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improve handling of freqencies above sample rate
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@baines
baines committed May 14, 2017
1 parent df3865f commit a35caac
Showing 1 changed file with 64 additions and 56 deletions.
120 changes: 64 additions & 56 deletions audio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -49,7 +49,7 @@ static struct chan {

// square
int duty;
float fval;
int duty_counter;

// noise
uint16_t lfsr_reg;
Expand All @@ -58,7 +58,7 @@ static struct chan {

// wave
int sample_cursor;
float sample;
uint8_t sample;
} chans[4];

#define FREQ 44100.0f
Expand All @@ -69,7 +69,7 @@ static size_t nsamples;
static float* samples;

static SDL_AudioDeviceID audio;
static const float duty[] = { 0.125, 0.25, 0.5, 0.75 };
static const int duty_lookup[] = { 1, 2, 4, 6 };
static float logbase;
static float vol_l, vol_r;
static const char* notes[] = {
Expand Down Expand Up @@ -119,7 +119,6 @@ void update_env(struct chan* c){
}

void update_len(struct chan* c){

if(c->len.enabled){
c->len.counter += c->len.inc;
if(c->len.counter > 1.0f){
Expand All @@ -129,16 +128,18 @@ void update_len(struct chan* c){
}
}

int update_freq(struct chan* c){
c->freq_counter += c->freq_inc;
bool update_freq(struct chan* c, float* pos){
float inc = c->freq_inc - *pos;
c->freq_counter += inc;

int result = 0;
while(c->freq_counter > 1.0f){
c->freq_counter -= 1.0f;
result++;
if(c->freq_counter > 1.0f){
*pos = c->freq_inc - (c->freq_counter - 1.0f);
c->freq_counter = 0.0f;
return true;
} else {
*pos = c->freq_inc;
return false;
}

return result;
}

void update_sweep(struct chan* c){
Expand All @@ -152,23 +153,21 @@ void update_sweep(struct chan* c){
c->freq += inc;
c->sweep.freq = c->freq;

set_note_freq(c, 4194304 / (float)((2048 - c->freq) << 5));
set_note_freq(c, 4194304.0f / (float)((2048 - c->freq) << 5));
c->freq_inc *= 8.0f;
} else {
c->enabled = 0;
}
c->sweep.counter -= 1.0f;
}
}

float lerp(float a, float b, float t){
return a + (b - a) * t;
}

void update_square(bool ch2){
struct chan* c = chans + ch2;
if(!c->powered) return;

set_note_freq(c, 4194304.0f / (float)((2048 - c->freq) << 5));
c->freq_inc *= 8.0f;

for(int i = 0; i < nsamples; i+=2){
update_len(c);
Expand All @@ -177,16 +176,21 @@ void update_square(bool ch2){
update_env(c);
if(!ch2) update_sweep(c);

float d;
if(update_freq(c)){
c->fval = (2*c->freq_counter - c->freq_inc) / c->freq_inc;
} else if((d = c->freq_counter - duty[c->duty]) > 0.0f){
c->fval = MAX(-1.0f, (c->freq_inc - 2*d) / c->freq_inc);
} else {
c->fval = 1.0f;
}
float pos = 0.0f;
float prev_pos = 0.0f;
float sample = 0.0f;

float sample = hipass(c, c->fval * (c->volume / 15.0f));
while(update_freq(c, &pos)){
c->duty_counter = (c->duty_counter + 1) & 7;
sample += ((pos - prev_pos) / c->freq_inc) * (float)c->val;

if(!c->duty_counter || c->duty_counter == c->duty){
c->val = ~(c->val-1);
}
prev_pos = pos;
}
sample += ((pos - prev_pos) / c->freq_inc) * (float)c->val;
sample = hipass(c, sample * (c->volume / 15.0f));

if(!c->user_mute){
samples[i+0] += sample * 0.25f * c->on_left * vol_l;
Expand All @@ -200,34 +204,41 @@ void update_wave(void){
struct chan* c = chans + 2;
if(!c->powered) return;

set_note_freq(c, 4194304 / (float)((2048 - c->freq) << 5));
c->freq_inc *= 16.0f;
float freq = 4194304.0f / (float)((2048 - c->freq) << 5);
set_note_freq(c, freq);

