demod_9600.c

//
//    This file is part of Dire Wolf, an amateur radio packet TNC.
// 
//    Copyright (C) 2011, 2012, 2013, 2015, 2019, 2021  John Langner, WB2OSZ
//
//    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 2 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/>.
//


//#define DEBUG4 1	/* capture 9600 output to log files */


/*------------------------------------------------------------------
 *
 * Module:      demod_9600.c
 *
 * Purpose:   	Demodulator for baseband signal.
 *		This is used for AX.25 (with scrambling) and IL2P without.
 *		
 * Input:	Audio samples from either a file or the "sound card."
 *
 * Outputs:	Calls hdlc_rec_bit() for each bit demodulated.  
 *
 *---------------------------------------------------------------*/

#include "direwolf.h"

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <unistd.h>
#include <sys/stat.h>
#include <string.h>
#include <assert.h>
#include <ctype.h>

// Fine tuning for different demodulator types.
// Don't remove this section.  It is here for a reason.

#define DCD_THRESH_ON 32                // Hysteresis: Can miss 0 out of 32 for detecting lock.
                                        // This is best for actual on-the-air signals.
                                        // Still too many brief false matches.
#define DCD_THRESH_OFF 8                // Might want a little more fine tuning.
#define DCD_GOOD_WIDTH 1024             // No more than 1024!!!

#include "fsk_demod_state.h"		// Values above override defaults.

#include "tune.h"
#include "hdlc_rec.h"
#include "demod_9600.h"
#include "textcolor.h"
#include "dsp.h"




static float slice_point[MAX_SUBCHANS];


/* Add sample to buffer and shift the rest down. */

__attribute__((hot)) __attribute__((always_inline))
static inline void push_sample (float val, float *buff, int size)
{
	memmove(buff+1,buff,(size-1)*sizeof(float));
	buff[0] = val; 
}


/* FIR filter kernel. */

__attribute__((hot)) __attribute__((always_inline))
static inline float convolve (const float *__restrict__ data, const float *__restrict__ filter, int filter_size)
{
	float sum = 0.0f;
	int j;

//#pragma GCC ivdep				// ignored until gcc 4.9
	for (j=0; j<filter_size; j++) {
	    sum += filter[j] * data[j];
	}
	return (sum);
}

/* Automatic gain control. */
/* Result should settle down to 1 unit peak to peak.  i.e. -0.5 to +0.5 */

__attribute__((hot)) __attribute__((always_inline))
static inline float agc (float in, float fast_attack, float slow_decay, float *ppeak, float *pvalley)
{
	if (in >= *ppeak) {
	  *ppeak = in * fast_attack + *ppeak * (1.0f - fast_attack);
	}
	else {
	  *ppeak = in * slow_decay + *ppeak * (1.0f - slow_decay);
	}

	if (in <= *pvalley) {
	  *pvalley = in * fast_attack + *pvalley * (1.0f - fast_attack);
	}
	else  {   
	  *pvalley = in * slow_decay + *pvalley * (1.0f - slow_decay);
	}

	if (*ppeak > *pvalley) {
	  return ((in - 0.5f * (*ppeak + *pvalley)) / (*ppeak - *pvalley));
	}
	return (0.0);
}


/*------------------------------------------------------------------
 *
 * Name:        demod_9600_init
 *
 * Purpose:     Initialize the 9600 (or higher) baud demodulator.
 *
 * Inputs:      modem_type	- Determines whether scrambling is used.
 *
 *		samples_per_sec	- Number of samples per second for audio.
 *
 *		upsample	- Factor to upsample the incoming stream.
 *				  After a lot of experimentation, I discovered that
 *				  it works better if the data is upsampled.
 *				  This reduces the jitter for PLL synchronization.
 *
 *		baud		- Data rate in bits per second.
 *
 *		D		- Address of demodulator state.
 *
 * Returns:     None
 *		
 *----------------------------------------------------------------*/

void demod_9600_init (enum modem_t modem_type, int original_sample_rate, int upsample, int baud, struct demodulator_state_s *D)
{	
	float fc;
	int j;
	if (upsample < 1) upsample = 1;
	if (upsample > 4) upsample = 4;


	memset (D, 0, sizeof(struct demodulator_state_s));
	D->modem_type = modem_type;
	D->num_slicers = 1;

// Multiple profiles in future?

