Yes, that has some mystical effect on me. I think I saw it when you shares it the other day. How does the AM capture the sound? What exactly generates the sound waves?
It's electromagnetic interference from the electronics. A hulking powerful "big iron" PDP-7 makes a much better RFI broadcast antenna than an iPhone, but it broadcasts at much lower frequencies, that an AM radio can pick up.
You can hear a little bit of interference on an FM radio, but it sounds much stronger and more interesting on the AM dial.
PDP-7's were actually relatively sleek fast inexpensive computers for the time, and were only 18 bits compared to the PDP-6's and PDP-10's 36 bits. ("If you're not working with 36 bits, you're not working with a full DEC!" -DIGEX) But physically they still have a lot of "big iron" parts and use lots of power like mainframes.
The other sibling post by nitrogen referred to "tempest", which is a (no longer) secret NSA codename for "Telecommunications Electronics Materials Protected from Emanating Spurious Transmissions", a certification for shielding equipment from spying on it via electromagnetic radiation.
The slower the computer, the more "details" you can hear of the computation. Modern computers are so fast and have so much going on and are so much better shielded (they need to be, since they have their own radios they don't want their own RFI to interfere with), that they aren't nearly as interesting to listen to on AM radio. I loved to listen to my Apple ][ on AM radio, especially compiling my FORTH system and applications (it's satisfying to listen to your own code you wrote compiling and running)!
Thank you, this is a very interesting topic to me. I remember ,when I was a kid in the 90s and had the Sound Blaster card, 1.0 or something quite cheap and in certain situations I would hear very similar but quiet interference on the headphones when there was no sound playing.
