Can I pipe input from a Linux soundcard into 2 different applications?

Question

I have an Ubuntu 16.04.3 LTS system with

• an ASUS Xonar DGX soundcard, and
• a generic USB sound card.

At our church, we want to be able to broadcast a service as well as record it as a WAV file. To accomplish this, I have the 3.5mm audio output cable from the sound board connected to a splitter, so that there are two cables to plug into the "Line In" connector on each soundcard.

Then, I use "arecord" to record the audio on one card, and "darkice" to create an MP3 stream on the other.

Ideally, I'd like to be able to have one soundcard handle both the recording and streaming simultaneously. Is such a thing possible? Or am I stuck using two soundcards with two applications?

Answer 1

Yes, it's easily possible. Are you really only running only ALSA, or are you also running Pulseaudio (the default on basically all distros for many years)?

With Pulseaudio, it should work out of the box.

With ALSA, make sure you are using the plughw device name (which internally adds a dsnoop pluging) instead of the hw device name. The dsnoop plugin will split the input stream into multiple streams for different applications.

Edit

As I said, instead of -D hw:0 etc. (modify for your hardware address), just use -D plughw:0 for arecord, and similar for the device name in darkice.

A condition is that no other program uses hw:0 directly. All of them must use plughw:0.

You don't really need to know details about dsnoop, ALSA does this automatically for you. (But you can read up details on the plugin page I linked).

You don't have to setup your own ~/.asoundrc, you don't have to modify any configuration files.

Answer 2

I accepted dirkt's answer which got me most of the way there. Turns out I needed to create a dsnoop interface:

/etc/asound.conf

pcm.custom_snoop_card {
    type dsnoop
    ipc_key 420042
    ipc_key_add_uid 1
    slave {
        pcm "hw:0,0"   # the actual device
        format S16_LE
        channels 2
        rate 44100
    }
}

Now I can specify device = custom_snoop_card in my darkice config, and simultaneously make a WAV recording with arecord -D custom_snoop_card -f S16_LE -c2 -r 44100 -vv myfile.wav