Real-time DSP effects on C64: how is this possible?

Question

I just saw this and this is one of the most amazing things I've ever seen:

http://www.youtube.com/watch?v=MDrqBYkco-Y

I can’t even figure it out. What's going on here?

Answer 1

This document provides a detailed explanation of what is happening. The primary technique is voice compression, which works just like a music sequencer or tracker, but designed for voice. This makes it easy to adjust the pitch and speed (as the tracker does). Follow some typical C64 tricks to sync everything and use every processor cycle.

Answer 2

Atari 2600. 76 - 46 30 . :

  out1 = table1[phase1] + table2[phase2];
  out2 = table3[phase3] + table4[phase4];
  phase1 = (phase1 + freq1) mod length1;
  phase2 = (phase2 + freq2) mod length2;
  phase3 = (phase3 + freq3) mod length3;
  phase4 = (phase4 + freq4) mod length4;

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