Ken Shirriff has written an excellent in-depth look at the 76477 sound effects chip:
The 76477 Complex Sound Generation chip (1978) provided sound effects for Space Invaders1 and many other video games. It was also a popular hobbyist chip, easy to experiment with and available at Radio Shack. I reverse-engineered the chip from die photos and found some interesting digital circuitry inside. Perhaps the most interesting is a shift register based white noise generator, useful for drums, gunshots, explosions and other similar sound effects. The chip also uses a digital mixer to combine the chip’s different sound generators. An unusual feature of the chip is that it uses Integrated Injection Logic (I2L), a type of digital logic developed in the 1970s with the goal of high-density, high-speed chips. (I wrote about the chip’s analog circuitry last year in this article.)
See the full post on his blog here.