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Commented and fixed mistake that would equate any noise divider > 32768 with 0.

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This commit is contained in:
Thomas Harte 2016-10-19 21:51:35 -04:00
parent 101f168ea4
commit 6073906c39
2 changed files with 20 additions and 5 deletions
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23
Components/AY38910/AY38910.cpp
View File

@ -74,15 +74,22 @@ void AY38910::get_samples(unsigned int number_of_samples, int16_t *target)
{
for(int c = 0; c < number_of_samples; c++)
{
// a master divider divides the clock by 16
// a master divider divides the clock by 16;
// resulting_steps will be 1 if a tick occurred, 0 otherwise
int former_master_divider = _master_divider;
_master_divider++;
int resulting_steps = ((_master_divider ^ former_master_divider) >> 4) & 1;
// Bluffer's guide to the stuff below: I wanted to avoid branches. If I avoid branches then
// I avoid stalls.
//
// Repeating patterns are:
// (1) decrement, then shift a high-order bit right and mask to get 1 for did underflow, 0 otherwise;
// (2) did_underflow * a + (did_underflow ^ 1) * b to pick between reloading and not reloading
int did_underflow;
#define shift(x, r) \
x -= resulting_steps; \
did_underflow = (x >> 15)&1; \
did_underflow = (x >> 16)&1; \
x = did_underflow * r + (did_underflow^1) * x;
#define step_channel(c) \
@ -94,13 +101,15 @@ void AY38910::get_samples(unsigned int number_of_samples, int16_t *target)
step_channel(1);
step_channel(2);
// ... the noise generator
// ... the noise generator. This recomputes the new bit repeatedly but harmlessly, only shifting
// it into the official 17 upon divider underflow.
shift(_noise_divider, _output_registers[6]&0x1f);
_noise_output ^= did_underflow&_noise_shift_register&1;
_noise_shift_register |= ((_noise_shift_register ^ (_noise_shift_register >> 3))&1) << 17;
_noise_shift_register >>= did_underflow;
// ... and the envelope generator
// ... and the envelope generator. Table based for pattern lookup, with a 'refill' step — a way of
// implementing non-repeating patterns by locking them to table position 0x1f.
shift(_envelope_divider, _envelope_period);
_envelope_position += did_underflow;
int refill = _envelope_overflow_masks[_output_registers[13]] * (_envelope_position >> 5);
@ -110,6 +119,10 @@ void AY38910::get_samples(unsigned int number_of_samples, int16_t *target)
#undef step_channel
#undef shift
// The output level for a channel is:
// 1 if neither tone nor noise is enabled;
// 0 if either tone or noise is enabled and its value is low.
// (which is implemented here with reverse logic, assuming _channel_output and _noise_output are already inverted)
#define level(c, tb, nb) \
(((((_output_registers[7] >> tb)&1)^1) & _channel_output[c]) | ((((_output_registers[7] >> nb)&1)^1) & _noise_output)) ^ 1
@ -120,6 +133,7 @@ void AY38910::get_samples(unsigned int number_of_samples, int16_t *target)
};
#undef level
// Channel volume is a simple selection: if the bit at 0x10 is set, use the envelope volume; otherwise use the lower four bits
#define channel_volume(c) \
((_output_registers[c] >> 4)&1) * envelope_volume + (((_output_registers[c] >> 4)&1)^1) * (_output_registers[c]&0x1f)
@ -130,6 +144,7 @@ void AY38910::get_samples(unsigned int number_of_samples, int16_t *target)
};
#undef channel_volume
// Mix additively. TODO: non-linear volume.
target[c] = (int16_t)((
volumes[0] * channel_levels[0] +
volumes[1] * channel_levels[1] +

2
Components/AY38910/AY38910.hpp
View File

@ -36,7 +36,7 @@ class AY38910: public ::Outputs::Filter<AY38910> {
int _selected_register;
uint8_t _registers[16], _output_registers[16];
uint16_t _tone_generator_controls[3];
int _tone_generator_controls[3];
int _channel_dividers[3];
int _channel_output[3];