I thought I'd try mucking around with some audio stuff#

Thu, 10 Mar 2005 02:45:26 +0000

I thought I'd try mucking around with some audio stuff. Chiefly I'd like to be able to put mixes together for the FNS music bike; it remains to be seen whether I actually can do this - or whether anyone likes my (lack of) music taste - but first I needed software. My findings, summarised and leaving out a lot of blind alleys:

1. The Linux MP3-HOWTO is woefully out of date. Of the two plausible-sounding apps it lists, emixer has disappeared from the planet, and GDAM (a) doesn't build without some slight hacking up

   GDAM_ARG_DOUBLE_ARRAY(arg).array[0] = spatial->##prefix##_x;
   

   and (b) after fixing this, segfaults on starting due (I think) to some problem with missing or incorrect XML files.

2. Mixxx seems like it ought to work, and even compiles (after installing a stack of libraries that I've hitherto not had the need for, but fair enough). But it needs a soundcard with a second output for cueing - although it can be told to use Left for cue and Right for speakers, my motherboard audio (VIA VT8237) allegedly can do six channels, so why not go for two stereo outputs?

3. (It also complains at startup about not having hardware GL, then incessantly thereafter about GLXBadLargeRequest, but for the moment I am choosing to deal with this by redirecting stderr. The claimed workaround of disabling waveform display actually causes it to segfault)

4. The ALSA fun starts here. With the aid of the motherboard manual I figured that the microphone jack doubles as rear speaker output; with a combination of amixer (which has a command-line interface that actually lets you see what's going on, not some crappy curses thing) and speaker-test -Dsurround40 -c4 I found correct mixer settings to get front channels coming through speakers and rear channels through the headphones plugged into the mic jack. Look out especially for Spread Front to Surround and Center/LFE and Surround Jack as Input, both of which should be disabled.

5. This is the best ALSA documentation I have ever read, and also the thing that gave me a pointer to Using surroundXX PCMs with JACK. With that and with

   jackd -d alsa -S -d jack40 -n3 --inchannels 2 --outchannels 4
   

   (less than 3 periods and I get enough "xrun" spewage that xterm's scrolling shows up in top(1)) all that remains is to tell mixxx's preference screeen to use alsapcm:playback[1234] in that order for each channel it wants to send stuff down.

For my next trick I'm almost tempted to configure GL after all. The video card (Voodoo 3 PCI) is old enough that it probably ought to have Linux drivers by now, after all.