In fact, if you use Jackd (and perhaps Pulse) you can change the sound card used by Spotify, but this is overkill for my needs (mainly cause, I use Spotify on a litte old netbook and Jackd is a nightmare to configure)

After some strace dump, Alsa (libasound2) source reading, I decided to fix it by myself. Let’s go for some Monkey Patching. Monkey patching on closed source binary isn’t that easy, the hard step : find the right function to hack. Here the Alsa tutorial come to rescue (ldd of course too). So we need to modify the snd_pcm_open call.

Let’s give it a try : First we need to dynamically re-route this call to a wrapper. The easier way to do this, is to build a special library and to force Spotify to use the wrapper with an LD_PRELOAD hack. (see code below). In fact, Spotify call the snd_pcm_open with name=”default” which is the default sound card, so we can change this to “usb” for example, and fix the .asoundrc to point “usb” sound card to the right hardware card.

That’s enough, let’s code :

#define _GNU_SOURCE

#include <dlfcn.h>
#include <stdio.h>
#include <alsa/asoundlib.h>

static int (*wrap_snd_pcm_open)(snd_pcm_t **pcm, const char *name, snd_pcm_stream_t stream, int mode) = NULL;

void * wrap(void * func,char *name)
{
    char *msg;
    if (func == NULL) {
        func = dlsym(RTLD_NEXT, name);
        fprintf(stderr, "** wrapping %s =>  %p\n", name,func);
        if ((msg = dlerror()) != NULL) {
            fprintf(stderr, "** %s: dlopen failed : %s\n", name,msg);
            exit(1);
        } else {
            fprintf(stderr, "** %s: wrapping done\n",name);
        }
    }
    return func;
}

int snd_pcm_open(snd_pcm_t **pcm, const char *name, snd_pcm_stream_t stream, int mode)
{
    int temp;

    sprintf(name,"usb");

    wrap_snd_pcm_open = wrap(wrap_snd_pcm_open,"snd_pcm_open");
    temp = wrap_snd_pcm_open(pcm,name,stream,mode);
    fprintf(stderr, "** Calling snd_pcm_open for path:[%s] => fd:[%d]\n",name,temp);
    fflush(stderr);
    return temp;

}

As you can see in the code, I use a generic wrapper function because during my code session, I used this function to re-route some other functions. To build the library :

gcc -g -fPIC -shared -Wl,-soname,preload.so -ldl -o preload.so  main.c

Load the lib, with export LD_PRELOAD=./preload.so before running Spotify. Spotify (and all other software using Alsa) will use “usb” as default sound card.

Note: C reader will note that name is a const char pointer, and so we can’t change it, but even if gcc raise a big warning, the code do trick fine.

Enjoy good quality sound even w/ closed source

Here on my shuttle .. not working

aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: IXP [ATI IXP], device 0: ATI IXP AC97 [ATI IXP AC97]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: IXP [ATI IXP], device 1: ATI IXP IEC958 [ATI IXP IEC958 (AC97)]
Subdevices: 1/1
Subdevice #0: subdevice #0

Here on another box (VIA C3 based)

aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: V8235 [VIA 8235], device 0: VIA 8235 [VIA 8235]
Subdevices: 4/4
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1
Subdevice #2: subdevice #2
Subdevice #3: subdevice #3
card 0: V8235 [VIA 8235], device 1: VIA 8235 [VIA 8235]
Subdevices: 1/1
Subdevice #0: subdevice #0

So yes the C3 can play 4 stream at the same time, while the AC97 doesn’t.

Another way to do this is to use software mixing

Update:

To use the software mix in Alsa: simply change the pcm.default to pcm.dmix

#pcm.default cards.pcm.default
pcm.default pcm.dmix

For more infos check:

• http://base-art.net/wk/Article/6.html Phil blog
• http://respyre.org/pyalsa.html PyAlsa webpage

– Enjoy sound :)

Take care that alsa-base ask you which sound card you have. This is needed for building the init-script.

Next:

cd /usr/src/
tar xvfj  alsa-driver.tar.bz2
cd modules/alsa-driver
./configure
make
make install
/etc/init.d/alsa start

Take care that by default all mixer are off !!

– Enjoy using gnomemeeting