path: root/src/rds_mod.c

/*
 * RDS Modulator from:
 * PiFmRds - FM/RDS transmitter for the Raspberry Pi
 * https://github.com/ChristopheJacquet/PiFmRds
 * 
 * Copyright (C) 2014 by Christophe Jacquet, F8FTK
 *
 * adapted for use with fl2k_fm:
 * Copyright (C) 2018 by Steve Markgraf <steve@steve-m.de>
 *
 * SPDX-License-Identifier: GPL-3.0+
 *
 * 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>
#include <string.h>
#include <time.h>
#include <stdlib.h>

#define RT_LENGTH	64
#define PS_LENGTH	8
#define GROUP_LENGTH	4

extern double waveform_biphase[576];

struct {
	uint16_t pi;
	int ta;
	char ps[PS_LENGTH];
	char rt[RT_LENGTH];
} rds_params = { 0 };

/* The RDS error-detection code generator polynomial is
   x^10 + x^8 + x^7 + x^5 + x^4 + x^3 + x^0
*/
#define POLY		0x1B9
#define POLY_DEG	10
#define MSB_BIT		(1 << 15)
#define BLOCK_SIZE	16

#define BITS_PER_GROUP (GROUP_LENGTH * (BLOCK_SIZE+POLY_DEG))
#define SAMPLES_PER_BIT 192
#define FILTER_SIZE (sizeof(waveform_biphase)/sizeof(double))
#define SAMPLE_BUFFER_SIZE (SAMPLES_PER_BIT + FILTER_SIZE)


uint16_t offset_words[] = { 0x0FC, 0x198, 0x168, 0x1B4 };
// We don't handle offset word C' here for the sake of simplicity

/* Classical CRC computation */
uint16_t crc(uint16_t block)
{
	uint16_t crc = 0;
	int i, bit, msb;

	for (i = 0; i < BLOCK_SIZE; i++) {
		bit = (block & MSB_BIT) != 0;
		block <<= 1;

		msb = (crc >> (POLY_DEG-1)) & 1;
		crc <<= 1;

		if ((msb ^ bit) != 0)
			crc = crc ^ POLY;
	}

	return crc;
}

/* Possibly generates a CT (clock time) group if the minute has just changed
   Returns 1 if the CT group was generated, 0 otherwise
*/
int get_rds_ct_group(uint16_t *blocks)
{
	static int latest_minutes = -1;
	int l, mjd, offset;

	// Check time
	time_t now;
	struct tm *utc;

	now = time(NULL);
	utc = gmtime(&now);

	if(utc->tm_min != latest_minutes) {
		// Generate CT group
		latest_minutes = utc->tm_min;

		l = utc->tm_mon <= 1 ? 1 : 0;
		mjd = 14956 + utc->tm_mday + 
		      (int)((utc->tm_year - l) * 365.25) +
		      (int)((utc->tm_mon + 2 + l*12) * 30.6001);

		blocks[1] = 0x4400 | (mjd>>15);
		blocks[2] = (mjd<<1) | (utc->tm_hour>>4);
		blocks[3] = (utc->tm_hour & 0xF)<<12 | utc->tm_min<<6;

		utc = localtime(&now);

		//'struct tm' has no member named 'tm_gmtoff' on Windows+MinGW
		#if defined(__APPLE__) || defined(__FreeBSD__)
		offset = utc->tm_gmtoff / (30 * 60);
		#else
		offset = time(NULL) / (30 * 60);
		#endif

		blocks[3] |= abs(offset);
		if (offset < 0)
			blocks[3] |= 0x20;

		return 1;
	} else {
		return 0;
	}
}

/* Creates an RDS group. This generates sequences of the form 0A, 0A, 0A, 0A, 2A, etc.
   The pattern is of length 5, the variable 'state' keeps track of where we are in the
   pattern. 'ps_state' and 'rt_state' keep track of where we are in the PS (0A) sequence
   or RT (2A) sequence, respectively.
*/
void get_rds_group(int *buffer)
{
	static int state = 0;
	static int ps_state = 0;
	static int rt_state = 0;
	uint16_t blocks[GROUP_LENGTH] = { rds_params.pi, 0, 0, 0 };
	uint16_t block, check;
	int i, j;