//	switch (profile) {

//	  case 'J':			// upsample x2 with filtering.
//	  case 'K':			// upsample x3 with filtering.
//	  case 'L':			// upsample x4 with filtering.


	    D->lp_filter_len_bits =  1.0;	// -U4 = 61 	4.59 samples/symbol

	    // Works best with odd number in some tests.  Even is better in others.
	    //D->lp_filter_size = ((int) (0.5f * ( D->lp_filter_len_bits * (float)original_sample_rate / (float)baud ))) * 2 + 1;

	    // Just round to nearest integer.
	    D->lp_filter_size = (int) (( D->lp_filter_len_bits * (float)original_sample_rate / baud) + 0.5f);

	    D->lp_window = BP_WINDOW_COSINE;

	    D->lpf_baud = 1.00;

	    D->agc_fast_attack = 0.080;
	    D->agc_slow_decay =  0.00012;

	    D->pll_locked_inertia = 0.89;
	    D->pll_searching_inertia = 0.67;

//	    break;
//	}

#if 0
	text_color_set(DW_COLOR_DEBUG);
	dw_printf ("----------  %s  (%d, %d)  -----------\n", __func__, samples_per_sec, baud);
	dw_printf ("filter_len_bits = %.2f\n", D->lp_filter_len_bits);
	dw_printf ("lp_filter_size = %d\n", D->lp_filter_size);
	dw_printf ("lp_window = %d\n", D->lp_window);
	dw_printf ("lpf_baud = %.2f\n", D->lpf_baud);
	dw_printf ("samples per bit = %.1f\n", (double)samples_per_sec / baud);
#endif


	// PLL needs to use the upsampled rate.

        D->pll_step_per_sample = 
		(int) round(TICKS_PER_PLL_CYCLE * (double) baud / (double)(original_sample_rate * upsample));


#ifdef TUNE_LP_WINDOW
	D->lp_window = TUNE_LP_WINDOW;
#endif

#if TUNE_LP_FILTER_SIZE
	D->lp_filter_size = TUNE_LP_FILTER_SIZE;
#endif

#ifdef TUNE_LPF_BAUD
	D->lpf_baud = TUNE_LPF_BAUD;
#endif	

#ifdef TUNE_AGC_FAST
	D->agc_fast_attack = TUNE_AGC_FAST;
#endif

#ifdef TUNE_AGC_SLOW
	D->agc_slow_decay = TUNE_AGC_SLOW;
#endif

#if defined(TUNE_PLL_LOCKED)
	D->pll_locked_inertia = TUNE_PLL_LOCKED;
#endif

#if defined(TUNE_PLL_SEARCHING)
	D->pll_searching_inertia = TUNE_PLL_SEARCHING;
#endif

	// Initial filter (before scattering) is based on upsampled rate.

	fc = (float)baud * D->lpf_baud / (float)(original_sample_rate * upsample);

	//dw_printf ("demod_9600_init: call gen_lowpass(fc=%.2f, , size=%d, )\n", fc, D->lp_filter_size);

	gen_lowpass (fc, D->u.bb.lp_filter, D->lp_filter_size * upsample, D->lp_window);

