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Generate volume ramp.

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This commit is contained in:
Thomas Harte 2024-04-13 20:23:47 -04:00
parent 88bb16f261
commit 779794632e
1 changed files with 32 additions and 1 deletions
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33
Machines/Acorn/Archimedes/Sound.hpp
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@ -11,13 +11,44 @@
#include "../../../Concurrency/AsyncTaskQueue.hpp"
#include "../../../Outputs/Speaker/Implementation/LowpassSpeaker.hpp"
#include <array>
#include <cstdint>
namespace Archimedes {
// Generate lookup table for sound output levels, and hold it only once regardless
// of how many template instantiations there are of @c Sound.
static constexpr std::array<int16_t, 256> generate_levels() {
std::array<int16_t, 256> result{};
// There are 8 segments of 16 steps; each segment is a linear
// interpolation from its start level to its end level and
// each level is double the previous.
//
// Bit 7 provides a sign.
for(size_t c = 0; c < 256; c++) {
const bool is_negative = c & 128;
const auto point = static_cast<int>(c & 0xf);
const auto chord = static_cast<int>((c >> 4) & 7);
const int start = (1 << chord) - 1;
const int end = (chord == 7) ? 247 : ((start << 1) + 1);
const int level = start * (16 - point) + end * point;
result[c] = static_cast<int16_t>((level * 32767) / 3832);
if(is_negative) result[c] = -result[c];
}
return result;
}
struct SoundLevels {
static constexpr auto levels = generate_levels();
};
/// Models the Archimedes sound output; in a real machine this is a joint efort between the VIDC and the MEMC.
template <typename InterruptObserverT>
struct Sound {
struct Sound: private SoundLevels {
Sound(InterruptObserverT &observer, const uint8_t *ram) : ram_(ram), observer_(observer) {
speaker_.set_input_rate(1'000'000);
speaker_.set_high_frequency_cutoff(2'200.0f);