filters.h

/*
 * JMPXRDS, an FM MPX signal generator with RDS support on
 * top of Jack Audio Connection Kit - Various filter implementations
 *
 * Copyright (C) 2015 Nick Kossifidis <mickflemm@gmail.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#include <stdint.h>		/* For typed integers */
#include <fftw3.h>		/* For FFTW support */
#include <jack/jack.h>		/* For jack-related types */

/* A generic FFT-based FIR low pass filter */
struct lpf_filter_data {
	uint16_t period_size;
	uint16_t num_bins;
	uint16_t middle_bin;
	uint16_t cutoff_bin;
	uint32_t sample_rate;
	float bin_bw;
	fftwf_complex *filter_resp;
	fftwf_complex *complex_buff;
	float *real_in;
	float *real_out;
	uint16_t overlap_len;
	fftwf_plan dft_plan;
	fftwf_plan ift_plan;
};

/*
 * Filtering on the frequency domain introduces errors
 * when going back to time domain due to the fixed number
 * of frequency bins used (time domain is continuous, freq
 * domain is discrete). Such errors introduce noise on the
 * filter's output (time aliasing). To reduce the noise we
 * use a method known as overlap-discard where we re-use
 * an ammount of frames from the previous input and discard
 * the same amount of frames (the overlap) on the output.
 * In other words we use a sliding window for doing FFT that
 * spawns across multiple periods to make the filter's output
 * more continuous in time domain. As a result of this we'll
 * obviously introduce a systematic delay between input and
 * output (output will be delayed by overlap samples).
 */
#define	AFLT_LPF_OVERLAP_FACTOR 3
#define SSB_LPF_OVERLAP_FACTOR 3

void lpf_filter_destroy(const struct lpf_filter_data *);
int lpf_filter_init(struct lpf_filter_data *, uint32_t, uint32_t, uint16_t, uint8_t);
int lpf_filter_apply(const struct lpf_filter_data *, const float*, float*, uint16_t, float);


/* FM Preemphasis IIR filter */
struct fmpreemph_filter_data {
	float last_in;
	float last_out[2];
	float ataps_50[2];
	float btaps_50[2];
	float ataps_75[2];
	float btaps_75[2];
};

enum fmpreemph_mode {
	LPF_PREEMPH_50US = 0,	/* E.U. / WORLD */
	LPF_PREEMPH_75US = 1,	/* U.S. */
	LPF_PREEMPH_NONE = 2,
	LPF_PREEMPH_MAX = 3
};

int
fmpreemph_filter_init(struct fmpreemph_filter_data *, float);

float
fmpreemph_filter_apply(struct fmpreemph_filter_data *,
		       float, enum fmpreemph_mode);

#define AFLT_CUTOFF_FREQ 16750

/* Hilbert transformer for the Hartley modulator (SSB) */
struct hilbert_transformer_data {
	uint16_t num_bins;
	fftwf_complex *complex_buff;
	float *real_buff;
	fftwf_plan dft_plan;
	fftwf_plan ift_plan;
};

int hilbert_transformer_init(struct hilbert_transformer_data *ht, uint16_t);
void hilbert_transformer_destroy(const struct hilbert_transformer_data *ht);
int hilbert_transformer_apply(const struct hilbert_transformer_data *ht, const float *, uint16_t);