Saturn Custom Sound Processor

Saturn Custom Sound Processor
Designer: Yamaha

The Yamaha YMF292, also known as the Saturn Custom Sound Processor (abbreviated as SCSP) is a sound chip created by Yamaha for the Sega Saturn, before also making its way into the Sega Model 2A-CRX, Model 2B-CRX, and Sega Model 3 arcade boards.

Hardware

The SCSP contains 32 channels of sample-based audio. In addition to playing back samples with a variety of different loop settings, each SCSP channel can further shape the sound of its samples using ADSR envelopes and an LFO generator (used for hardware vibrato or tremolo effects), and can send its output audio directly to the speakers and to one of the 16 input slots on the chip's Digital Signal Processor (DSP). The DSP itself is a programmable effects unit that can perform up to 128 microcode operations on its input slots to create delay, filtering, and many other audio effects.

One rarely-used feature unique to the SCSP is its FM functionality, which allows any channel to have its sample data be modulated by the stored audio output of up to two other channels (including itself). Because of the freeform nature of this operation, there is no limit to how the SCSP's channels can be arranged—one song can use all 32 of its channels for direct PCM playback, another song can split its channels into 16 2-OP PCM/FM instruments, while another still could use 3 5-OP instruments, 4 3-OP instruments, and 7 direct playback channels. Additionally, songs that use the DSP may reserve some channels for effects inputs (like an LFO for a chorus effect) instead of as playback channels or modulators.

The Sega Saturn's audio RAM is connected directly to the SCSP, and is used to store the samples, sound programs (e.g. Cybersound), and any other data needed by the music and sound effects. The entire chip is controlled by a single Motorola 68EC000 16-bit CPU running at 11.3Mhz, which is essentially the same as (and compatible with) the M68000 used in the Mega Drive and several other consoles, but without the 8-bit MC6800 interface. The SCSP can also be directly accessed by both SH-2s in the Saturn, although it was more common to leave as much of the sound processing to the 68EC000 as possible.

The SCSP only operates on raw 8-bit or 16-bit big-endian PCM data, with no native support for compressed samples. Many later Sega Saturn games, however, would store longer sound effects or music as compressed ADX data, which would be decompressed and played by the 68EC000 in real time. Although the SCSP supported MIDI inputs through the Saturn MIDI Interface Box, the only games to take advantage of this feature were Saturn Music School and its sequel.

Technical information

• 44.1 kHz sampling rate (CD quality)
• 22.6 MHz Yamaha FH1 digital signal processor
• All 32 slots can be used for PCM sample playback
• Any of 32 slots can be linked together for multiple operators per FM sound channel

Documentation

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  User's Manual (ST-077-R2-052594)