// ----------------------------------------------------------------------------
// Set chip model.
// ----------------------------------------------------------------------------
public void set_chip_model(siddefs.chip_model model)//chip_model model)
{
wave.set_chip_model(model);
envelope.set_chip_model(model);
if (model == siddefs.chip_model.MOS6581)
{
// The waveform D/A converter introduces a DC offset in the signal
// to the envelope multiplying D/A converter. The "zero" level of
// the waveform D/A converter can be found as follows:
//
// Measure the "zero" voltage of voice 3 on the SID audio output
// pin, routing only voice 3 to the mixer ($d417 = $0b, $d418 =
// $0f, all other registers zeroed).
//
// Then set the sustain level for voice 3 to maximum and search for
// the waveform output value yielding the same voltage as found
// above. This is done by trying out different waveform output
// values until the correct value is found, e.g. with the following
// program:
//
// lda #$08
// sta $d412
// lda #$0b
// sta $d417
// lda #$0f
// sta $d418
// lda #$f0
// sta $d414
// lda #$21
// sta $d412
// lda #$01
// sta $d40e
//
// ldx #$00
// lda #$38 ; Tweak this to find the "zero" level
//l cmp $d41b
// bne l
// stx $d40e ; Stop frequency counter - freeze waveform output
// brk
//
// The waveform output range is 0x000 to 0xfff, so the "zero"
// level should ideally have been 0x800. In the measured chip, the
// waveform output "zero" level was found to be 0x380 (i.e. $d41b
// = 0x38) at an audio output voltage of 5.94V.
//
// With knowledge of the mixer op-amp characteristics, further estimates
// of waveform voltages can be obtained by sampling the EXT IN pin.
// From EXT IN samples, the corresponding waveform output can be found by
// using the model for the mixer.
//
// Such measurements have been done on a chip marked MOS 6581R4AR
// 0687 14, and the following results have been obtained:
// * The full range of one voice is approximately 1.5V.
// * The "zero" level rides at approximately 5.0V.
//
wave_zero = 0x380;
}
else
{
// No DC offsets in the MOS8580.
wave_zero = 0x800;
}
}
