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Programmable Sound Generator (PSG)
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A programmable sound generator (PSG) is a chip that generates analog sound waveforms, noise and, in some cases, is able to process digital samples. PSGs were largely used as the main sound processors in computers and video game consoles in the 80s and 90s alongside single chip synthesizers. The main signals that a PSG can generate are square waves, triangle waves, saw waves and noise. Each PSG model differs in the offer of waves and sounds it can produce. As an example, the technical capabilities of the 2A03 Ricoh Processor will be explained.
The 2A03 Processor was an 8-bit microprocessor developed by Ricoh for the Nintendo Entertainment System/FAMICOM. Although it was a full integrated processor, it had specific sound generating capabilities.
The 2A03 processor had 4 sound generating channels: 2 square waves, 1 triangle wave and a noise channel. It also contained a fifth digital sample channel that could playback .
For the square channels it was capable of giving the waves 12.5%, 25%, 50% and 75% duty cycle (this means the fraction in which the signal is active in the period, which changes its harmonic content and overall sound). As it can be seen in the examples below, the square waves with 25% and 75% duty cycle are identical just with the phase flipped.
The processor was also capable of changing the envelope of the wave and the pitch which can result in effects such as vibrato. The 2A03 processor wasn’t technically capable of producing effects, since it didn’t have a DSP. However, with clever uses of the envelope and channels it was possible to “fake” effects such as delay.
Demonstration of "faux" effect with a 2A03 emulator In the video above it is possible to see an example of how fake effects were created. In this case, the envelope is programed so that it gives an illusion of delay or echo in the square wave signal.
Another of the capabilities of this processor was playing back digital audio samples. Because digital audio and overall computing processing was at early stages at this point, the digital samples that the 2A03 could process had to be heavily compressed to 7-bit resolution. The audio examples below compare a drum beat uncompressed with the orignal samples against a compressed one passed through a 2A03 emulator.
