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wfm: add a wideband FM modulator and demodulator

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Model the pair on fmd.c / fmmod.c, but with the parts that a 75 kHz
deviation forces:

  - the demodulator discriminates with arg(x[n] * conj(x[n-1])) rather
    than a PLL; an omegaN in the tens of kHz cannot track 75 kHz.
  - emphasis is a one-pole RC (tau 75 us) on both ends, not fmd's 1/f
    fc_impulse FIR.  A 1/f FIR from f_low = 20 Hz would sit ~+57 dB at
    20 Hz, where broadcast FM specifies flat below the corner.
  - TXA_WFM leaves the shared preemph block off; wfmmod carries its own.
  - wfmmod clamps bp_fc = deviation + f_high to 0.45 * samplerate, since
    +/-90 kHz exceeds Nyquist at the rates the narrowband modes use.

Scope is mono: no 19 kHz pilot, no 38 kHz stereo subcarrier, no RDS.

Both mode enums are appended to so the ABI stays stable for clients.
The JNI bindings are deliberately left alone.

Verified against a wfmmod -> wfmd loopback: the discriminator is exact,
the dc-removal one-pole tracks |H_lp(f)| * ain * sdelta to ratio 1.000,
and a 700 + 1900 Hz two-tone comes back with THD+N ~ 3e-5 % at +/-37.5
kHz deviation.  Note the demodulator aliases unless samplerate exceeds
2 * deviation * |aud|max, so 192 kHz is the practical floor.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Uladzimir Karpenka
2026-07-10 07:43:07 +03:00
parent f39eea6b01
commit 65cb3c386e
12 changed files with 871 additions and 8 deletions
Download Patch File Download Diff File Expand all files Collapse all files
TXA.c
+35 -5
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@@ -357,7 +357,22 @@ void create_txa (int channel)
1, // run bandpass filter
max(2048, ch[channel].dsp_size), // number coefficients for bandpass filter
0); // minimum phase flag
txa[channel].wfmmod.p = create_wfmmod (
0, // run - OFF by default
ch[channel].dsp_size, // size
txa[channel].midbuff, // pointer to input buffer
txa[channel].midbuff, // pointer to output buffer
ch[channel].dsp_rate, // samplerate
75000.0, // deviation
20.0, // low cutoff frequency
15000.0, // high cutoff frequency
1, // run pre-emphasis
75.0e-6, // pre-emphasis time constant
1, // run bandpass filter
max(2048, ch[channel].dsp_size), // number coefficients for bandpass filter
0); // minimum phase flag
txa[channel].gen1.p = create_gen (
0, // run
ch[channel].dsp_size, // buffer size
@@ -490,6 +505,7 @@ void destroy_txa (int channel)
destroy_meter (txa[channel].alcmeter.p);
destroy_uslew (txa[channel].uslew.p);
destroy_gen (txa[channel].gen1.p);
destroy_wfmmod (txa[channel].wfmmod.p);
destroy_fmmod (txa[channel].fmmod.p);
destroy_ammod (txa[channel].ammod.p);
destroy_wcpagc (txa[channel].alc.p);
@@ -544,6 +560,7 @@ void flush_txa (int channel)
flush_wcpagc (txa[channel].alc.p);
flush_ammod (txa[channel].ammod.p);
flush_fmmod (txa[channel].fmmod.p);
flush_wfmmod (txa[channel].wfmmod.p);
flush_gen (txa[channel].gen1.p);
flush_uslew (txa[channel].uslew.p);
flush_meter (txa[channel].alcmeter.p);
@@ -580,6 +597,7 @@ void xtxa (int channel)
xammod (txa[channel].ammod.p); // AM Modulator
