[Simh] Sounds

[Bob Supnik]

And then there was early computer music...

When Applied Data Research got its PDP-7 in 1966, there was a DECUS 
program to get it to play music by toggling the lower order 4 bits of 
the MQ (and the MQ lights) to generate square waves. If you wired that 
up to an audio player, you got electronic "music" of a blatting sort, in 
four part harmony. (It really needed some analog filtering to flatten 
the square waves into curves, but that was beyond me.) Anyway, I spent 
much of my free time that summer programming as much of the original 
piano score of "Pictures at an Exhibition" as would fit into four parts. 
With the buzzing tones, the completely constant volume, and the coarse 
controls over note lengths, it sounded utterly bizarre, but... the 
computer was playing music! As was said back then, "The marvel is not 
that the bear dances well, but that the bear dances at all."

The PDP-7 had DECtapes, and they had their own unique rhythms. DEC's 
software was poorly written and could only read a block at a time, so 
you'd hear the tape start, reverse, read, stop; rinse and repeat. A 
brilliant colleague named Avram Caspy figured out how to insert 
optimized routines underneath DEC's software (he used all 8KW extended 
memory as a buffer). With his routines, the DECtapes would start, 
reverse, and then whoosh at high speed for up to 30 blocks before 
stopping. DEC's paper tape routines were equally poor and would 
stutter-read; use of interrupts and a short circular buffer made that 
continuous and quieter as well.

Another fun set of devices were the very high-speed vacuum pick card 
readers that the mainframe companies made. They would blow air through 
the card deck to separate the cards and then vacuum pick the top card, 
reading and expelling it at breakneck speed (the best readers did 1000 
cards per minute or more). Of course, when they broke, you got a totally 
different sound, as cards were blown all over the machine room, 
typically with the front-edges curled, making them unreadable.

/Bob