Improve audio anti-aliasing, allow 2MHz output

Core/apu.c

@@ -211,7 +211,7 @@ static void render(GB_gameboy_t *gb)
             }
         }
 
-        if (likely(gb->apu_output.last_update[i] == 0)) {
+        if (likely(gb->apu_output.last_update[i] == 0 || gb->apu_output.cycles_since_render == 0)) {
             output.left += gb->apu_output.current_sample[i].left * multiplier;
             output.right += gb->apu_output.current_sample[i].right * multiplier;
         }
@@ -633,11 +633,14 @@ static void step_lfsr(GB_gameboy_t *gb, unsigned cycles_offset)
 
 void GB_apu_run(GB_gameboy_t *gb, bool force)
 {
-    uint32_t clock_rate = GB_get_clock_rate(gb) * 2;    
+    uint32_t clock_rate = GB_get_clock_rate(gb);
+    bool orig_force = force;
+    
+restart:;
     uint16_t cycles = gb->apu.apu_cycles;
 
     if (force ||
-        (gb->apu.apu_cycles > 0x400) ||
+        (cycles + gb->apu_output.cycles_since_render >= gb->apu_output.max_cycles_per_sample) ||
         (gb->apu_output.sample_cycles >= clock_rate) ||
         (gb->apu.square_sweep_calculate_countdown || gb->apu.channel_1_restart_hold || gb->apu.square_sweep_calculate_countdown_reload_timer) ||
         (gb->model <= GB_MODEL_CGB_E && (gb->apu.wave_channel.bugged_read_countdown || (gb->apu.wave_channel.enable && gb->apu.wave_channel.pulsed)))) {
@@ -646,9 +649,46 @@ void GB_apu_run(GB_gameboy_t *gb, bool force)
     if (!force) {
         return;
     }
+    
+    /* Force renders to never be more than max_cycles_per_sample apart by spliting runs. */
+    while (cycles + gb->apu_output.cycles_since_render > gb->apu_output.max_cycles_per_sample) {
+        /* We're already past max_cycles_per_sample. This can happen when changing clock rates, etc.
+           Let this sample render normally. */
+        if (unlikely(gb->apu_output.cycles_since_render > gb->apu_output.max_cycles_per_sample)) break;
+        
+        gb->apu.apu_cycles = gb->apu_output.max_cycles_per_sample - gb->apu_output.cycles_since_render;
+        
+        if (gb->apu.apu_cycles) {
+            // Run for just enough cycles to reach max_cycles_per_sample
+            cycles -= gb->apu.apu_cycles;
+            GB_apu_run(gb, true);
+            // Re-evaluate force if needed
+            if (!orig_force) {
+                force = false;
+                gb->apu.apu_cycles = cycles;
+                goto restart;
+            }
+            // Check if we need another batch
+            continue;
+        }
+        
+        // Render if needed
+        if (gb->apu_output.sample_cycles >= clock_rate) {
+            gb->apu_output.sample_cycles -= clock_rate;
+            render(gb);
+        }
+        break;
+    }
 
     gb->apu.apu_cycles = 0;
-    if (!cycles) return;
+    if (!cycles) {
+        /* This can happen in pre-CGB stop mode */
+        while (unlikely(gb->apu_output.sample_cycles >= clock_rate)) {
+            gb->apu_output.sample_cycles -= clock_rate;
+            render(gb);
+        }
+        return;
+    }
     
     if (unlikely(gb->apu.wave_channel.bugged_read_countdown)) {
         uint16_t cycles_left = cycles;
@@ -1633,6 +1673,10 @@ void GB_set_sample_rate(GB_gameboy_t *gb, unsigned sample_rate)
     gb->apu_output.sample_rate = sample_rate;
     if (sample_rate) {
         gb->apu_output.highpass_rate = pow(0.999958,  GB_get_clock_rate(gb) / (double)sample_rate);
+        gb->apu_output.max_cycles_per_sample = ceil(GB_get_clock_rate(gb) / 2.0 / sample_rate);
+    }
+    else {
+        gb->apu_output.max_cycles_per_sample = 0x400;
     }
 }
 
@@ -1645,6 +1689,7 @@ void GB_set_sample_rate_by_clocks(GB_gameboy_t *gb, double cycles_per_sample)
     }
     gb->apu_output.sample_rate = GB_get_clock_rate(gb) / cycles_per_sample * 2;
     gb->apu_output.highpass_rate = pow(0.999958, cycles_per_sample);
+    gb->apu_output.max_cycles_per_sample = ceil(cycles_per_sample / 4);
 }
 
 unsigned GB_get_sample_rate(GB_gameboy_t *gb)

Core/apu.h

@@ -169,6 +169,7 @@ typedef struct {
     unsigned sample_rate;
 
     unsigned sample_cycles; // Counts by sample_rate until it reaches the clock frequency
+    unsigned max_cycles_per_sample;
 
     // Samples are NOT normalized to MAX_CH_AMP * 4 at this stage!
     unsigned cycles_since_render;

Core/gb.c

@@ -171,6 +171,7 @@ GB_gameboy_t *GB_init(GB_gameboy_t *gb, GB_model_t model)
 #endif
     gb->cartridge_type = &GB_cart_defs[0]; // Default cartridge type
     gb->clock_multiplier = 1.0;
+    gb->apu_output.max_cycles_per_sample = 0x400;
     
     if (model & GB_MODEL_NO_SFC_BIT) {
         /* Disable time syncing. Timing should be done by the SFC emulator. */

Core/timing.c

@@ -254,6 +254,7 @@ static void timers_run(GB_gameboy_t *gb, uint8_t cycles)
         if (GB_is_cgb(gb)) {
             gb->apu.apu_cycles += 1 << !gb->cgb_double_speed;
         }
+        gb->apu_output.sample_cycles += (gb->apu_output.sample_rate << !gb->cgb_double_speed) << 1;
         return;
     }
     
@@ -267,6 +268,7 @@ static void timers_run(GB_gameboy_t *gb, uint8_t cycles)
         advance_tima_state_machine(gb);
         GB_set_internal_div_counter(gb, gb->div_counter + 4);
         gb->apu.apu_cycles += 1 << !gb->cgb_double_speed;
+        gb->apu_output.sample_cycles += (gb->apu_output.sample_rate << !gb->cgb_double_speed) << 1;
         GB_SLEEP(gb, div, 2, 4);
     }
 }
@@ -476,7 +478,6 @@ void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
     if (likely(gb->io_registers[GB_IO_LCDC] & GB_LCDC_ENABLE)) {
         gb->double_speed_alignment += cycles;
     }
-    gb->apu_output.sample_cycles += cycles * gb->apu_output.sample_rate;
     gb->cycles_since_last_sync += cycles;
     gb->cycles_since_run += cycles;