// New in 1.7 -
// Use a polyphase filter to reduce the CPU load.
// Originally I used zero stuffing to upsample.
// Here is the general idea.
//
// Suppose the input samples are 1 2 3 4 5 6 7 8 9 ...
// Filter coefficients are a b c d e f g h i ...
//
// With original sampling rate, the filtering would involve multiplying and adding:
//
// 	1a 2b 3c 4d 5e 6f ...
//
// When upsampling by 3, each of these would need to be evaluated
// for each audio sample:
//
//	1a 0b 0c 2d 0e 0f 3g 0h 0i ...
//	0a 1b 0c 0d 2e 0f 0g 3h 0i ...
//	0a 0b 1c 0d 0e 2f 0g 0h 3i ...
//
// 2/3 of the multiplies are always by a stuffed zero.
// We can do this more efficiently by removing them.
//
//	1a       2d       3g       ...
//	   1b       2e       3h    ...
//	      1c       2f       3i ...
//
// We scatter the original filter across multiple shorter filters.
// Each input sample cycles around them to produce the upsampled rate.
//
//	a d g ...
//	b e h ...
//	c f i ...
//
// There are countless sources of information DSP but this one is unique
// in that it is a college course that mentions APRS.
// https://www2.eecs.berkeley.edu/Courses/EE123
//
// Was the effort worthwhile?  Times on an RPi 3.
//
// command:   atest -B9600  ~/walkabout9600[abc]-compressed*.wav
//
// These are 3 recordings of a portable system being carried out of
// range and back in again.  It is a real world test for weak signals.
//
//	options		num decoded	seconds		x realtime
//			1.6	1.7	1.6	1.7	1.6	1.7
//			---	---	---	---	---	---
//	-P-		171	172	23.928	17.967	14.9	19.9
//	-P+		180	180	54.688	48.772	6.5	7.3
//	-P- -F1		177	178	32.686	26.517	10.9	13.5
//
// So, it turns out that -P+ doesn't have a dramatic improvement, only
// around 4%, for drastically increased CPU requirements.
// Maybe we should turn that off by default, especially for ARM.
//

	int k = 0;
	for (int i = 0; i < D->lp_filter_size; i++) {
	    D->u.bb.lp_polyphase_1[i] = D->u.bb.lp_filter[k++];
	    if (upsample >= 2) {
	        D->u.bb.lp_polyphase_2[i] = D->u.bb.lp_filter[k++];
	        if (upsample >= 3) {
	            D->u.bb.lp_polyphase_3[i] = D->u.bb.lp_filter[k++];
	            if (upsample >= 4) {
	                D->u.bb.lp_polyphase_4[i] = D->u.bb.lp_filter[k++];
	            }
	        }
	    }
	}


	/* Version 1.2: Experiment with different slicing levels. */
	// Really didn't help that much because we should have a symmetrical signal.

	for (j = 0; j < MAX_SUBCHANS; j++) {
	  slice_point[j] = 0.02f * (j - 0.5f * (MAX_SUBCHANS-1));
	  //dw_printf ("slice_point[%d] = %+5.2f\n", j, slice_point[j]);
	}

} /* end fsk_demod_init */



/*-------------------------------------------------------------------
 *
 * Name:        demod_9600_process_sample
 *
 * Purpose:     (1) Filter & slice the signal.
 *		(2) Descramble it.
 *		(2) Recover clock and data.
 *
 * Inputs:	chan	- Audio channel.  0 for left, 1 for right.
 *
 *		sam	- One sample of audio.
 *			  Should be in range of -32768 .. 32767.
 *
 * Returns:	None 
 *
 * Descripion:	"9600 baud" packet is FSK for an FM voice transceiver.
 *		By the time it gets here, it's really a baseband signal.
 *		At one extreme, we could have a 4800 Hz square wave.
 *		A the other extreme, we could go a considerable number
 *		of bit times without any transitions.
 *
 *		The trick is to extract the digital data which has
 *		been distorted by going thru voice transceivers not
 *		intended to pass this sort of "audio" signal.
 *
 *		For G3RUH mode, data is "scrambled" to reduce the amount of DC bias.
 *		The data stream must be unscrambled at the receiving end.
 *
 *		We also have a digital phase locked loop (PLL)
 *		to recover the clock and pick out data bits at
 *		the proper rate.
 *
 *		For each recovered data bit, we call:
 *
 *			  hdlc_rec (channel, demodulated_bit);
 *
 *		to decode HDLC frames from the stream of bits.
 *
 * Future:	This could be generalized by passing in the name
 *		of the function to be called for each bit recovered
 *		from the demodulator.  For now, it's simply hard-coded.
 *
 *		After experimentation, I found that this works better if
 *		the original signal is upsampled by 2x or even 4x.
 *
 * References:	9600 Baud Packet Radio Modem Design
 *		http://www.amsat.org/amsat/articles/g3ruh/109.html
 *
 *		The KD2BD 9600 Baud Modem
 *		http://www.amsat.org/amsat/articles/kd2bd/9k6modem/
 *
 *		9600 Baud Packet Handbook
 * 		ftp://ftp.tapr.org/general/9600baud/96man2x0.txt
 *
 *
 *--------------------------------------------------------------------*/

