Sunsoft 5B audio

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!

Registers

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

Audio Register Select ($C000)

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

Audio Register Write ($E000)

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

Internal audio registers

The AY-3-8910 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 enable on channels C/B/A, Tone enable 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	Envelope high period
$0D	---- CAaH	Envelope 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 used by any of the three channels.

Tone

TODO frequency formula

Noise

TODO noise frequency / generator

Envelope

TODO envelope period formula

The envelope 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 4-bit signal which is then converted to analog with a logarithmic DAC.

TODO details of log conversion, and final mixing.