Possible to route audio directly from usb-audio line-in to same usb audio line-out?

Question

I have a Raspberry Pi 3 - and was able to get my ASUS USB audio device to work both with mic/line-in and output to speakers.

Right now I can do it it in two steps:

• arecord will record to a wav file
• aplay will play the wav file

Just wondering if it's possible with a simple config setting to somehow have the audio pass thru directly from line-in to speaker-out?

Hoping to avoid using avconv or ffmepg to redirect -seems like a lot of cpu just to do pass-thru audio.

If I can do this, I'd like to be running a program in the background that would interrupt the audio for a message and then return back to passthru audio

Answer 1

I found a simple solution using just ALSA and uses little CPU - around 3-5% on RPi 3 using latest Jessie build and a USB Audio Device

It appears that recent builds all come with ALSA's dmix plugin built-in. This allows you to share and output audio output stream

Create New .asoundrc

pi@raspberrypi:~ $ sudo nano .asoundrc

Copy/Paste this based on USB Audio Device is the second card in system

pcm.LineOut {
    type dmix # Kernel PCM
    ipc_key 1112233  # Any Unique Number Number
    slave {
        pcm "hw:1,0" # Card # of USB Audio from aplay -l
        period_time 0
        period_size 1024
        buffer_size 8192
        rate 48000
        #periods 128
     }
}

pcm.!default {
        type hw
        card 1
}
ctl.!default {
        type hw
        card 1
}

Modify ALSA settings to make the USB Audio device the default using

alsamixer
<F6>
Select the USB Sound Card
Change Inout to Line if you have a high end Audio device, otherwise leave as Mic
Modify volumes to 40-50%

On exit, save settings

alsactl store

Start the stream playback from Audio In direct to Audio Out (& will return to command line)

arecord -f dat | aplay -D "LineOut" &
<CR>

Reduce volume of line input:

 amixer set Line,0 0% 

Use dmix shared pcm device to insert wav file previously recorded from line in:

aplay -D LineOut -R 48000 hiVol.wav

When it's all done restore the volume back to normal:

amixer set Line,0 50% 

Answer 2

You can use module-loopback functionality of PulseAudio. First identify the names for your sound card's line in and speakers using pactl list sources and pactl list sinks respectively. Then create the loopback:

pactl load-module module-loopback source=src_name sink=sink_name

If your soundcard is detected with correct default source and sink, simply running pactl load-module module-loopback may be enough.

Once the loopback is created, the sink and source it uses can be modified with pavucontrol.