Skip to content

Sound Card (UAC)

A UAC1 audio function is a stereo speaker (isochronous OUT) plus a mono microphone (isochronous IN), 48 kHz / 16-bit. Provide a microphone source and a speaker sink - or omit mic_source and the class plays a built-in 440 Hz tone.

A tone-generator sound card

import time, usbip
from usbip.device import USBDevice
from usbip.classes.device import UAC

def spk_sink(data):          # PCM the host played to the speaker
    pass

dev = USBDevice(0x1209, 0x0007, product="USBIP Audio")
dev.add(UAC(spk_sink=spk_sink))        # mic = built-in tone
dev.set_iso_pacing(True)               # play at real time (see below)
dev.plug()                             # -> an ALSA card
time.sleep(3600)                       # serve (Ctrl-C to stop)

#include <unistd.h>
#include "usbip-device.h"
#include "classes/uac.h"

static void spk(void *u, const uint8_t *d, int n) { 
    (void)u; (void)d; (void)n; 
}

int main(void) {
    usbip_device *g = usbip_device_create(0x1209, 0x0007);
    uac_opts opts = {
        .spk_sink = spk,     /* mic_source NULL -> 440 Hz tone */
    };
    uac_add(g, &opts);
    usbip_device_set_iso_pacing(g, 1);    /* play at real time */
    usbip_device_plug(g, NULL);           /* -> an ALSA card */
    for (;;) sleep(1);
}

Record and play

Once a client has imported the device, the host's in-box audio driver binds it and it becomes an ordinary sound device - recording from it yields the microphone stream (the 440 Hz tone unless you supply one), and playing to it lands in spk_sink().

snd-usb-audio binds it as an ALSA card:

aplay -l                                      # find the card
arecord -D plughw:CARD -d 3 mic.wav           # record the device's mic stream
aplay   -D plughw:CARD song.wav               # play into spk_sink()

usbaudio.sys binds it. Pick it under Settings → Sound, then record with Voice Recorder or play anything to it.

Enable isochronous pacing for audio

Without dev.set_iso_pacing(True) / usbip_device_set_iso_pacing(g, 1) audio free-runs many times too fast - USB/IP has no bus frame clock.

Full program: examples/audio_device.py (with --mic-in/--out WAV); in C, examples/device/audio/. API: UAC.