Custom IC 54 - Galaga/Bosconian/Xevious Noise Generator

Work started on this chip a long while back and got to the point of both A & B noise channels working pretty well, but the channel C with an adjustable noise period was a real pain in the A to get right. Not having a totally complete implementation caused this project languish as I did more interesting things. BUT now with the original MB8842 to 'reference' it makes life a whole lot easier when implementing the final missing 'C' channel.  The 'C' channel was used to generate the squealing tire sound in Pole Position and the shoot sound in Bosco, it wasn't used at all in Xevious or Galaga.