util/spkmodem-decode: separate silence check

i conflated two separate tests in a previous change.

the silence check was defeated by still checking f
alongside it, which would be set, thus satisfying
the condition, and proceeding, which defeats the
purpose of the silence check (ignore false signal
that is actually noise) - so in the original patch
that i wrote, the extra checks actually do nothing.

this patch fixes that, and makes the logic a bit
clearer.

Signed-off-by: Leah Rowe <leah@libreboot.org>

1 files changed, 16 insertions, 11 deletions

diff --git a/util/spkmodem_decode/spkmodem-decode.c b/util/spkmodem_decode/spkmodem-decode.c
index 08e7b17a..ff94e90e 100644
--- a/util/spkmodem_decode/spkmodem-decode.c
+++ b/util/spkmodem_decode/spkmodem-decode.c
@@ -460,25 +460,30 @@ auto_detect_tone(struct decoder_state *st)
 {
 	int f;
 
+	if (st->learn_samples >= LEARN_SAMPLES)
+		return;
+
 	/*
-	 * Don't also run auto-detect during decode,
-	 * otherwise it would run for every sample.
+	 * Ignore silence / near silence.
+	 * Both FIR windows will be near zero when no signal exists.
 	 */
-	if (st->learn_samples >= LEARN_SAMPLES)
+	if (st->freq_data <= 2 && st->freq_separator <= 2) {
+		st->learn_samples++;
 		return;
+	}
 
 	/*
-	 * Check both FIR windows.
-	 * Inside separator frames, the separator window contains tone,
-	 * during data frames the data window does; a minimum of
-	 * the two captures the lowest active tone cluster more reliably.
+	 * Choose the lowest active tone.
+	 * Separator frames carry tone in the separator window,
+	 * data frames carry tone in the data window.
 	 */
 	f = st->freq_data;
-	if (st->freq_separator > 0 && st->freq_separator < f)
+
+	if (f <= 0 || (st->freq_separator > 0 &&
+	    st->freq_separator < f))
 		f = st->freq_separator;
-	if (f > 0 || /* prevent noise from corrupting tone-learning */
-	    st->freq_data > 2 || /* <--- stop from */
-	    st->freq_separator > 2) { /* learning silence if no signal */
+
+	if (f > 0) {
 		if (f < st->freq_min)
 			st->freq_min = f;