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Substitutes a more efficient inner loop for audio generation.

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This commit is contained in:
Thomas Harte 2019-07-17 14:54:06 -04:00
parent 22ee51c12c
commit b9a34bee51
2 changed files with 29 additions and 9 deletions
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38
Machines/Apple/Macintosh/Audio.cpp
View File

@ -12,7 +12,8 @@ using namespace Apple::Macintosh;
namespace {
// The sample_length is coupled with the clock rate selected within the Macintosh proper.
// The sample_length is coupled with the clock rate selected within the Macintosh proper;
// as per the header-declaration a divide-by-two clock is expected to arrive here.
const std::size_t sample_length = 352 / 2;
}
@ -36,6 +37,7 @@ void Audio::set_volume(int volume) {
// Post the volume change as a deferred event.
task_queue_.defer([=] () {
volume_ = volume;
set_volume_multiplier();
});
}
@ -47,6 +49,7 @@ void Audio::set_enabled(bool on) {
// Post the enabled mask change as a deferred event.
task_queue_.defer([=] () {
enabled_mask_ = int(on);
set_volume_multiplier();
});
}
@ -58,7 +61,12 @@ bool Audio::is_zero_level() {
void Audio::set_sample_volume_range(std::int16_t range) {
// Some underflow here doesn't really matter.
volume_multiplier_ = range / (7 * 255);
output_volume_ = range / (7 * 255);
set_volume_multiplier();
}
void Audio::set_volume_multiplier() {
volume_multiplier_ = int16_t(output_volume_ * volume_ * enabled_mask_);
}
void Audio::get_samples(std::size_t number_of_samples, int16_t *target) {
@ -66,14 +74,24 @@ void Audio::get_samples(std::size_t number_of_samples, int16_t *target) {
// in fact it uses pulse-width modulation. But the scale for pulses isn't specified, so
// that's something to return to.
// TODO: temporary implementation. Very inefficient. Replace.
for(std::size_t sample = 0; sample < number_of_samples; ++sample) {
target[sample] = volume_multiplier_ * int16_t(sample_queue_.buffer[sample_queue_.read_pointer] * volume_ * enabled_mask_);
++subcycle_offset_;
while(number_of_samples) {
// Determine how many output samples will be at the same level.
const auto cycles_left_in_sample = std::min(number_of_samples, sample_length - subcycle_offset_);
if(subcycle_offset_ == sample_length) {
subcycle_offset_ = 0;
sample_queue_.read_pointer = (sample_queue_.read_pointer + 1) % sample_queue_.buffer.size();
}
// Determine the output level, and output that many samples.
// (Hoping that the copiler substitutes an effective memset16-type operation here).
const int16_t output_level = volume_multiplier_ * (int16_t(sample_queue_.buffer[sample_queue_.read_pointer]) - 128);
for(size_t c = 0; c < cycles_left_in_sample; ++c) {
target[c] = output_level;
}
target += cycles_left_in_sample;
// Advance the sample pointer.
subcycle_offset_ += cycles_left_in_sample;
sample_queue_.read_pointer = (sample_queue_.read_pointer + (subcycle_offset_ / sample_length)) % sample_queue_.buffer.size();
subcycle_offset_ %= sample_length;
// Decreate the number of samples left to write.
number_of_samples -= cycles_left_in_sample;
}
}

2
Machines/Apple/Macintosh/Audio.hpp
View File

@ -75,9 +75,11 @@ class Audio: public ::Outputs::Speaker::SampleSource {
// thread only.
int volume_ = 0;
int enabled_mask_ = 0;
std::int16_t output_volume_ = 0;
std::int16_t volume_multiplier_ = 0;
std::size_t subcycle_offset_ = 0;
void set_volume_multiplier();
};
}