mFSK Modem

An M-ary 2FSK and 4FSK Modem based on the FSK Modem designed by David Rowe. I have changed it to a Dynamic Library. I'm using the Netbeans IDE and have included the export so you can import and compile any changes. You can also just burst the ZIP's (for example on a Raspberry Pi) and type make.

Build Instructions

First copy the libfsk.so file to /usr/local/lib Make sure the permissions are set correctly (644).

sudo cp libfsk.so /usr/local/lib
sudo ldconfig

Then copy all the header files to /usr/local/include

sudo cp header/* /usr/local/include

To compile the test file

c99 mod-test.c -o mod-test -lfsk -lm

Executing the mod-test program will create a RAW audio file /tmp/fsksignal.raw and you can use audacity to view the spectrum, or play with the audio.

Examples using Audacity

Included is a couple of PNG graphics from the modulator tests, and the resulting RAW audio samples. The audio samples are 16-bit signed PCM, little-endian, at an 8000 sample rate.

The test signal has an F1 frequency of 1100 Hz, a shift frequency of 200 Hz, and a symbol rate of 100 Baud.

In 2FSK this results in a 0-bit producing 1100 Hz, and a 1-bit producing 1300 Hz.

In 4FSK this results in a 00-bits producing 1100 Hz, 01-bits producing 1300 Hz, 10-bits producing 1500 Hz, and 11-bits producing 1700 Hz

Project Development

Currently the modem compiles without error, and the modulator/demodulator seems to work.

There is a little testing program I used for the CRC, Scrambler, and Interleaver actually work. It shows they produce the right data, and is bidirectional.

Library Calls

In order to use the FSK library, the following functions are available, and the internal data structure is shown in structures:

struct FSK {
    complex float phase[MAX_TONES];
    float f_est[MAX_TONES];
    float norm_rx_timing;
    float EbNodB;
    float ppm;
    int Ndft;
    int Fs;
    int N;
    int Rs;
    int Ts;
    int Nmem;
    int P;
    int Nsym;
    int Nbits;
    int f1;
    int shift;
    int mode;
    int est_min;
    int est_max;
    int est_space;
    int nstash;
    int nin;
    bool burst_mode;
    fft_cfg fftcfg;
    float *fft_est;
    complex float *samp_old;
};

struct MODULATE {
    complex float oscillator[MAX_TONES];
    complex float phase;
    float fs;
    float rs;
    float f1;
    float shift;
    int mode;
    int cycles;
};

struct STATS {
    fft_cfg fftcfg;
    float snr_est; /* estimated SNR of rx signal in dB (3 kHz noise BW) */
    float foff; /* estimated freq offset in Hz */
    float rx_timing; /* estimated optimum timing offset in samples */
    float clock_offset; /* Estimated tx/rx sample clock offset in ppm */
    float scale_dB;
    float fftbuffer[STATS_FFTSIZE];
};

Stats

struct STATS *stats_open(void);
void stats_close(struct STATS *);
void stats_spectrum(struct STATS *, float [], complex float [], int);

float stats_get_snr_est(struct STATS *);
float stats_get_foff(struct STATS *);
float stats_get_rx_timing(struct STATS *);
float stats_get_clock_offset(struct STATS *);

float *stats_get_fft_buf_ptr(struct STATS *);

void stats_set_snr_est(struct STATS *, float);
void stats_set_foff(struct STATS *, float);
void stats_set_rx_timing(struct STATS *, float);
void stats_set_clock_offset(struct STATS *, float);

Fsk

struct FSK *fsk_create(int, int, int, int);
struct FSK *fsk_create_hbr(int, int, int, int, int, int);
void fsk_destroy(struct FSK *);

int fsk_get_nin(struct FSK *);
int fsk_get_N(struct FSK *);
int fsk_get_Nmem(struct FSK *);
int fsk_get_Nbits(struct FSK *);
int fsk_get_Ts(struct FSK *);
float fsk_get_f_est(struct FSK *, int);
void fsk_set_nsym(struct FSK *, int);
void fsk_set_est_limits(struct FSK *, int, int);
void fsk_set_estimators(struct FSK *);

Modulate

struct MODULATE *mod_create(int, int, int, int, int);
void mod_destroy(struct MODULATE *);
void modulate(struct MODULATE *, complex float [], int);

Demodulate

void fsk_demod(struct FSK *, uint8_t [], complex float []);