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"
#include "modem_settings.h"
#include "packet.h"

SDL_AudioDeviceID dev;
AudioData encodedAudioData;

Encoder encoder;


const uint8_t syncSymbols[] =
{
    2,0,
    2,2,
    6,2,
    1,2
};

static void encoder_set_symbol(
        AudioData *audio,
        Encoder *enc,
        unsigned symbol)
{
    /*
       symbol:
       0 ... number of FSK tones-1
    */

    uint16_t freq =
        enc->startFreq +
        symbol * enc->dataSpacing;


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


    SynthVoice *v =
        &audio->synthVoices[DATA_VOICE];


    v->frequency = freq;

    v->phaseStep =
        ((uint64_t)freq << 32) /
        enc->sampleRate;

    v->volume = 255;
}

static int encoder_get_symbol(
        Encoder *enc)
{
    unsigned symbol=0;


    for(unsigned i=0;
        i<enc->bitsPerSymbol;
        i++)
    {

        if(enc->packetIndex >= enc->packetSize)
            return -1;


        int bit =
        (enc->packet[enc->packetIndex]
        >>enc->bitIndex)&1;


        symbol |= bit<<i;


        enc->bitIndex++;


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


    return symbol;
}

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) /
        (enc->sampleRate ? enc->sampleRate : SAMPLE_RATE);
    clock->volume = 255;

    // Debug: print clock toggle
    //printf("ENCODER CLOCK: toggled to %u Hz (state=%d)\n", freq, enc->clockState);
}

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

    encoder_clock(enc,audio);


    switch(enc->state)
    {

        case TX_PREAMBLE:
        {
            unsigned symbol =
                enc->preambleSymbols %
                enc->toneCount;


            encoder_set_symbol(
                audio,
                enc,
                symbol);


            enc->preambleSymbols++;


            if(enc->preambleSymbols>=PREAMBLE_SYMBOLS)
            {
                enc->packetIndex=0;
                enc->bitIndex=0;

                enc->state=TX_PACKET;
            }

            break;
        }


        case TX_PACKET:
        {
            int symbol =
                encoder_get_symbol(enc);


            if(symbol<0)
            {
                enc->state=TX_END;
                break;
            }


            encoder_set_symbol(
                audio,
                enc,
                symbol);

            break;
        }


        case TX_END:
            stopEncoder(enc);
            break;


        default:
            break;
    }


    enc->samplesRemaining=
        enc->symbolSamples;
}

void startEncoder(Encoder *enc)
{
    encoder_build_packet(enc, enc->buffer, enc->bufferSize);
    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<2;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<2; 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;
    }
}

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


    encoder.bufferSize = 0;


    /*
       8-FSK:
       3 bits per symbol
       8 frequencies
    */
    encoder.bitsPerSymbol = 3;
    encoder.toneCount = 8;


    encoder.byteIndex = 0;


    encoder.startFreq =
        DEFAULT_START_FREQ;

    encoder.endFreq =
        DEFAULT_END_FREQ;


    uint16_t freqRange =
        encoder.endFreq -
        encoder.startFreq;


    /*
       N tones need N-1 intervals
    */
    encoder.dataSpacing =
        freqRange /
        (encoder.toneCount-1);


    encoder.symbolSamples =
        SAMPLE_RATE /
        DEFAULT_SYMBOL_RATE;


    encoder.clock0 =
        goertzel_bin_freq(
            DEFAULT_CLOCK0_FREQ,
            SAMPLE_RATE,
            encoder.symbolSamples);


    encoder.clock1 =
        goertzel_bin_freq(
            DEFAULT_CLOCK1_FREQ,
            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 = 960;  // requested buffer
    spec.callback = audio_callback;
    encodedAudioData.encoder = &encoder;
    spec.userdata = &encodedAudioData;

    SDL_AudioSpec realSpec = {0};
    dev = SDL_OpenAudioDevice(NULL, 0, &spec, &realSpec, 0);
    printf("Encoder device opened: dev=%u (requested %dHz/%d samples)\n", dev, spec.freq, spec.samples);
    if (dev == 0) {
        printf("Failed to open audio encoder: %s\n", SDL_GetError());
        SDL_Quit();
    }

    // Update encoder sample rate and symbolSamples based on actual device
    encoder.sampleRate = realSpec.freq;
    encoder.symbolSamples = realSpec.freq / DEFAULT_SYMBOL_RATE;
    encoder.clock0 = goertzel_bin_freq(
        DEFAULT_CLOCK0_FREQ,
        encoder.sampleRate,
        encoder.symbolSamples
    );
    encoder.clock1 = goertzel_bin_freq(
        DEFAULT_CLOCK1_FREQ,
        encoder.sampleRate,
        encoder.symbolSamples
    );

    printf("Encoder real spec: freq=%d channels=%d samples=%d -> symbolSamples=%u\n",
           realSpec.freq, realSpec.channels, realSpec.samples, encoder.symbolSamples);

    SDL_PauseAudioDevice(dev, 0);
}