Chapter 2 Overview of SCSP

■ 2.1 LSI overview

Since SCSP was developed with multimedia support in mind, the audio functions are more sophisticated and sound quality than ever before. The arithmetic unit also has performance close to that of a commercially available synthesizer, and can produce a large number of sounds. In addition, various interfaces are available for future extensibility. The DSP section can also create multiple sound fields, and is effective for various sound field reproduction and special effects such as 3D sound localization.
The features of LSI are described below.

● Sampling frequency

• Sound source resampling frequency (constant playback rate) 44.1KHz
• Waveform data sampling frequency 0 (CD) ~ 44.1KHz

● Features

• It is equipped with 32 slots for both FM and PCM.
• A total of 32 LFOs are installed, one for each slot (completely independent).
• It is equipped with 32 4-segment EGs.
• It has two built-in CPU interfaces, a sound CPU and a main CPU.
• It has a built-in MIDI interface.
• The DSP of 128 steps is carried.
• It has a built-in input mixer that selects the DSP input.
• It has a built-in output mixer that mixes sound source output and DSP output.
• The new FM calculation system is carried.

■ 2.2 LSI specifications

The detailed specifications of the sound CPU (MC68EC000) and SCSP are described below.

● Sound CPU specifications

• SOUND CPU MC 68EC000-12 12.5MHz version
• Operating frequency
  11.2896MHz
• CPU instruction
  Fully compatible instruction system with MC68000 CPU Note 1
• Interrupt signal
  Level 1-7 interrupts by autovector
• Hardware configuration
  Configuration with the MC6800 (8-bit) interface removed from the MC68000 CPU

Note 1:

Some orders are prohibited in Sega Saturn.

• RESET instruction
• TAS instruction

● SCSP LSI specifications

• SOUND LSI SCSP (SATURN CUSTOM SOUND PROCESSOR)
• Operating frequency
  22.5792MHz

◆ Sound source

• Speech synthesis method PCM speech synthesis and FM speech synthesis method Noise sound source (mixed use possible)
  For FM speech synthesis, not only the conventional 4-operator (slot) type, but also 2-32 operators can be used to make any FM connection. You can also set the feedback as you like.
• Sound source resampling frequency
  Fixed at 44.1KHz, first-order interpolation

• Waveform data format
  16-bit straight line and 8-bit straight line 2'S complement method
• Number of audio processing slots (number of installed: 32)
  Equivalent to 1 operator (1 slot) of PCM sound source and FM sound source
• Maximum (maximum) number of simultaneous pronunciations (number of devices: 1 to 32)
  All 4 operator (slot) type FM sounds are 8 sounds, and all PCM sounds are 32 sounds. The number of slots used for one FM sound can be set arbitrarily. It is also possible to use a mixture of FM sound and PCM sound. Any combination will do as long as the total number of slots used does not exceed 32.
• Waveform loop function
  Select from normal, reverse, and alternative loops for each slot
• EG (Number of mounted: 32)
  4-segment AMPLITUDE-EG (one in each slot)
• LFO (number of units installed: 32)
  One in each slot (for amplitude / frequency modulation)
• Types of LFO output waveforms (number of mounted: 4)
  4 waveforms: sawtooth wave, square wave, triangle wave, and white noise
• Sound source oscillation frequency
  1024 steps (non-linear) can be set between -8 to +7 octaves octaves

◆ DSP section for effects

• DSP processing speed
  128 steps / Fs (multi-parallel processing DSP Fs = 44.1KHz)
• Program RAM (capacity: 128W)
  128 words x 60 bits
• Coefficient data RAM (capacity: 64W)
  64 words x 13 bits
• Temporary (general purpose) RAM (capacity: 128W)
  128 words x 24 bits
• Multiplier accuracy
  24-bit x 13-bit = 26-bit output
• Adder accuracy
  26 bits + 26 bits = 26 bits output
• DSP internal bus width
  24-bit data bus

◆ Digital mixer section

• Number of output level adjustment steps
  8 stages (for each slot sound and DSP output sound)
• Number of pan pot adjustment stages
  31 stages (1 stage in the center, 15 stages each on the left and right)
• Number of DSP effect send channels (number of installed: 16)
  Multiple slot outputs can be mixed and stored in each channel
• DSP effect Number of return channels (number of installed: 16)
  Level and pan pot setting for each channel
• Master volume setting function
  Stereo compatible

◆ CPU interface

• Main CPU interface (number of units installed: 1)
  SCU B-BUS interface
• Sound CPU interface (equipped: 1)
  MC68EC000 interface

◆ Other functions and interfaces

• Timer (number of mounted: 3)
  8-bit timer with prescaler
• DMAC (Number of units installed: 1)
  DMA controller for transfer between SCSP and DRAM
• Interrupt controller (number of units installed: 1)
  Interrupt controller for main sound CPU
• MIDI interface
  IN: 1 OUT: 1
• Digital audio interface (number of units installed: 1)
  Input stereo interface: 1
• Interface for sound memory (number of units installed: 1)
  DRAM interface
• DAC output interface (number of units installed: 1)
  16-bit / 18-bit stereo DAC interface
• External interrupt signal interface (equipped: 3)
  Unconnected / unused at Sega Saturn

◆ DSP program library

Various effect programs

• REVERB (HALL, ROOM, VOCAL, PLATE, etc.)
• EARLY REFLECTION
• ECHO / DELAY (STEREO, MONORAL)
• PITCH SHIFTER (SINGLE, DOUBLE, TRIPLE)
• CHORUS
• FLANGER
• SYMPHONIC
• SURROUND
• VOICE CANCEL
• AUTO PAN
• PHASER
• DISTORTION
• FILTER
• PARAMETRIC EQUALIZER

■ 2.3 SCSP chip block

Figure 2.1 shows the SCSP chip block.

Figure 2.1 SCSP chip block diagram

Sound CPU interface		LFO
Main CPU interface		Phase generator
DMA transfer interface		Address pointer
MIDI interface		Memory controller
External input		Interpolator
Envelope generator		Level calculation unit
DSP		Output mixer

● Sound CPU interface

An interface for connecting the MC68EC000.

● Main CPU interface

An interface for connecting a system controller (SCU).

● DMA transfer interface

The DMA controller with built-in SCSP enables data transfer between SCSP and sound memory.

● MIDI interface

A serial interface that supports the MIDI standard. However, some external circuitry is required to support the MIDI connector.

● External input (external digital audio input interface)

An interface that inputs digital audio signals from an external device (external device).

● LFO (Low Frequency Oscillator)

It is a function generator that generates waveforms with frequencies lower than the range that can be heard by the human ear, and is used as various modulation waveforms.

● Phase generator

It is a block that calculates and calculates frequency information according to the sound frequency setting of PCM data.

● Address pointer

Generates a waveform address value based on the phase information from the phase generator.

● Memory controller

It is a block that controls the sound memory, such as outputting the address generated by the address pointer to the memory and reading the data.

● INTERPOLATOR

A block that performs interpolation operations for waveform data.

● Envelope Generator (EG)

A function calculation block that controls the audio output level over time.

● Level calculation unit

This block calculates the waveform output level based on the level coefficients generated by EG, TL (Total Level), and ALFO.

● DSP

This is an audio DSP that handles the special acoustic effects of SCSP.

● Output mixer

This is the final block with various localization calculation functions and level adjustment functions for combining each audio output into a stereo output.