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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]
| Address | Name | Description | Mask |
|---|---|---|---|
| $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 |
| $0808 | LFO Frequency | (global) | $FF |
| $0809 | LFO Control | bit 7 = LFO trigger (?)
bit 0&1 = LFO Control (?) |
$83 |
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. [/code]