SimH PDP-8 simulator plays music

Kyle Owen kylevowen at
Wed Apr 5 09:43:17 CDT 2017

I suspected that I could somehow get some music out of the SimH PDP-8
simulator for a while now, if I could only make it run real time and toggle
a GPIO pin fast enough say, on a Raspberry Pi. That may still be doable in
the future, but I also had a suspicion that I could generate music not in
real time.

I finally got around to trying out my idea last night. A few lines were
added to pdp8_cpu.c to spit out the elapsed instruction cycles every time a
CAF instruction is executed, the default "noise" instruction in the MUSIC.PA

That's all I did to the simulator. I then ran MUSIC with a given .MU file
and watched as many integers are spit out onto the screen. These were
copied and pasted into a new text file and saved.

The rest of it is in a single C program that I cobbled together. It reads
in this new text file and generates a series of pulses as an array of
floats. Each interval is about 1.93 microseconds, which I calculated to be
the average number of pulses for the music program to be "in tune" with
A=440 Hz, plus or minus. This value is subject to change, particularly as
the notes get higher in frequency, but only by perhaps 6% or so from my
experiments. One detail to note is that per the recommendation of the
MUSIC.PA manual, these pulses are extended to roughly 6 microseconds, or
three time intervals in my program.

This array of floats is then downsampled use libsamplerate to 44.1 kHz
(from 1/1.93 microseconds, or roughly 520 kHz) and output to a canonical
WAV file, 16-bit single channel.

What do you know, it worked! Here's a sample:

My code can be found here, for those interested:

Presumably, this technique could be used to generate music from any given
computer simulator.



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