I think it's the COVOX 8-bit thing. Not sure what the specs are, but if you can load samples and loops and set them off at specific intervals, you can accomplish a whole lot with almost no burden to the main computer.
Edit: nope. The COVOX was a parallel port DAC. The computer has to send each sample out. I suspect some light compression, pre-rendered audio segments and an interrupt-driven routine to get data from the HDD and send enough samples per second the sound doesn't get distorted.
> and send enough samples per second the sound doesn't get distorted
That's kind of the awesome thing here, this machine isn't even capable of 1MIPs, so it has to pump audio out of the COVOX and read, decode and display the video.
From the pouet page from one of the authors:
"1. This computer has no DMA. We have to read all data "manually" from HDD registers, word by word. As well as switch heads, cylinders and sectors on fly.
2. This computer is very slow, it spends up to 72 CPU cycles to move a number from one memory cell to another. And it runs on 4 MHz only."
I remember making a knock-off and having doubts if something this simple would work. But it did and it was absolutely magnificent. The PC stopped beeping and started making proper sounds. The sunset of the age of tangible computer wonders :)
Is the music also produced by the computer? I suppose that it is but it's hard to believe given the quality. Impressive work.