atari2600/source/tia-audio.cpp

#include "tia-audio.h"

namespace tia
{
    namespace audio
    {
        static const uint16_t BIT_PATTERN_15 = 0x7556;
        static const uint32_t BIT_PATTERN_31 = 0x74567254;
        static const uint8_t LFSR511_TABLE[511] = {
            1,0,0,0,0,0,0,0,0,1,0,0,0,1,0,0,0,0,1,1,0,0,0,1,1,1,0,0,1,0,1,
            0,0,0,1,0,1,1,0,0,1,1,0,1,0,0,1,1,1,1,0,0,0,0,0,1,0,1,0,1,0,0,
            1,0,0,1,0,0,0,1,0,0,1,1,0,0,0,1,0,1,1,1,0,0,1,1,1,0,1,0,1,1,0,
            1,0,0,0,1,1,0,1,1,0,0,0,1,1,0,0,1,0,0,1,0,1,1,1,1,1,0,0,0,1,0,
            0,0,1,1,0,1,1,1,1,0,1,0,1,0,0,0,0,1,1,0,0,1,1,0,0,1,1,1,0,0,0,
            1,0,0,0,0,1,0,0,1,0,1,0,1,1,1,1,1,1,1,1,0,0,0,0,1,1,1,1,1,0,1,
            1,1,0,1,0,0,0,1,0,1,0,0,1,1,1,0,1,1,1,1,1,0,1,1,1,0,0,0,0,0,0,
            1,0,0,1,1,1,1,1,1,0,1,1,0,1,1,1,0,0,1,0,0,0,0,1,1,1,0,1,1,0,1,
            1,1,0,0,1,0,1,1,0,1,1,0,0,1,1,1,1,1,0,0,1,1,0,1,0,1,0,1,0,0,0,
            1,0,1,1,1,1,0,0,1,0,1,0,0,0,1,0,1,1,0,1,0,1,1,1,0,1,1,0,0,0,0,
            1,0,1,1,0,0,1,0,1,0,1,1,1,1,0,1,1,0,1,0,0,1,0,1,0,0,1,0,1,1,0,
            0,0,0,0,1,0,0,0,1,1,1,1,1,1,0,1,0,0,0,0,0,1,1,1,0,0,0,1,0,1,0,
            1,1,0,1,1,0,1,0,1,0,0,1,0,0,1,1,0,1,0,0,0,0,0,0,0,1,1,0,1,1,1,
            0,1,1,0,1,0,0,1,1,0,0,1,0,1,0,0,0,1,1,1,1,0,1,1,1,1,0,0,1,1,1,
            1,0,1,0,0,1,1,1,0,0,1,0,0,0,0,0,1,0,1,1,0,1,0,0,1,0,0,0,1,1,0,
            1,0,1,1,0,1,1,1,1,1,1,0,1,0,1,0,1,0,1,1,0,0,1,1,0,1,1,1,0,1,0,
            1,0,0,0,0,0,1,1
        };

        uint8_t get_lfsr511_bit(uint32_t step)
        {
            return LFSR511_TABLE[step % 511];
        }

        uint8_t get_poly_mode_bit(uint8_t audc, uint32_t step)
        {
            switch (audc & 0x0F) {
                case 0:  
                case 11: 
                    return 1; // Direct volume (PCM output)

                case 1: 
                    // 4-bit Poly (15 steps)
                    return (BIT_PATTERN_15 >> (step % 15)) & 1;

                case 2: 
                    // 5-bit Poly -> 4-bit Poly divisor (31 steps)
                    return (BIT_PATTERN_31 >> (step % 31)) & 1;

                case 3: {
                    // 5-bit Poly XOR 4-bit Poly (465 steps)
                    uint8_t b5 = (BIT_PATTERN_31 >> (step % 31)) & 1;
                    uint8_t b4 = (BIT_PATTERN_15 >> (step % 15)) & 1;
                    return b5 ^ b4;
                }

                case 4: 
                case 5: 
                    // Pure Tone: Divide frequency clock by 2
                    return (step & 1);

                case 6: 
                case 10: 
                    // Divide by 31 then divide by 2
                    return ((step / 31) & 1);

                case 7: 
                case 9: 
                    // 5-bit Poly output variant
                    return ((BIT_PATTERN_31 >> ((step / 2) % 31)) & 1);

                case 8: 
                    // White Noise (9-bit Poly / 511 steps)
                    return get_lfsr511_bit(step);

                case 12: 
                case 13: 
                    // Pure Tone: Divide frequency clock by 6
                    return ((step / 3) & 1);

                case 14: 
                    // Divide by 93 (31 * 3) then divide by 2
                    return ((step / 93) & 1);

                case 15: 
                    // 5-bit Poly clocked down by 6
                    return ((BIT_PATTERN_31 >> ((step / 6) % 31)) & 1);

                default:
                    return 0;
            }
        }

        struct channel
        {
            uint8_t control = 0;
            uint8_t frequency = 0;
            uint8_t volume = 0;
            uint8_t freq_counter = 0;
            uint32_t poly_counter = 0;

        };

        channel channels[2];

        void tick()
        {
            for (int i=0; i<2; ++i) {
                if (channels[i].freq_counter == 0) {
                    channels[i].freq_counter = channels[i].frequency + 1;
                    channels[i].poly_counter++;
                } else {
                    channels[i].freq_counter--;
                }
            }
        }

        void write(uint16_t address, uint8_t value)
        {
            switch (address) {
                case 0x15: // AUDC0
                    channels[0].control = value & 0x0f; break;
                case 0x16: // AUDC1
                    channels[1].control = value & 0x0f; break;
                case 0x17: // AUDF0
                    channels[0].frequency = value & 0x1f; break;
                case 0x18: // AUDF1
                    channels[1].frequency = value & 0x1f; break;
                case 0x19: // AUDV0
                    channels[0].volume = value & 0x0f; break;
                case 0x1A: // AUDV1
                    channels[1].volume = value & 0x0f; break;
            }
        }

        uint8_t get_sample0() { return get_sample(0); }
        uint8_t get_sample1() { return get_sample(1); }

        uint8_t get_sample(uint8_t channel)
        {
            if (channels[channel].volume == 0) return 0;

            bool bit_out = false;

            switch (channels[channel].control) {
                case 0:  
                case 11: 
                    // Set to 1: Direct volume output (PCM mode)
                    bit_out = true; 
                    break;

                case 1: 
                    // 4-bit Poly (15-step repeating sequence)
                    bit_out = (BIT_PATTERN_15 >> (channels[channel].poly_counter % 15)) & 1;
                    break;

                case 2: 
                    // 5-bit Poly (31-step repeating sequence)
                    bit_out = (BIT_PATTERN_31 >> (channels[channel].poly_counter % 31)) & 1;
                    break;

                case 4: 
                case 5: 
                    // Pure tone (Divide by 2 square wave)
                    bit_out = (channels[channel].poly_counter & 1);
                    break;

                case 8: 
                    // White noise (9-bit Poly / 511-step sequence)
                    bit_out = get_lfsr511_bit(channels[channel].poly_counter % 511);
                    break;

                case 12: 
                case 13: 
                    // Pure tone (Divide by 6 square wave)
                    bit_out = ((channels[channel].poly_counter / 3) & 1);
                    break;

                default:
                    // Handle remaining poly modes (3, 6, 7, 9, 10, 14, 15) via lookup table
                    bit_out = get_poly_mode_bit(channels[channel].control, channels[channel].poly_counter);
                    break;
            }

            return bit_out ? channels[channel].volume * 17 : 0;
        }

    }
}