if(freq >= 131079) c->enabled = false;
else c->freq_inc *= 16.0f;

for(int i = 0; i < nsamples; i+=2){
update_len(c);

if(c->enabled){
float t = 1.0f;
float pos = 0.0f;
float prev_pos = 0.0f;
float sample = 0.0f;

if(update_freq(c)){
while(update_freq(c, &pos)){
c->sample_cursor = c->val;
c->val = (c->val + 1) & 31;
t = c->freq_counter / c->freq_inc;
}

uint8_t s = mem[0xFF30 + c->sample_cursor / 2];
if(c->sample_cursor & 1){
s &= 0xF;
} else {
s >>= 4;
c->sample = mem[0xFF30 + c->sample_cursor / 2];
if(c->sample_cursor & 1){
c->sample &= 0xF;
} else {
c->sample >>= 4;
}

c->sample = (c->volume) ? (c->sample >> (c->volume-1)) : 0;
sample += ((pos - prev_pos) / c->freq_inc) * c->sample;
prev_pos = pos;
}
sample += ((pos - prev_pos) / c->freq_inc) * c->sample;

if(c->volume > 0){
s >>= (c->volume - 1);
float diff = (float[]){ 7.5f, 3.75f, 1.5f }[c->volume - 1];
c->sample = lerp(c->sample, (float)s, t);
sample = hipass(c, (sample - diff) / 7.5f);

float sample = hipass(c, (c->sample - diff) / 7.5f);
if(!c->user_mute){
samples[i+0] += sample * 0.25f * c->on_left * vol_l;
samples[i+1] += sample * 0.25f * c->on_right * vol_r;
Expand All @@ -250,24 +261,24 @@ void update_noise(void){
if(c->enabled){
update_env(c);

float sample = c->val;
int count = update_freq(c);
for(int j = 0; j < count; ++j){
float pos = 0.0f;
float prev_pos = 0.0f;
float sample = 0.0f;

while(update_freq(c, &pos)){
c->lfsr_reg = (c->lfsr_reg << 1) | (c->val == 1);

if(c->lfsr_wide){
c->val = !(((c->lfsr_reg >> 14) & 1) ^ ((c->lfsr_reg >> 13) & 1)) ? 1 : -1;
} else {
c->val = !(((c->lfsr_reg >> 6 ) & 1) ^ ((c->lfsr_reg >> 5 ) & 1)) ? 1 : -1;
}
sample += c->val;
sample += ((pos - prev_pos) / c->freq_inc) * c->val;
prev_pos = pos;
}

if(count){
sample /= (float)count;
}

sample += ((pos - prev_pos) / c->freq_inc) * c->val;
sample = hipass(c, sample * (c->volume / 15.0f));

if(!c->user_mute){
samples[i+0] += sample * 0.25f * c->on_left * vol_l;
samples[i+1] += sample * 0.25f * c->on_right * vol_r;
Expand Down Expand Up @@ -330,6 +341,7 @@ void audio_reset(void){
memset(chans, 0, sizeof(chans));
memset(samples, 0, nsamples * sizeof(float));
SDL_ClearQueuedAudio(audio);
chans[0].val = chans[1].val = 1;
}

void audio_init(void){
Expand Down Expand Up @@ -420,18 +432,14 @@ void chan_trigger(int i){

if(i == 2){ // wave
len_max = 256;
c->val = 0;
} else if(i == 3){ // noise
c->lfsr_reg = 0xFFFF;
c->val = -1;
}

c->len.inc = (256.0f / (float)(len_max - c->len.load)) / FREQ;
c->len.counter = 0.0f;

if(i < 2){
c->val = 1;
} else {
c->val = 0;
}
}

void audio_write(uint16_t addr, uint8_t val){
Expand Down Expand Up @@ -479,7 +487,7 @@ void audio_write(uint16_t addr, uint8_t val){
case 0xFF16:
case 0xFF20:
chans[i].len.load = val & 0x3f;
chans[i].duty = val >> 6;
chans[i].duty = duty_lookup[val >> 6];
break;

case 0xFF1B:
Expand Down

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