The PC Engine contains a Programmable Sound Generator or 'PSG', like many other third-generation consoles analysed on this website. However, this PSG particularly relies on the waveform memory (also called 'waveform buffer') approach to synthesise sound, rather than using a predefined set of waveforms (e.g., pulse, triangle, etc.). Waveform memory enables programmers to define their own waveforms, which gives greater flexibility for soundtrack arrangements.
Video - Oscilloscope display during a game of R-Type (1987).
The system features six audio channels , each of which is configured by writing to a set of registers. These store the attributes of the channels, including:
- Waveform shape: The waveform cycle is generated using thirty-two 5-bit values, each corresponding to the amplitude of the wave at a specific time.
- Some games, such as Fire Pro Wrestling 2 and Bloody Wolf, even altered these values during playback to obtain unique sounds (at the expense of audible clicks during transfers).
- Frequency control: Two 8-bit registers adjust the frequency of the channel to produce different musical notes using the same waveform.
- Amplitude level: A single 8-bit register stores two 4-bit values that determine the volume of the channel. These values correspond to the 'Left' and 'Right' panning control, a pioneering feature compared to what the competition offered.
Extra functions
Some groups of channels have additional modes of operation. For instance, the last two channels are connected to a noise generator. Moreover, the second channel can act as a Low-Frequency Oscillator (LFO) to modulate the first channel. I initially assumed this to be a synonym for FM synthesis, but LFOs are more of a 'subset' that produce a vibrato effect.
Speaking of modes, there's another one available called Direct D/A (DDA), which enables the CPU to write directly to the audio buffer (bypassing the PSG). Consequently, the PC Engine can play Pulse-Code Modulation (PCM) samples, which are still 5-bit and tightly dependent on CPU cycles. Nevertheless, two channels can be combined to reproduce 10-bit samples! .
All in all, this makes the PC Engine's PSG an exceptionally flexible piece of hardware.
Final sound
The PSG mixes all audio channels and outputs a stereo signal. But again, unless you find a way to bypass the RF port, you will only hear mono audio from your telly.