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minimodem: refine fsk frame position on carrier acquisition 

FSK_ANALYZE_NSTEPS was previously 10.

Now FSK_ANALYZE_NSTEPS_CARRIER_LOCK is 100, and FSK_ANALYZE_NSTEPS is
reduced to 4.

(Also, try_confidence_search_limit = INFINITY on carrier acquisition frame).

Effects: Better initial phase lock on signal in the first
place; sloppier tracking of fast/slow signals; reduced longterm CPU load.
This commit is contained in:
Kamal Mostafa 2012-09-03 23:31:16 -07:00
parent aa1124d6e7
commit 8731b54bc2
1 changed files with 35 additions and 14 deletions
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49
src/minimodem.c
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@ -928,9 +928,12 @@ main( int argc, char*argv[] )
try_max_nsamples = nsamples_per_bit;
try_max_nsamples += nsamples_overscan;
#define FSK_ANALYZE_NSTEPS 10 /* accuracy vs. performance */
// Note: FSK_ANALYZE_NSTEPS has subtle effects on the
// "rate perfect" calculation. oh well.
// FSK_ANALYZE_NSTEPS Try 4 frame positions across the try_max_nsamples
// range. Using a larger nsteps allows for more accurate tracking of
// fast/slow signals (at decreased performance). Note also
// FSK_ANALYZE_NSTEPS_CARRIER_LOCK below, which refines the frame
// position upon first acquiring carrier.
#define FSK_ANALYZE_NSTEPS 4
unsigned int try_step_nsamples = try_max_nsamples / FSK_ANALYZE_NSTEPS;
if ( try_step_nsamples == 0 )
try_step_nsamples = 1;
@ -941,19 +944,11 @@ main( int argc, char*argv[] )
* prev_stop bit begins (since the "frame" includes the prev_stop). */
unsigned int frame_start_sample = 0;
// If we don't have carrier, then set this try_confidence_search_limit
// to infinity (search for best possible frame) so to get the decoder
// into phase with the signal, so the next try_first_sample will match
// up with where the next frame should be.
unsigned int try_first_sample;
float try_confidence_search_limit;
if ( carrier ) {
try_first_sample = nsamples_overscan;
try_confidence_search_limit = fsk_confidence_search_limit;
} else {
try_first_sample = 0;
try_confidence_search_limit = INFINITY;
}
try_confidence_search_limit = fsk_confidence_search_limit;
try_first_sample = carrier ? nsamples_overscan : 0;
confidence = fsk_find_frame(fskp, samplebuf, expect_nsamples,
try_first_sample,
@ -1019,6 +1014,32 @@ main( int argc, char*argv[] )
carrier_nsamples -= nsamples_overscan;
} else {
// We just acquired carrier.
if ( confidence < INFINITY && try_step_nsamples > 1 ) {
// FSK_ANALYZE_NSTEPS_CARRIER_LOCK:
// Scan again, but try harder to find the best frame upon
// acquiring carrier. Since we found a valid confidence frame
// in the "sloppy" fsk_find_frame() call already, we're sure
// to find one at least as good this time.
#define FSK_ANALYZE_NSTEPS_CARRIER_LOCK 100
try_step_nsamples = try_max_nsamples / FSK_ANALYZE_NSTEPS_CARRIER_LOCK;
if ( try_step_nsamples == 0 )
try_step_nsamples = 1;
try_confidence_search_limit = INFINITY;
confidence = fsk_find_frame(fskp, samplebuf, expect_nsamples,
try_first_sample,
try_max_nsamples,
try_step_nsamples,
try_confidence_search_limit,
expect_bits_string,
&bits,
&amplitude,
&frame_start_sample
);
}
if ( !quiet_mode ) {
if ( bfsk_data_rate >= 100 )
fprintf(stderr, "### CARRIER %u @ %.1f Hz ###\n",