View source for PSG (TG-16)

(the following is an excerpt from the research notes of David Shadoff, for TGHack)

== Register Map ==

Note: all addresses are segment $FF-relative.

[code]
{| class="wikitable"
|-
|$0800
|Voice Select
|select voice #
|$07 (only 3 bits used)
|-
|$0801
|Main Volume
|

Most sig.  4 bits = left channel

Least sig. 4 bits = right channel

not a voice-dependent register
|$FF
|-
|$0802
|Frequency (low)
|Least sig. 8 bits of 12-bit frequency counter, for voice selected by 'Voice Select'
|$FF
|-
|$0803
|Frequency (high)
|Most sig. 4 bits of 12-bit frequency counter, for voice selected by 'Voice Select'
|$0F
|-
|$0804
|Channel on/dda/volume
|
bit 7   = voice enabled

bit 6   = 'dda' - 'Direct digital-to-analogue'(?)  With this flag set, any value written into the 'wave data' location will directly appear on the channel's output, rather than be stored in the circular wave buffer

bit 0-4 = voice volume

voice-dependent register
|$CF
|-
|$0805
|Pan volume ('balance')
|

Most sig.  4 bits = left channel

Least sig. 4 bits = right channel

voice-dependent register
|$FF
|-
|$0806
|Wave data
|Samples are inserted into a 32-address circular buffer when this address is written to.  Only 5 data bits are used.  (voice-dependent register)
|$1F
|-
|$0807
|Noise
|(available only to voices 5 & 6)

bit 7   = noise enable

bit 0-4 = noise frequency (for more details, see patent)

voice-dependent register
|$9F
|}

The LFO is not a per-voice attribute.
Unfortunately, I don't know much else about it, or understand how these
affect sound:

{| class="wikitable"
|-
!Address
!Name
!Mask
!Description
|-
|$0808
|LFO Frequency
|$FF
|
|-
|$0809
|LFO Control
|$83
|bit 7   = LFO trigger (?)
bit 0&1 = LFO Control (?)
|}      

About the 'frequency' values:

First, I discovered that the values are inverse - a higher value means a
lower tone.  Second, I had to use the value $1b4 to get a 256Hz tone from
the PC-Engine.  From this, I worked out that the sound chip has a base
clock of 3.58MHz (common in these systems, since this is the NTSC colorburst
frequency), and uses the 'frequency' value as a down-counter (or divider).
Once the value reaches 0, a 'step' is performed.  In this case, the 'step'
means advancing to the next sample in the 5-bit, 32-sample waveform for that
voice.

So, 3.58Mhz / $1b4 (436 decimal) / 256Hz = 32 samples/cycle

This all starts to make sense when you realize that the PCE put the sound
generator inside the CPU -- it's all simple digital stuff, up until the
D/A output.
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