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Fixed rounding errors and typos in FM pre-emphasis

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Fixed rounding errors
Fixed a typo for the b1 parameter
The pre-emphasis value has been adjusted from 75us to 50us for global compatibility

This fixes the pre-emphasis for FM completely, verified with RTL-SDR
This commit is contained in:
Kristijan Gjoshev
2021-07-29 00:11:04 +02:00
committed by GitHub
parent cce1fe6acf
commit d86308ec0d
1 changed files with 5 additions and 5 deletions
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10
src/pifmrds/fm_mpx.c
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@@ -128,16 +128,16 @@ int fm_mpx_open(char *filename, size_t len) {
// IIR pre-emphasis filter
// Reference material: http://jontio.zapto.org/hda1/preempiir.pdf
double tau=75e-6;
double delta=1.96e-6;
double tau=50e-6; // 50us default pre-emphasis, TODO: make it adjustable for the US to 75us
double delta=1/(2*PI*20000); // 20KHz frequency max for the emphasis function
double taup, deltap, bp, ap, a0, a1, b1;
taup=1.0/(2.0*(in_samplerate*FIR_PHASES))/tan( 1.0/(2*tau*(in_samplerate*FIR_PHASES) ));
deltap=1.0/(2.0*(in_samplerate*FIR_PHASES))/tan( 1.0/(2*delta*(in_samplerate*FIR_PHASES) ));
bp=sqrt( -taup*taup + sqrt(taup*taup*taup*taup + 8.0*taup*taup*deltap*deltap) ) / 2.0 ;
ap=sqrt( 2*bp*bp + taup*taup );
a0=( 2.0*ap + 1/(in_samplerate*FIR_PHASES) )/(2.0*bp + 1/(in_samplerate*FIR_PHASES) );
a1=(-2.0*ap + 1/(in_samplerate*FIR_PHASES) )/(2.0*bp + 1/(in_samplerate*FIR_PHASES) );
b1=( 2.0*bp + 1/(in_samplerate*FIR_PHASES) )/(2.0*bp + 1/(in_samplerate*FIR_PHASES) );
a0=( 2.0*ap + 1.0/(in_samplerate*FIR_PHASES) )/(2.0*bp + 1.0/(in_samplerate*FIR_PHASES) );
a1=(-2.0*ap + 1.0/(in_samplerate*FIR_PHASES) )/(2.0*bp + 1.0/(in_samplerate*FIR_PHASES) );
b1=( 2.0*bp - 1.0/(in_samplerate*FIR_PHASES) )/(2.0*bp + 1.0/(in_samplerate*FIR_PHASES) );
double x=0,y=0;
for(int i=0; i<FIR_TAPS; i++) {