msk/acars: add support for noise floor estimator

This introduces a basic, low cpu-cost NF estimator which works by using
an exponential moving average over the magnitude of the received signal
updated only outside of ACARS message processing.

The computational overhead of this extra estimator is virtually nil, as
the signal power estimator has been reworked to use the same input as
the noise floor estimator.

Fixes: TLeconte#84

Author: f00b4r0

acars.c

@@ -213,6 +213,7 @@ static void *blk_thread(void *arg)
 				{ .type = STATSD_UCOUNTER, .name = "decoder.msg.good", .value.u = 1 },
 				{ .type = STATSD_LGAUGE, .name = "decoder.msg.errs", .value.l = blk->err },
 				{ .type = STATSD_FGAUGE, .name = "decoder.msg.lvl", .value.f = blk->lvl },
+				{ .type = STATSD_FGAUGE, .name = "decoder.msg.nf", .value.f = blk->nf },
 				{ .type = STATSD_LGAUGE, .name = "decoder.msg.len", .value.l = blk->txtlen },
 			};
 			// use the frequency if available, else the channel number
@@ -251,18 +252,22 @@ int initAcars(channel_t *ch)
 
 void decodeAcars(channel_t *ch)
 {
+	const float mag = ch->MskMag;
 	uint8_t r = ch->outbits;
 	//vprerr("#%d r: %x, count: %d, st: %d\n", ch->chn+1, r, ch->count, ch->Acarsstate);
 
 	ch->nbits = 8;	// by default we'll read another byte next
 
+	/* update power level exp moving average. Average over last 16 bytes */
+	ch->MskPwr = ch->MskPwr - (1.0F/16.0F * (ch->MskPwr - mag * mag));
+
 	switch (ch->Acarsstate) {
 	case PREKEY:
 		if (ch->count >= 16 && 0xFF != r) {	// we have our first non-0xFF byte after a sequence
 			uint8_t q = ~r;	// avoid type promotion in calling ffs(~r)
 			int l = ffs(q);		// find the first (LSb) 0 in r
 
-			vprerr("#%d synced, count: %d, r: %x, fz: %d, lvl: %5.1f\n", ch->chn+1, ch->count, r, l, 10 * log10(ch->MskLvl));
+			vprerr("#%d synced, count: %d, r: %x, fz: %d, lvl: %5.1f\n", ch->chn+1, ch->count, r, l, 10 * log10(ch->MskPwr));
 			ch->count = 0;
 			ch->Acarsstate = SYNC;
 
@@ -299,6 +304,8 @@ void decodeAcars(channel_t *ch)
 				ch->count++;
 				break;
 			default:
+				/* outside of msgs, update noise floor moving average: long term (10^3 'bytes') magnitude exp moving average */
+				ch->MskNF = ch->MskNF - (1e-3F * (ch->MskNF - mag));
 				ch->count = 0;
 				break;
 			}
@@ -347,7 +354,8 @@ void decodeAcars(channel_t *ch)
 			}
 			gettimeofday(&(ch->blk->tv), NULL);
 			ch->Acarsstate = TXT;
-			ch->blk->lvl = 10 * log10(ch->MskLvl);
+			ch->blk->lvl = 10 * log10(ch->MskPwr);
+			ch->blk->nf = 20 * log10(ch->MskNF);
 			ch->blk->chn = ch->chn;
 			ch->blk->txtlen = 0;
 			ch->blk->err = 0;

acarsdec.h

@@ -54,7 +54,7 @@
 typedef struct mskblk_s {
 	struct mskblk_s *prev;
 	struct timeval tv;
-	float lvl;
+	float lvl, nf;
 	uint8_t chn;	// there will never be 255 channels
 	uint8_t txtlen;
 	uint8_t err;
@@ -88,7 +88,7 @@ typedef struct {
 	char be;
 	char msn[4];		// only for libacars - null-terminated copy of msg.no[0-3]
 	int err;
-	float lvl;
+	float lvl, nf;
 #ifdef HAVE_LIBACARS
 	la_reasm_status reasm_status;
 	la_proto_node *decoded_tree;
@@ -104,7 +104,9 @@ typedef struct {
 	float complex *inb;
 	double MskPhi;
 	double MskDf;
-	float MskLvl;
+	float MskMag;				// signal magnitude moving average
+	float MskPwr;				// signal power moving average (average of magnitude squared)
+	float MskNF;				// noise floor moving average (average of magnitude outside of msg blocks)
 	float MskClk;
 	unsigned int MskS, idx;
 

msk.c

@@ -54,12 +54,10 @@ int initMsk(channel_t *ch)
 static inline void putbit(float v, channel_t *ch)
 {
 	ch->outbits >>= 1;
-	if (v > 0) {
+	if (v > 0)
 		ch->outbits |= 0x80;
-	}
 