xemphp (txa[channel].preemph.p, 1); // FM pre-emphasis (second option)
xfmmod (txa[channel].fmmod.p); // FM Modulator
xwfmmod (txa[channel].wfmmod.p); // WFM Modulator (pre-emphasis is internal)
xgen (txa[channel].gen1.p); // output signal generator (TUN and Two-tone)
xuslew (txa[channel].uslew.p); // up-slew for AM, FM, and gens
xmeter (txa[channel].alcmeter.p); // ALC Meter
@@ -653,6 +671,7 @@ void setDSPSamplerate_txa (int channel)
setSamplerate_wcpagc (txa[channel].alc.p, ch[channel].dsp_rate);
setSamplerate_ammod (txa[channel].ammod.p, ch[channel].dsp_rate);
setSamplerate_fmmod (txa[channel].fmmod.p, ch[channel].dsp_rate);
setSamplerate_wfmmod (txa[channel].wfmmod.p, ch[channel].dsp_rate);
setSamplerate_gen (txa[channel].gen1.p, ch[channel].dsp_rate);
setSamplerate_uslew (txa[channel].uslew.p, ch[channel].dsp_rate);
setSamplerate_meter (txa[channel].alcmeter.p, ch[channel].dsp_rate);
@@ -723,6 +742,8 @@ void setDSPBuffsize_txa (int channel)
setSize_ammod (txa[channel].ammod.p, ch[channel].dsp_size);
setBuffers_fmmod (txa[channel].fmmod.p, txa[channel].midbuff, txa[channel].midbuff);
setSize_fmmod (txa[channel].fmmod.p, ch[channel].dsp_size);
setBuffers_wfmmod (txa[channel].wfmmod.p, txa[channel].midbuff, txa[channel].midbuff);
setSize_wfmmod (txa[channel].wfmmod.p, ch[channel].dsp_size);
setBuffers_gen (txa[channel].gen1.p, txa[channel].midbuff, txa[channel].midbuff);
setSize_gen (txa[channel].gen1.p, ch[channel].dsp_size);
setBuffers_uslew (txa[channel].uslew.p, txa[channel].midbuff, txa[channel].midbuff);
@@ -758,6 +779,7 @@ void SetTXAMode (int channel, int mode)
txa[channel].mode = mode;
txa[channel].ammod.p->run = 0;
txa[channel].fmmod.p->run = 0;
txa[channel].wfmmod.p->run = 0;
txa[channel].preemph.p->run = 0;
switch (mode)
{
@@ -779,6 +801,10 @@ void SetTXAMode (int channel, int mode)
txa[channel].fmmod.p->run = 1;
txa[channel].preemph.p->run = 1;
break;
case TXA_WFM:
// wfmmod carries its own RC pre-emphasis; the shared emphp stays off
txa[channel].wfmmod.p->run = 1;
break;
default:
break;
@@ -818,10 +844,11 @@ void TXAResCheck (int channel)
int TXAUslewCheck (int channel)
{
return (txa[channel].ammod.p->run == 1) ||
(txa[channel].fmmod.p->run == 1) ||
(txa[channel].gen0.p->run == 1) ||
(txa[channel].gen1.p->run == 1);
return (txa[channel].ammod.p->run == 1) ||
(txa[channel].fmmod.p->run == 1) ||
(txa[channel].wfmmod.p->run == 1) ||
(txa[channel].gen0.p->run == 1) ||
(txa[channel].gen1.p->run == 1);
}
void TXASetupBPFilters (int channel)
@@ -855,6 +882,7 @@ void TXASetupBPFilters (int channel)
case TXA_AM:
case TXA_SAM:
case TXA_FM:
case TXA_WFM:
if (txa[channel].compressor.p->run)
{
CalcBandpassFilter (txa[channel].bp0.p, 0.0, txa[channel].f_high, 2.0);
@@ -914,6 +942,7 @@ void TXASetNC (int channel, int nc)
SetTXAFMEmphNC (channel, nc);
SetTXAEQNC (channel, nc);
SetTXAFMNC (channel, nc);
SetTXAWFMNC (channel, nc);
SetTXACFIRNC (channel, nc);
SetChannelState (channel, oldstate, 0);
}
@@ -925,6 +954,7 @@ void TXASetMP (int channel, int mp)
SetTXAFMEmphMP (channel, mp);
SetTXAEQMP (channel, mp);
SetTXAFMMP (channel, mp);
SetTXAWFMMP (channel, mp);
}
PORT