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T6W28: fix noise emulation

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thanks to Burnt Fishy and The Beesh-Spweesh! for information
This commit is contained in:
tildearrow 2025-08-19 20:58:08 -05:00
parent 3b37a5334a
commit ee64dd6a16
3 changed files with 13 additions and 9 deletions
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18
src/engine/platform/sound/t6w28/T6W28_Apu.cpp
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@ -145,8 +145,8 @@ static const int noise_periods [3] = { 0x100, 0x200, 0x400 };
void T6W28_Noise::reset()
{
period = &noise_periods [0];
shifter = 0x4000;
tap = 13;
shifter = 0xfffe;
tap = 12;
period_extra = 0;
T6W28_Osc::reset();
}
@ -201,9 +201,13 @@ void T6W28_Noise::run( sms_time_t time, sms_time_t end_time )
do
{
int changed = (l_shifter + 1) & 2; // set if prev and next bits differ
l_shifter = (((l_shifter << 14) ^ (l_shifter << tap)) & 0x4000) | (l_shifter >> 1);
if ( changed )
int prev_l_shifter=l_shifter;
if (tap==16) {
l_shifter = ((l_shifter >> 14) & 1) | (l_shifter << 1);
} else {
l_shifter = ((((l_shifter >> 15)&1) ^ ((l_shifter >> 12)&1)) & 1) | (l_shifter << 1);
}
if ( (prev_l_shifter^l_shifter)&1 )
{
delta_left = -delta_left;
blip_add_delta( output_left, time, delta_left );
@ -370,8 +374,8 @@ void T6W28_Apu::write_data_right( sms_time_t time, int data )
noise.period = &noise.period_extra;
int const tap_disabled = 16;
noise.tap = (data & 0x04) ? 13 : tap_disabled;
noise.shifter = 0x4000;
noise.tap = (data & 0x04) ? 12 : tap_disabled;
noise.shifter = 0xfffe;
}
}