inline static void nudge_pll (int chan, int subchan, int slice, float demod_out, struct demodulator_state_s *D);

static void process_filtered_sample (int chan, float fsam, struct demodulator_state_s *D);


__attribute__((hot))
void demod_9600_process_sample (int chan, int sam, int upsample, struct demodulator_state_s *D)
{
	float fsam;

#if DEBUG4
	static FILE *demod_log_fp = NULL;
	static int log_file_seq = 0;		/* Part of log file name */
#endif

	int subchan = 0;

	assert (chan >= 0 && chan < MAX_CHANS);
	assert (subchan >= 0 && subchan < MAX_SUBCHANS);

	/* Scale to nice number for convenience. */
	/* Consistent with the AFSK demodulator, we'd like to use */
	/* only half of the dynamic range to have some headroom. */
	/* i.e.  input range +-16k becomes +-1 here and is */
	/* displayed in the heard line as audio level 100. */

	fsam = (float)sam / 16384.0f;

	// Low pass filter
	push_sample (fsam, D->u.bb.audio_in, D->lp_filter_size);

	fsam = convolve (D->u.bb.audio_in, D->u.bb.lp_polyphase_1, D->lp_filter_size);
	process_filtered_sample (chan, fsam, D);
	if (upsample >= 2) {
	    fsam = convolve (D->u.bb.audio_in, D->u.bb.lp_polyphase_2, D->lp_filter_size);
	    process_filtered_sample (chan, fsam, D);
	    if (upsample >= 3) {
	        fsam = convolve (D->u.bb.audio_in, D->u.bb.lp_polyphase_3, D->lp_filter_size);
	        process_filtered_sample (chan, fsam, D);
	        if (upsample >= 4) {
	            fsam = convolve (D->u.bb.audio_in, D->u.bb.lp_polyphase_4, D->lp_filter_size);
	            process_filtered_sample (chan, fsam, D);
	        }
	    }
	}
}


__attribute__((hot))
static void process_filtered_sample (int chan, float fsam, struct demodulator_state_s *D)
{

	int subchan = 0;

/*
 * Version 1.2: Capture the post-filtering amplitude for display.
 * This is similar to the AGC without the normalization step.
 * We want decay to be substantially slower to get a longer
 * range idea of the received audio.
 * For AFSK, we keep mark and space amplitudes.
 * Here we keep + and - peaks because there could be a DC bias.
 */

// TODO:  probably no need for this.  Just use  D->m_peak, D->m_valley

	if (fsam >= D->alevel_mark_peak) {
	  D->alevel_mark_peak = fsam * D->quick_attack + D->alevel_mark_peak * (1.0f - D->quick_attack);
	}
	else {
	  D->alevel_mark_peak = fsam * D->sluggish_decay + D->alevel_mark_peak * (1.0f - D->sluggish_decay);
	}

	if (fsam <= D->alevel_space_peak) {
	  D->alevel_space_peak = fsam * D->quick_attack + D->alevel_space_peak * (1.0f - D->quick_attack);
	}
	else {
	  D->alevel_space_peak = fsam * D->sluggish_decay + D->alevel_space_peak * (1.0f - D->sluggish_decay);
	}

/* 
 * The input level can vary greatly.
 * More importantly, there could be a DC bias which we need to remove.
 *
 * Normalize the signal with automatic gain control (AGC). 
 * This works by looking at the minimum and maximum signal peaks
 * and scaling the results to be roughly in the -1.0 to +1.0 range.
 */
	float demod_out;
	int demod_data;				/* Still scrambled. */

	demod_out = agc (fsam, D->agc_fast_attack, D->agc_slow_decay, &(D->m_peak), &(D->m_valley));

// TODO: There is potential for multiple decoders with one filter.

