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Attempt to use the most-significant relevant bits for sample position.

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This commit is contained in:
Thomas Harte 2020-11-19 22:13:09 -05:00
parent 4d9372c52f
commit 3512352c32
1 changed files with 6 additions and 2 deletions
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8
Machines/Apple/AppleIIgs/Sound.cpp
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@ -233,13 +233,17 @@ void GLU::generate_audio(size_t number_of_samples, std::int16_t *target, int16_t
} }
uint8_t GLU::EnsoniqState::Oscillator::sample(uint8_t *ram) { uint8_t GLU::EnsoniqState::Oscillator::sample(uint8_t *ram) {
// Determines how many you'd have to shift a 16-bit pointer to the right for,
// in order to hit only the position-supplied bits.
const int pointer_shift = 8 - ((table_size >> 3) & 7);
// Table size mask should be 0x8000 for the largest table size, and 0xff00 for // Table size mask should be 0x8000 for the largest table size, and 0xff00 for
// the smallest. // the smallest.
const uint16_t table_size_mask = 0xffff >> (8 - ((table_size >> 3) & 7)); const uint16_t table_size_mask = 0xffff >> pointer_shift;
// The pointer should use (at most) 15 bits; starting with bit 1 for resolution 0 // The pointer should use (at most) 15 bits; starting with bit 1 for resolution 0
// and starting at bit 8 for resolution 7. // and starting at bit 8 for resolution 7.
const uint16_t table_pointer = uint16_t(position >> (1 + table_size&7)); const uint16_t table_pointer = uint16_t(position >> ((table_size&7) + pointer_shift));
// The full pointer is composed of the bits of the programmed address not touched by // The full pointer is composed of the bits of the programmed address not touched by
// the table pointer, plus the table pointer. // the table pointer, plus the table pointer.