Poor Man's Theremin

If you're interested in gaining some experience playing the theremin but don't want to spend a lot of money or build the kit, try the “Poor Man's Theremin” (PMT), written by Seth David Schoen.

The PMT turns a laptop computer with an 802.11b card into a theremin-like instrument, using the signal strength reported by the card to control the pitch of a note. To try it, first compile this C program, called ttone, using the command cc -o ttone -lm ttone.c.

#include <math.h>
#include <linux/kd.h>
const int A = 440;
const float r = 1.05946;
int pitch(int base, int observed){
    return (int) A * pow(r, (observed-base));
    }
int main(int argc, char *argv[]){
    int base, observed;
    if (strcmp("off", argv[1])){
        base = atoi(argv[1]);
        observed = atoi(argv[2]);
        ioctl(0, KIOCSOUND, 1190000 / pitch(base, observed) );
    } else {
        ioctl(0, KIOCSOUND, 0);
    };
}

Put the ttone executable in a directory that's in your PATH.

Now run the following shell script, called pmt.sh, and move into range of an 802.11b access point. You can change the pitch by moving closer to or further away from the access point or by moving your hand over the 802.11b antenna in a theremin-player-like manner.

#!/bin/sh
# Poor Man's Theremin
m=100
oldQ=foo
[ $1 ] && m=$1
while :
do
Q=$(iwconfig 2</dev/null | grep Link.Quality | cut -d: -f2 | cut -d/ -f1)
if [ $oldQ != $Q ]
then
    ./ttone $m $Q
    fi
    oldQ=$Q
    done

The Poor Man's Theremin does not have the volume control of a proper theremin, and the pitch changes in discrete steps instead of continuously. Implementing these features is left as an exercise for the reader.

Don Marti is editor in chief of Linux Journal.