//dw_printf ("peak=%.2f valley=%.2f fsam=%.2f norm=%.2f\n", D->m_peak, D->m_valley, fsam, norm);

	if (D->num_slicers <= 1) {

	  /* Normal case of one demodulator to one HDLC decoder. */
	  /* Demodulator output is difference between response from two filters. */
	  /* AGC should generally keep this around -1 to +1 range. */

	  demod_data = demod_out > 0;
	  nudge_pll (chan, subchan, 0, demod_out, D);
	}
	else {
	  int slice;

	  /* Multiple slicers each feeding its own HDLC decoder. */

	  for (slice=0; slice<D->num_slicers; slice++) {
	    demod_data = demod_out - slice_point[slice] > 0;
	    nudge_pll (chan, subchan, slice, demod_out - slice_point[slice], D);
	  }
	}

	// demod_data is used only for debug out.
	// suppress compiler warning about it not being used.
	(void) demod_data;

#if DEBUG4

	if (chan == 0) {

	  if (1) {
	  //if (D->slicer[slice].data_detect) {
	    char fname[30];
	    int slice = 0;

	    if (demod_log_fp == NULL) {
	      log_file_seq++;
	      snprintf (fname, sizeof(fname), "demod/%04d.csv", log_file_seq);
	      //if (log_file_seq == 1) mkdir ("demod", 0777);
	      if (log_file_seq == 1) mkdir ("demod");

	      demod_log_fp = fopen (fname, "w");
	      text_color_set(DW_COLOR_DEBUG);
	      dw_printf ("Starting demodulator log file %s\n", fname);
	      fprintf (demod_log_fp, "Audio, Filtered,  Max,  Min, Normalized, Sliced, Clock\n");
	    }

	    fprintf (demod_log_fp, "%.3f, %.3f, %.3f, %.3f, %.3f, %d, %.2f\n",
			fsam + 6,
			fsam + 4,
			D->m_peak + 4,
			D->m_valley + 4,
			demod_out + 2,
			demod_data + 2,
			(D->slicer[slice].data_clock_pll & 0x80000000) ? .5 : .0);

	    fflush (demod_log_fp);
	  }
	  else {
	    if (demod_log_fp != NULL) {
	      fclose (demod_log_fp);
	      demod_log_fp = NULL;
	    }
	  }
	}
#endif

} /* end demod_9600_process_sample */


/*-------------------------------------------------------------------
 *
 * Name:        nudge_pll
 *
 * Purpose:	Update the PLL state for each audio sample.
 *
 *		(2) Descramble it.
 *		(2) Recover clock and data.
 *
 * Inputs:	chan	- Audio channel.  0 for left, 1 for right.
 *
 *		subchan	- Which demodulator.  We could have several running in parallel.
 *
 *		slice	- Determines which Slicing level & HDLC decoder to use.
 *
 *		demod_out_f - Demodulator output, possibly shifted by slicing level
 *				It will be compared with 0.0 to bit binary value out.
 *
 *		D	- Demodulator state for this channel / subchannel.
 *
 * Returns:	None
 *
 * Description:	A PLL is used to sample near the centers of the data bits.
 *
 *		D->data_clock_pll is a SIGNED 32 bit variable.
 *		When it overflows from a large positive value to a negative value, we
 *		sample a data bit from the demodulated signal.
 *
 *		Ideally, the the demodulated signal transitions should be near
 *		zero we we sample mid way between the transitions.
 *
 *		Nudge the PLL by removing some small fraction from the value of
 *		data_clock_pll, pushing it closer to zero.
 *
 *		This adjustment will never change the sign so it won't cause
 *		any erratic data bit sampling.
 *
 *		If we adjust it too quickly, the clock will have too much jitter.
 *		If we adjust it too slowly, it will take too long to lock on to a new signal.
 *
 *		I don't think the optimal value will depend on the audio sample rate
 *		because this happens for each transition from the demodulator.
 *
 * Version 1.4:	Previously, we would always pull the PLL phase toward 0 after
 *		after a zero crossing was detetected.  This adds extra jitter,
 *		especially when the ratio of audio sample rate to baud is low.
 *		Now, we interpolate between the two samples to get an estimate
 *		on when the zero crossing happened.  The PLL is pulled toward
 *		this point.
 *
 *		Results???  TBD
 *
 * Version 1.6:	New experiment where filter size to extract clock is not the same
 *		as filter to extract the data bit value.
 *
 *--------------------------------------------------------------------*/

