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"Listening" to the PDP-11

Michael Robinson

Michael Robinson

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Joined
Oct 4, 2021
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23
Location
Silver Spring, MD
I've often thought about the experiential difference between running emulators and "actual" hardware. Surely the emulator is easier to configure and can give more debugging information, but the actual machine has "presence". Right? It's big, makes noise, keeps my basement warm during the oncoming winter, and puts on a nice show of lights on the front panel.

But one underappreciated difference between emulators and actual hardware is that actual hardware throws off lots of electromagnetic interference. This is usually thought to be a problem, but I've heard stories of using this idea to craft a program that "plays music" on a nearby AM radio based on the pattern of memory accesses. I've even done that myself!

In any case, not to make a short story too long, I wondered if I might be able to "hear" the differences between different operating systems. In fact, you can! You can glean the difference in behavior of an eager boot prompt (a high pitched whine in some cases) versus a somewhat busy timesharing round-robin cycle (MIT SITS is apparently the most ferocious-sounding).

For this experiment, I raided my junk box to build a simple two-channel AM radio receiver with no front end filtering. This way, you're just listening to audio frequency electromagnetics. The two channels allow you to drop an antenna in different parts of the machine for a bit of a stereo effect. I placed one near the core memory and one near the CPU. If you're interested, the plans are here: https://github.com/kb1dds/electrophone

With the electrophone in place, I went through and collected up as many operating systems as I could, and recorded their front panel behavior in time with the audio recording:

You can spend quite a bit of time puzzling over what the machine is doing, and why the resulting audio is what results. Or just be amused by the cacophony of odd sounds.

Enjoy!
Michael
 
That takes me back a while...

I used to listen to the 'clicking' of the analogue multiplexer relay patterns on our machines at work and I could tell if the software was misbehaving.

Also, on each of the I/O cards, there is an 'accessed' LED. I would also look at the light patterns and could tell a similar thing for the other cards.

Dave
 
That was very cool, thanks! I will have to do that experiment too :)
Coincidentally, I just watched Star Trek: The Next Generation, episode "11001001", in which the Bynars hijack the Enterprise to save their world. They needed the Enterprise, because it is the only ship with the memory capacity to save all. Their world's computer would not survive the EMP from the nearby supernova, so they shut it down and need some "offline" storage. When the Bynars talk to each other, it would not surprise me that they were inspired by AM picked up from a PDP8. Your PDP-11/45 is the "modern version" Really cool.
 
That was very cool, thanks! I will have to do that experiment too :)
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Please share what you find if you do! One my students who heard my recordings asked if different PDP-11 CPUs running similar OS's, but perhaps with different devices, might be noticeably different.

When the Bynars talk to each other, it would not surprise me that they were inspired by AM picked up from a PDP8. Your PDP-11/45 is the "modern version" Really cool.
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Thanks! I definitely recall that TNG episode, but hadn't made the connection to the sound effects. To my ear, I had thought that they did some audio tweakage on human speech.

That's a pretty sick industrial track you are laying down!
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No kidding! I actually sent earlier recordings of the same machine running XXDP diagnostics to one of my musician friends who has been playing with the samples in his compositions. Play it at a bit slower speed for a different (more trance-y) effect. You can pick out different bursts of activity that probably correlate nicely to subroutines, etc., if you have some source code to follow.

I have also spent (thus far) a happy few hours messing with resamples of the 2.9 BSD recording at different speeds. Some of the device polling portions flow nicely into a rhythmic groove that's pretty catchy. Running the Unix v6 post-login activity very slow (~15x slower) makes the 60Hz clock interrupt into a very creepy heartbeat sound, with periodic bursts of additional dissonant musical (?) phrases interspersed.
 
The PDP-12 has a speaker connected to the MSB of the AC register. When running the OS you hear sound patterns, and get used to the normal sound. When running the DECtape diagnostics there are some "bell" sounds when everything is working OK.
 
That was great.

I'd love to hear one of the Unixen running 'make' on a small code base.

When you were logging in to one of the systems, I could almost see in my head someone talking to the machine, like R2-D2. You typing in the commands, the machine responding in buzzes and chirps.
 
At the risk of becoming a PDP-11 audio jockey... A request!

I'd love to hear one of the Unixen running 'make' on a small code base.
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Here's a quick and easy snippet. Compiling a hello world C program on Unix v6:
Code: 
$ cc hello.c

Put on some stereo headphones!


Compilation is mostly on one ear, while linking is mostly on the other. Not sure why that is, exactly, but you can definitely hear the difference between the two stages. And mostly throughout, you can hear the clock interrupt running along the background.
 
We used to do something similar with the HP2116C in high school... one of the nerds put together a programming guide so that it was relatively easy to compose tunes for it... fun, fun!
 
Interesting video. That first bit sounds much like an Atari 810 disk drive loading. The Atari's audio chip also runs the serial disk I/O and so makes audible noise. Coincidentally, Atari used the PDP-11 for the 800's OS development.
 
The Atari's audio chip also runs the serial disk I/O and so makes audible noise.
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Ah! You solved a mystery I forgot about. As a child, my family had an Atari 800XL. I distinctly recall the sound of loading things from floppy, but never connected the audio chip with disk accesses. At the time, I thought the sounds were "added for effect" rather than being a "side effect" of the disk activity. Nifty!
 
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