Real time audio effects with a RaspberryPi using Pure Data, with buttons and potentiometers integration.
A standard Raspbian (Debian Wheezy) works just fine.
Since the Pi doesn't have any audio inputs (and the quality of its output jack is rather bad), a USB audio adapter is required. There are tons of cheap options (under $5), the one I have is listed by lsusb as ID 0d8c:013c C-Media Electronics, Inc. CM108 Audio Controller.
Making it work properly wasn't easy. First, it must be set up so that it becomes the default soundcard. This is done by running sudo nano /etc/modprobe.d/alsa-base.conf and removing the comment that prevents snd-usb-audio from loading. It should end up looking like this:
options snd-usb-audio index=0
The default audio configuration must also be changed via sudo nano /etc/asound.conf to
pcm.!default {
type plug
slave {
pcm "hw:0,0"
}
}
ctl.!default {
type hw
card 0
}
You should also update the Pi's firmware. Instructions to do so can be found here.
Finally, USB devices should be forced to run in 1.1 mode by running sudo nano /boot/cmdline.txt and adding dwc_otg.speed=1 at the end. This causes most keyboards to stop working on the RPi, so be sure to set up SSH or VNC remote access beforehand.
Simply install via sudo apt-get install puredata pd-zexy pd-comport pd-cyclone. Pure Data must be started from the command line, using sudo pd -lib zexy -lib comport -lib cyclone/coll (plus whatever other options you might use, such as -nogui). pd-zexy is required for the multiplex~ and demultiplex~ objects, pd-comport is required for the comport object, and pd-cyclone is required for the coll object, inside convert.ascii2pd.
Make sure the correct number of input channels is set on Pure Data (Media/Audio Settings). The CM108 (as well as most cheap soundcards) only has one input channel, but the default value is 2. Failure to correct this setting will prevent Pure Data from properly reading the input signal from your USB interface.
I finally got tired of struggling to make the Pi's audio (in particular, the microphone inputs) work consistently. It can be made, and I successfully tried all of the effects the few times I got it working, but I don't want to waste any more time on the subject. Since neither size nor weight are constraints to me, I switched platforms to my old desktop PC, which was previously gathering dust in the basement.
It consists of an Asus M2N-MX SE Plus motherboard, and AMD Athlon X2 5200+ (clocked at 2.7 GHz), 2GB of RAM, and a 160GB HDD. I'm using the onboard sound card, and running Linux Mint 17.1 (Rebecca), with Xfce as a desktop environment. This setup is way overkill for the application, but it does leave me headroom to play with some more processor-intensive effects, and decrease system latency.
Getting the project running was very straightforward. Mint automatically installed drivers for the sound card, so all I had to do was clone the repo, install Pure Data and set the correct input and output devices. I expect this to be the case on most modern motherboards.
One of my main sources of inspiration, contained lots of helpful information, links and cool-sounding patches.
Ben also built one of these, and gave me the idea to reuse an old piece of equipment (in his case, a multimeter) with an LCD screen to display and control the different effects.