This sound is activated when you win a ship, usually at 1000 or 1500 points. It uses one half of the 556 timer as a 555 timer operating in astable mode. This is actually the same configuration of the step circuit (which uses the other half of the 556) but here the sound tone does not come from the timer itself, but rather from a separate 480 Hz oscillator. The output from these sources is combined using a 3-input "and" port (the first 7411 in the picture) but the 555 is programmed at a frequency so low that all it does is to turn on and off the 480 Hz sound, thus making it intermittent rather than continuous (the third port connector is controlled by the CPU, which can then turn everything on and off at will).
This circuit is also easily emulated, but there is a caveat. When you listen to the corresponding sample there is a "vibrato" effect that the analysis above cannot account for. I have performed some measurements on the sample and I have found a 60 Hz sine wave signal in it, so now I'm added it to the emulation code and the result is really nice. I am not sure where these 60 Hz come from, as all the board is designed around this frequency and at present I'm not able to get more information.
Copyright (c) 2004 Alessandro Scotti. All rights reserved.