-	ch->nbits--;
-	if (ch->nbits <= 0)
+	if (--ch->nbits == 0)
 		decodeAcars(ch);
 }
 
@@ -106,12 +104,11 @@ void demodMSK(channel_t *ch, int len)
 				v += h[o] * ch->inb[(j + idx) % FLEN];
 
 			/* normalize */
-			lvl = cabsf(v);
-			v /= lvl + 1e-8;
+			lvl = cabsf(v) + 1e-8F;
+			v /= lvl;
 
-			/* update level exp moving average. Average over last 16*8 bits */
-			lvl = lvl * lvl;
-			ch->MskLvl = ch->MskLvl - (1.0F/128.0F * (ch->MskLvl - lvl));
+			/* update magnitude exp moving average. Average over last 8 bits */
+			ch->MskMag = ch->MskMag - (1.0F/8.0F * (ch->MskMag - lvl));
 
 			if (ch->MskS & 1) {
 				vo = cimagf(v);

output.c

@@ -391,10 +391,10 @@ static int fmt_msg(acarsmsg_t *msg, int chn, struct timeval tv, char *buf, size_
 	int len = 0;
 
 	if (R.inmode >= IN_RTL)
-		len += snprintf(buf + len, bufsz - len, "[#%1d (F:%3.3f L:%+5.1f E:%1d) ", chn + 1,
-			R.channels[chn].Fr / 1000000.0, msg->lvl, msg->err);
+		len += snprintf(buf + len, bufsz - len, "[#%1d (F:%3.3f L:%+5.1f/%.1f E:%1d) ", chn + 1,
+			R.channels[chn].Fr / 1000000.0, msg->lvl, msg->nf, msg->err);
 	else
-		len += snprintf(buf + len, bufsz - len, "[#%1d (L:%+5.1f E:%1d) ", chn + 1, msg->lvl, msg->err);
+		len += snprintf(buf + len, bufsz - len, "[#%1d (L:%+5.1f/%.1f E:%1d) ", chn + 1, msg->lvl, msg->nf, msg->err);
 
 	if (R.inmode != IN_SNDFILE)
 		len += fmt_date(tv, buf + len, bufsz - len);
@@ -472,7 +472,7 @@ static int fmt_oneline(acarsmsg_t *msg, int chn, struct timeval tv, char *buf, s
 		if (*pstr == '\n' || *pstr == '\r')
 			*pstr = ' ';
 
-	len = snprintf(buf, bufsz, "#%1d (L:%+5.1f E:%1d) ", chn + 1, msg->lvl, msg->err);
+	len = snprintf(buf, bufsz, "#%1d (L:%+5.1f/%.1f E:%1d) ", chn + 1, msg->lvl, msg->nf, msg->err);
 
 	if (R.inmode != IN_SNDFILE)
 		len += fmt_date(tv, buf + len, bufsz - len);
@@ -575,6 +575,8 @@ static int fmt_json(acarsmsg_t *msg, int chn, struct timeval tv, char *buf, size
 	cJSON_AddRawToObject(json_obj, "freq", convert_tmp);
 	snprintf(convert_tmp, sizeof(convert_tmp), "%2.1f", msg->lvl);
 	cJSON_AddRawToObject(json_obj, "level", convert_tmp);
+	snprintf(convert_tmp, sizeof(convert_tmp), "%.1f", msg->nf);
+	cJSON_AddRawToObject(json_obj, "noise", convert_tmp);
 	cJSON_AddNumberToObject(json_obj, "error", msg->err);
 	snprintf(convert_tmp, sizeof(convert_tmp), "%c", msg->mode);
 	cJSON_AddStringToObject(json_obj, "mode", convert_tmp);
@@ -727,6 +729,7 @@ void outputmsg(const msgblk_t *blk)
 	/* fill msg struct */
 	memset(&msg, 0, sizeof(msg));
 	msg.lvl = blk->lvl;
+	msg.nf = blk->nf;
 	msg.err = blk->err;
 
 	msg.mode = blk->txt.d.mode;