__attribute__((hot))
inline static void nudge_pll (int chan, int subchan, int slice, float demod_out_f, struct demodulator_state_s *D)
{
	D->slicer[slice].prev_d_c_pll = D->slicer[slice].data_clock_pll;

	// Perform the add as unsigned to avoid signed overflow error.
	D->slicer[slice].data_clock_pll = (signed)((unsigned)(D->slicer[slice].data_clock_pll) + (unsigned)(D->pll_step_per_sample));

	if ( D->slicer[slice].prev_d_c_pll > 1000000000 && D->slicer[slice].data_clock_pll < -1000000000) {

	  /* Overflow.  Was large positive, wrapped around, now large negative. */

	  hdlc_rec_bit (chan, subchan, slice, demod_out_f > 0, D->modem_type == MODEM_SCRAMBLE, D->slicer[slice].lfsr);
	  pll_dcd_each_symbol2 (D, chan, subchan, slice);
	}

/*
 * Zero crossing?
 */
        if ((D->slicer[slice].prev_demod_out_f < 0 && demod_out_f > 0) ||
	    (D->slicer[slice].prev_demod_out_f > 0 && demod_out_f < 0)) {

	  // Note:  Test for this demodulator, not overall for channel.

	  pll_dcd_signal_transition2 (D, slice, D->slicer[slice].data_clock_pll);

	  float target = D->pll_step_per_sample * demod_out_f / (demod_out_f - D->slicer[slice].prev_demod_out_f);

	  if (D->slicer[slice].data_detect) {
	    D->slicer[slice].data_clock_pll = (int)(D->slicer[slice].data_clock_pll * D->pll_locked_inertia + target * (1.0f - D->pll_locked_inertia) );
	  }
	  else {
	    D->slicer[slice].data_clock_pll = (int)(D->slicer[slice].data_clock_pll * D->pll_searching_inertia + target * (1.0f - D->pll_searching_inertia) );
	  }
	}


#if DEBUG5

	//if (chan == 0) {
	if (D->slicer[slice].data_detect) {
	
	  char fname[30];

	  
	  if (demod_log_fp == NULL) {
	    seq++;
	    snprintf (fname, sizeof(fname), "demod96/%04d.csv", seq);
	    if (seq == 1) mkdir ("demod96"
#ifndef __WIN32__
					, 0777
#endif
						);

	    demod_log_fp = fopen (fname, "w");
	    text_color_set(DW_COLOR_DEBUG);
	    dw_printf ("Starting 9600 decoder log file %s\n", fname);
	    fprintf (demod_log_fp, "Audio, Peak, Valley, Demod, SData, Descram, Clock\n");
	  }
	  fprintf (demod_log_fp, "%.3f, %.3f, %.3f, %.3f, %.2f, %.2f, %.2f\n", 
			0.5f * fsam + 3.5,
			0.5f * D->m_peak + 3.5,
			0.5f * D->m_valley + 3.5,
			0.5f * demod_out + 2.0,
			demod_data ? 1.35 : 1.0,  
			descram ? .9 : .55,  
			(D->data_clock_pll & 0x80000000) ? .1 : .45);
	}
	else {
	  if (demod_log_fp != NULL) {
	    fclose (demod_log_fp);
	    demod_log_fp = NULL;
	  }
	}
	//}

#endif


/*
 * Remember demodulator output (pre-descrambling) so we can compare next time
 * for the DPLL sync.
 */
	D->slicer[slice].prev_demod_out_f = demod_out_f;

} /* end nudge_pll */


/* end demod_9600.c */