audiocodec/encoder.c

//
// Created by bruno on 13. 7. 2026.
//

#include "encoder.h"

#include <SDL.h>
#include <SDL_audio.h>
#include <SDL_stdinc.h>

#include "goertzel.h"

SDL_AudioDeviceID dev;
AudioData encodedAudioData;

Encoder encoder;

// Internal audio bus
float internalBusSamples[INTERNAL_BUFFER_SIZE] = {0};
volatile uint32_t internalBusWrite = 0;
volatile uint32_t internalBusRead = 0;

#define PREAMBLE_SYMBOLS 128

const uint8_t syncBytes[4] =
{
    'B',
    'R',
    'N',
    'Q'
};

static void encoder_set_bit(
    AudioData *audio,
    Encoder *enc,
    unsigned bitIndex,
    int bit)
{
    /*
    float spacing =
        (float)(enc->endFreq - enc->startFreq) /
        (enc->bitCount * 2 - 1);
        */

    uint16_t freq =
        enc->startFreq +
        enc->dataSpacing * (bitIndex * 2 + bit);

    freq = goertzel_bin_freq(
    freq,
    SAMPLE_RATE,
    enc->symbolSamples
);

    SynthVoice *v =
        &audio->synthVoices[bitIndex + 1];

    v->frequency = freq;
    v->phaseStep =
        ((uint64_t)freq << 32) / SAMPLE_RATE;
    v->volume = 255;
}

static int encoder_get_bit(Encoder *enc) {
    if (enc->byteIndex >= enc->bufferSize)
        return -1;

    int bit = (enc->buffer[enc->byteIndex] >> enc->bitIndex) & 1;

    enc->bitIndex++;

    if (enc->bitIndex == 8) {
        enc->bitIndex = 0;
        enc->byteIndex++;
    }

    return bit;
}

static void encoder_clock(
    Encoder *enc,
    AudioData *audio)
{
    enc->clockState ^= 1;

    SynthVoice *clock =
        &audio->synthVoices[0];

    uint16_t freq =
        enc->clockState ?
        enc->clock1 :
        enc->clock0;

    clock->frequency = freq;
    clock->phaseStep =
        ((uint64_t)freq << 32) /
        SAMPLE_RATE;
    clock->volume = 255;
}

void encoder_next_symbol(
    Encoder *enc,
    AudioData *audio)
{
    encoder_clock(enc, audio);

    switch(enc->state)
    {
        case TX_PREAMBLE:

            /* transmit all-zero symbol */

            for(unsigned i=0;i<enc->bitCount;i++)
                encoder_set_bit(audio,enc,i,0);

            enc->preambleSymbols++;

            if(enc->preambleSymbols >= PREAMBLE_SYMBOLS)
            {
                enc->syncIndex = 0;
                enc->bitIndex = 0;
                enc->state = TX_SYNC;
            }

            break;

        case TX_SYNC:
        {
            uint8_t byte =
                syncBytes[enc->syncIndex];

            for(unsigned i=0;i<enc->bitCount;i++)
            {
                encoder_set_bit(
                    audio,
                    enc,
                    i,
                    (byte >> i) & 1);
            }

            enc->syncIndex++;

            if(enc->syncIndex ==
               sizeof(syncBytes))
            {
                enc->state = TX_DATA;
                enc->byteIndex = 0;
                enc->bitIndex = 0;
            }

            break;
        }

        case TX_DATA:
        {
            int finished = 0;

            for(unsigned i=0;i<enc->bitCount;i++)
            {
                int bit =
                    encoder_get_bit(enc);

                if(bit < 0)
                {
                    bit = 0;
                    finished++;
                }

                encoder_set_bit(
                    audio,
                    enc,
                    i,
                    bit);
            }

            if(finished ==
               enc->bitCount)
            {
                enc->state = TX_END;
            }

            break;
        }

        case TX_END:

            stopEncoder(enc);
            break;

        default:
            break;
    }

    enc->samplesRemaining =
        enc->symbolSamples;
}

void startEncoder(Encoder *enc)
{
    enc->state = TX_PREAMBLE;
    enc->transmitting = true;

    enc->byteIndex = 0;
    enc->bitIndex = 0;
    enc->samplesRemaining = 0;
    enc->clockState = false;

    enc->preambleSymbols = 0;
    enc->syncIndex = 0;
}

void stopEncoder(Encoder *enc)
{
    enc->transmitting=false;
    enc->state=TX_IDLE;

    for(int i=0;i<MAX_BITS+1;i++)
    {
        encodedAudioData.synthVoices[i].volume=0;
    }
}

void audio_callback(void *userdata, Uint8 *stream, int len) {
    AudioData *audio = userdata;

    Encoder *enc = audio->encoder;

    if (enc->state == TX_END ||!enc->transmitting) {
        memset(stream, 0, len);
        return;
    }

    float *out = (float *) stream;
    int samples = len / sizeof(float);

    for (int i = 0; i < samples; i++) {
        if (enc->samplesRemaining == 0)
            encoder_next_symbol(enc, audio);

        enc->samplesRemaining--;

        float mix = 0.0f;
        int active = 0;

        for (unsigned v = 0; v < enc->bitCount + 1; v++) {
            SynthVoice *voice = &audio->synthVoices[v];

            if (voice->volume == 0)
                continue;

            voice->phase += voice->phaseStep;

            float sample = sinf(
                ((double) voice->phase / 4294967296.0) *
                (2.0 * M_PI));

            mix += sample * (voice->volume / 255.0f);

            active++;
        }

        float output = active ? mix / (active + 1) : 0.0f;
        out[i] = output;
        
        // Write to internal bus for loopback
        internalBusSamples[internalBusWrite] = output;
        internalBusWrite = (internalBusWrite + 1) % INTERNAL_BUFFER_SIZE;
    }
}

void initEncoder()
{
    memset(&encoder,0,sizeof(Encoder));

    encoder.bufferSize = 0;
    encoder.bitCount = 8;
    encoder.byteIndex = 0;

    encoder.startFreq = 1000;
    encoder.endFreq = 5000;
    encoder.dataSpacing = 250;
    encoder.symbolSamples = SAMPLE_RATE / 50;

    encoder.clock0 = goertzel_bin_freq(
    6000,
    SAMPLE_RATE,
    encoder.symbolSamples
);

    encoder.clock1 = goertzel_bin_freq(
        6300,
        SAMPLE_RATE,
        encoder.symbolSamples
    );


    encoder.state = TX_IDLE;
    encoder.transmitting = false;

}

void initAudioEncoder() {
    memset(&encodedAudioData, 0, sizeof(AudioData));
    initEncoder();

    SDL_AudioSpec spec = {0};
    spec.freq = SAMPLE_RATE;
    spec.format = AUDIO_F32SYS;
    spec.channels = 1;
    spec.samples = 4096;
    spec.callback = audio_callback;
    encodedAudioData.encoder = &encoder;
    spec.userdata = &encodedAudioData;

    dev = SDL_OpenAudioDevice(NULL, 0, &spec, NULL, 0);
    printf("Encoder device opened: dev=%u\n", dev);
    if (dev == 0) {
        printf("Failed to open audio encoder: %s\n", SDL_GetError());
        SDL_Quit();
    }
    SDL_PauseAudioDevice(dev, 0);
}