device (ctl).

Enable multiple input / output on default device.

source :https://stackoverflow.com/a/14398926/5530191

    pcm.dmixed {
        type dmix
        ipc_key 1024
        ipc_key_add_uid 0
        slave.pcm "hw:0,0"
    }
    pcm.dsnooped {
        type dsnoop
        ipc_key 1025
        slave.pcm "hw:0,0"
    }
    
    pcm.duplex {
        type asym
        playback.pcm "dmixed"
        capture.pcm "dsnooped"
    }
    
    # Instruct ALSA to use pcm.duplex as the default device
    pcm.!default {
        type plug
        slave.pcm "duplex"
    }
    ctl.!default {
        type hw
        card 0
    }

This does the following:

• creates a new device using the dmix plugin, which allows multiple apps to share the output stream
• creates another using dsnoop which does the same thing for the input stream
• merges these into a new duplex device that will support input and output using the asym plugin
• tell ALSA to use the new duplex device as the default device
• tell ALSA to use hw:0 to control the default device (alsamixer and so on)

Stick this in either ~/.asoundrc or /etc/asound.conf and you should be good to go.