Sunsoft 5B audio

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The Sunsoft 5B is a superset of the Sunsoft FME-7. It is identical to the FME-7 except it contains extra audio hardware. This audio hardware was only used in one game, Gimmick! Because this game did not use many features of the chip (e.g. noise, envelope), its features are often only partially implemented by emulators.

The audio hardware is a type of Yamaha YM2149F, which is itself a variant of the General Instrument AY-3-8910 PSG.

Registers

The two audio registers on the 5B correspond to writes to the internal YM2149F registers, first selecting the internal register through $C000, then writing to it through $E000.

Audio Register Select ($C000-$DFFF)

7......0
----RRRR
    ++++- The 4-bit internal register to select for use with $E000

Audio Register Write ($E000-$FFFF)

7......0
VVVVVVVV
++++++++- The 8-bit value to write to the internal register selected with $C000

Internal audio registers

The YM2149F has 16 internal audio registers, selected with $C000 and written to with $E000.

Register Bitfield Description
$00 LLLL LLLL Channel A low period
$01 ---- HHHH Channel A high period
$02 LLLL LLLL Channel B low period
$03 ---- HHHH Channel B high period
$04 LLLL LLLL Channel C low period
$05 ---- HHHH Channel C high period
$06 ---P PPPP Noise period
$07 --CB Acba Noise disable on channels C/B/A, Tone disable on channels c/b/a
$08 ---E VVVV Channel A envelope enable (E), volume (V)
$09 ---E VVVV Channel B envelope enable (E), volume (V)
$0A ---E VVVV Channel C envelope enable (E), volume (V)
$0B LLLL LLLL Envelope low period
$0C HHHH HHHH Envelope high period
$0D ---- CAaH Envelope reset and shape: continue (C), attack (A), alternate (a), hold (H)
$0E ---- ---- I/O port A (unused)
$0F ---- ---- I/O port B (unused)

Sound

There are three channels that output a square wave tone. In addition there is one noise generator, and one envelope generator, both of which may be shared by any of the three channels.

The 5B's audio is driven by the CPU clock (1.789773 MHz). It operates equivalent to a YM2149F with its SEL pin held low (see datasheet). This causes the tone and noise channels to operate at half the speed of an AY-3-8190 with an equivalent clock, but enables a smoother envelope with 32 steps instead of the AY's 16.

To use an AY-3-8910 as a substitute, you would need an external divider to reduce the clock speed by half.

Unlike the 2A03 and VRC6 pulse channels' frequency formulas, the formula for 5B does not add 1 to the period. A period value of 0 appears to produce the same result as a period value of 1, for tone[1], noise and envelope[2].

Tone

The tone generators produce a square wave with a period controlled by the CPU clock and the 12-bit period value in registers $00-05.

  • Frequency = Clock / (32 * Period)
  • Period = Clock / (32 * Frequency)

Register $07 controls the mixing of tone and noise components of each channel. A bit of 0 enables the noise/tone on the specified channel, and a bit of 1 disables it. If both bits are 1, the channel outputs a constant signal at the specified volume. If both bits are 0, the result is the logical and of noise and tone.

If bit 4 of registers $08-$0A is set, the volume of the channel is controlled by the envelope generator. Otherwise, it is controlled by the 4-bit value in bits 3-0 of the same register.

Noise

The noise generator produces a 1-bit random output with a period controlled by the CPU clock and the 5-bit period value in register $06.

  • Frequency = Clock / (32 * Period)
  • Period = Clock / (32 * Frequency)

It is implemented as a 17-bit linear feedback shift register with taps at bits 16 and 13.[3]

Envelope

The envelope produces a ramp that can be directed up or down, or to oscillate by various shape parameters.

Period

The ramp has a frequency controlled by the CPU clock and the 16-bit period value in registers $0B-0C. Note this formula is the frequency of a single step of the ramp.

  • Frequency = Clock / (16 * Period)
  • Period = Clock / (16 * Frequency)

The 5B divides each ramp into 32 steps, so for continued ("sawtooth") envelope shapes the resulting frequency will be 1/32 of the step frequency, and for the continued alternating ("triangle") envelope shapes it will sound at 1/64 of the step frequency.

Because the envelope is primarily intended for low (sub-audio) frequencies, its pitch control is not as accurate in audio frequency ranges as the tone channels.

Shape

Writing register $0D resets the envelope[4] and chooses its shape. The shape has four parameters: continue, attack, alternate, and hold.

  • Continue specifies whether the envelope continues to oscillate after the attack. If it is 0, the alternate and hold parameters have no effect.
  • Attack specifies whether the attack goes from high to low (0) or low to high (1).
  • Alternate specifies whether the signal continues to alternate up and down after the attack. If combined with hold it provides an immediate flip after the attack followed by the hold.
  • Hold specifies that the value shall be held after the attack. If combined with alternate, the value at the end of the attack will be immediately flipped before holding.
Value Continue Attack Alternate Hold Shape
$00 - $03 0 0 x x \_______
$04 - $07 0 1 x x /_______
$08 1 0 0 0 \\\\\\\\
$09 1 0 0 1 \_______
$0A 1 0 1 0 \/\/\/\/
$0B 1 0 1 1 \¯¯¯¯¯¯¯
$0C 1 1 0 0 ////////
$0D 1 1 0 1 /¯¯¯¯¯¯¯
$0E 1 1 1 0 /\/\/\/\
$0F 1 1 1 1 /_______

Output

The tone channels each produce a 5-bit signal which is then converted to analog with a logarithmic DAC. Note that the least significant bit cannot be controlled by the volume register, it is only used by the YM2149F's double-resolution envelope generator. The logarithmic curve increases by 1.5 decibels for each step in the 5-bit signal. This can easily be implemented as a lookup table.

Some emulator implementations that are based on the AY-3-8190 instead treat it as a 4-bit signal with a 3dB per step curve. Since the only extant 5B game does not use the envelope, the difference is unimportant unless accuracy is desired for homebrew 5B work.

The three output channels are mixed together linearly. The output is mixed with the 2A03 and amplified. It is very loud compared to other audio expansion carts.

The amplifier becomes nonlinear at higher amplitudes, and includes some filtering.[5]

References

  1. ↑ Period 0 verification for tone/noise
  2. ↑ Period 0 verification for envelope
  3. ↑ LFSR verification
  4. ↑ Envelope phase reset verification
  5. ↑ Sunsoft 5B amplifier investigation (ongoing)
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