	// Generate block content
	if (!get_rds_ct_group(blocks)) { // CT (clock time) has priority on other group types
		if (state < 4) {
			blocks[1] = 0x0400 | ps_state;

			if (rds_params.ta)
				blocks[1] |= 0x0010;

			blocks[2] = 0xCDCD;	 // no AF
			blocks[3] = rds_params.ps[ps_state*2] << 8 | rds_params.ps[ps_state*2+1];
			ps_state++;

			if (ps_state >= 4)
				ps_state = 0;
		} else { // state == 5
			blocks[1] = 0x2400 | rt_state;
			blocks[2] = rds_params.rt[rt_state*4+0] << 8 | rds_params.rt[rt_state*4+1];
			blocks[3] = rds_params.rt[rt_state*4+2] << 8 | rds_params.rt[rt_state*4+3];
			rt_state++;
			if (rt_state >= 16)
				rt_state = 0;
		}

		state++;
		if (state >= 5)
			state = 0;
	}
	
	// Calculate the checkword for each block and emit the bits
	for (i = 0; i < GROUP_LENGTH; i++) {
		block = blocks[i];
		check = crc(block) ^ offset_words[i];
		for (j = 0; j < BLOCK_SIZE; j++) {
			*buffer++ = ((block & (1 << (BLOCK_SIZE-1))) != 0);
			block <<= 1;
		}
		for (j = 0; j < POLY_DEG; j++) {
			*buffer++ = ((check & (1 << (POLY_DEG-1))) != 0);
			check <<= 1;
		}
	}
}

/* Get a number of RDS samples. This generates the envelope of the waveform using
   pre-generated elementary waveform samples, and then it amplitude-modulates the 
   envelope with a 57 kHz carrier, which is very efficient as 57 kHz is 4 times the
   sample frequency we are working at (228 kHz).
 */
void get_rds_samples(double *buffer, uint32_t count)
{
	static int bit_buffer[BITS_PER_GROUP];
	static int bit_pos = BITS_PER_GROUP;
	static double sample_buffer[SAMPLE_BUFFER_SIZE] = {0};

	static int prev_output = 0;
	static int cur_output = 0;
	static int cur_bit = 0;
	static int sample_count = SAMPLES_PER_BIT;
	static int inverting = 0;
	static int phase = 0;

	static unsigned int in_sample_index = 0;
	static unsigned int out_sample_index = SAMPLE_BUFFER_SIZE-1;

	unsigned int i, j, idx;
	double val, sample;
	double *src;

	for (i = 0; i < count; i++) {
		if (sample_count >= SAMPLES_PER_BIT) {
			if (bit_pos >= BITS_PER_GROUP) {
				get_rds_group(bit_buffer);
				bit_pos = 0;
			}

			// do differential encoding
			cur_bit = bit_buffer[bit_pos];
			prev_output = cur_output;
			cur_output = prev_output ^ cur_bit;

			inverting = (cur_output == 1);

			src = waveform_biphase;
			idx = in_sample_index;

			for (j = 0; j < FILTER_SIZE; j++) {
				val = *src++;
				if (inverting)
					val = -val;

				sample_buffer[idx++] += val;

				if (idx >= SAMPLE_BUFFER_SIZE)
					idx = 0;
			}

			in_sample_index += SAMPLES_PER_BIT;
			if (in_sample_index >= SAMPLE_BUFFER_SIZE)
				in_sample_index -= SAMPLE_BUFFER_SIZE;

			bit_pos++;
			sample_count = 0;
		}

		sample = sample_buffer[out_sample_index];
		sample_buffer[out_sample_index] = 0;
		out_sample_index++;
		if (out_sample_index >= SAMPLE_BUFFER_SIZE)
			out_sample_index = 0;

		// modulate at 57 kHz
		// use phase for this
		switch (phase) {
			case 0:
			case 2: sample = 0; break;
			case 1: break;
			case 3: sample = -sample; break;
		}
		phase++;
		if (phase >= 4)
			phase = 0;

		*buffer++ = sample;
		sample_count++;
	}
}

void set_rds_pi(uint16_t pi_code)
{
	rds_params.pi = pi_code;
}

void set_rds_rt(char *rt)
{
	int i;

	strncpy(rds_params.rt, rt, 64);

	for (i = 0; i < 64; i++) {
		if (rds_params.rt[i] == 0)
			rds_params.rt[i] = 32;
	}
}

void set_rds_ps(char *ps)
{
	int i;

	strncpy(rds_params.ps, ps, 8);

	for (i = 0; i < 8; i++) {
		if (rds_params.ps[i] == 0)
			rds_params.ps[i] = 32;
	}
}

void set_rds_ta(int ta)
{
	rds_params.ta = ta;
}