How to use Jack (or similar software) to route music played in the Pi as audio input (like a microphone)?

Question

I'm trying to make a small music reactive light project with the RPi. I'd like to play music (using whichever software in Raspbian), and to interpret that music as audio input to process while still playing out through the audio jack.

I've achieved it using MacOS and SoundFlower: set soundflower 2ch as both audio input/output, that drives the lights already; and then using soundflowerbed, set soundflower 2ch as built-in output so you recover sound as well.

I've been trying to clone this setup in the RPi using jackd & qjackctl, but i didn't manage.

Any thoughts? Thanks, Alvaro

Answer 1

You want to use ALSA for this, and load the snd_loopback module.

pi@raspberrypi ~ $ sudo modprobe snd-aloop pcm_substreams=1

This will create a loopback device on your system. Next you want to set the default ALSA audio output to this substream by editing /etc/asound.conf.

pi@raspberrypi ~ $ sudo nano /etc/asound.conf

Setup your configuration file:

# .asoundrc
pcm.multi {
    type route;
    slave.pcm {
        type multi;
        slaves.a.pcm "output";
        slaves.b.pcm "loopin";
        slaves.a.channels 2;
        slaves.b.channels 2;
        bindings.0.slave a;
        bindings.0.channel 0;
        bindings.1.slave a;
        bindings.1.channel 1;
        bindings.2.slave b;
        bindings.2.channel 0;
        bindings.3.slave b;
        bindings.3.channel 1;
    }

    ttable.0.0 1;
    ttable.1.1 1;
    ttable.0.2 1;
    ttable.1.3 1;
}

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

pcm.output {
    type hw
    card Headset
}

pcm.loopin {
    type plug
    slave.pcm "hw:Loopback,0,0"
}

pcm.loopout {
    type plug
    slave.pcm "hw:Loopback,1,0"
}

You need to make sure the output pcm points to the speaker/output device you want the audio to go to. You can use aplay -L to get a list of available output devices.

You can know use avconv or ffmpeg to record audio from any application and also routing the audio to an output device.

Here is an example:

pi@raspberrypi ~ $ avconv -f alsa -ac 2 -ar 44100 -i loopout test.wav

SOURCE:
https://trac.ffmpeg.org/wiki/Capture/ALSA