CubicSDR/src/modules/modem/analog/ModemUSB.cpp

#include "ModemUSB.h"

ModemUSB::ModemUSB() : ModemAnalog() {
    // half band filter used for side-band elimination
    demodAM_USB = ampmodem_create(0.5, 0.0, LIQUID_AMPMODEM_USB, 1);
    // options
    float fc = 0.25f;         // filter cutoff frequency
    float ft = 0.05f;         // filter transition
    float As = 90.0f;         // stop-band attenuation [dB]
    float mu = 0.0f;          // fractional timing offset
    
    // estimate required filter length and generate filter
    unsigned int h_len = estimate_req_filter_len(ft,As);
    float *h = (float *) malloc(h_len * sizeof(float));
    liquid_firdes_kaiser(h_len,fc,As,mu,h);
    ssbFilt = firfilt_crcf_create(h,h_len);
    ssbShift = nco_crcf_create(LIQUID_NCO);
    nco_crcf_set_frequency(ssbShift,  (2.0 * M_PI) * 0.25);
	free(h);
}

Modem *ModemUSB::factory() {
    return new ModemUSB;
}

std::string ModemUSB::getName() {
    return "USB";
}

ModemUSB::~ModemUSB() {
    firfilt_crcf_destroy(ssbFilt);
    nco_crcf_destroy(ssbShift);
    ampmodem_destroy(demodAM_USB);
}

int ModemUSB::checkSampleRate(long long sampleRate, int audioSampleRate) {
    if (sampleRate < MIN_BANDWIDTH) {
        return MIN_BANDWIDTH;
    }
    if (sampleRate % 2 == 0) {
        return sampleRate;
    }
    return sampleRate+1;
}

int ModemUSB::getDefaultSampleRate() {
    return 5400;
}

void ModemUSB::demodulate(ModemKit *kit, ModemIQData *input, AudioThreadInput *audioOut) {
    ModemKitAnalog *akit = (ModemKitAnalog *)kit;
    
    initOutputBuffers(akit,input);
    
    if (!bufSize) {
        input->decRefCount();
        return;
    }
    
    liquid_float_complex x, y;
    for (int i = 0; i < bufSize; i++) { // Reject lower band
        nco_crcf_step(ssbShift);
        nco_crcf_mix_down(ssbShift, input->data[i], &x);
        firfilt_crcf_push(ssbFilt, x);
        firfilt_crcf_execute(ssbFilt, &x);
        nco_crcf_mix_up(ssbShift, x, &y);
        ampmodem_demodulate(demodAM_USB, y, &demodOutputData[i]);
    }
    
    buildAudioOutput(akit, audioOut, true);
}