Added to README and --help as well

README.md

@@ -287,7 +287,12 @@ Other parameters were explained above at `firdes_lowpass_f`.
 	fir_decimate_cc <decimation_factor> [transition_bw [window]]
 
 It is a decimator that keeps one sample out of `decimation_factor` samples.
-To avoid aliasing, it runs a filter on the signal and removes spectral components above `0.5 × nyquist_frequency × decimation_factor`.
+To avoid aliasing, it runs a filter on the signal and removes spectral components above `0.5 × nyquist_frequency × decimation_factor` from the input signal.
+
+	fir_interpolate_cc <interpolation_factor> [transition_bw [window]]
+
+It is an interpolator that generates `interpolation_factor` number of output samples from one input sample.
+To avoid aliasing, it runs a filter on the signal and removes spectral components above `0.5 × nyquist_frequency / interpolation_factor` from the output signal.
 
 `transition_bw` and `window` are the parameters of the filter.
 

csdr.c

@@ -88,6 +88,7 @@ char usage[]=
 "    amdemod_cf\n"
 "    amdemod_estimator_cf\n"
 "    fir_decimate_cc <decimation_factor> [transition_bw [window]]\n"
+"    fir_interpolate_cc <interpolation_factor> [transition_bw [window]]\n"
 "    firdes_lowpass_f <cutoff_rate> <length> [window [--octave]]\n"
 "    firdes_bandpass_c <low_cut> <high_cut> <length> [window [--octave]]\n"
 "    agc_ff [hang_time [reference [attack_rate [decay_rate [max_gain [attack_wait [filter_alpha]]]]]]]\n"
@@ -860,7 +861,7 @@ int main(int argc, char *argv[])
 		sscanf(argv[2],"%d",&factor);
 		assert(factor >= 1);
 
-		float transition_bw = 0.01;
+		float transition_bw = 0.05;
 		if(argc>=4) sscanf(argv[3],"%g",&transition_bw);
 		assert(transition_bw >= 0 && transition_bw < 1.);
 

libcsdr.c

@@ -396,8 +396,8 @@ int fir_interpolate_cc(complexf *input, complexf *output, int input_size, int in
 			float accq=0;
 			//int tistart = (interpolation-ip)%interpolation; 
 			int tistart = (interpolation-ip); //why does this work? why don't we need the % part?
-			for(int ti=tistart, si=0; ti<taps_length; ti+=interpolation, si++) acci += (iof(input,i+si)) * taps[ti]; //@fir_interpolate_cc: i loop
-			for(int ti=tistart, si=0; ti<taps_length; ti+=interpolation, si++) accq += (qof(input,i+si)) * taps[ti]; //@fir_interpolate_cc: q loop
+			for(int ti=tistart, si=0; ti<taps_length; (ti+=interpolation), (si++)) acci += (iof(input,i+si)) * taps[ti]; //@fir_interpolate_cc: i loop
+			for(int ti=tistart, si=0; ti<taps_length; (ti+=interpolation), (si++)) accq += (qof(input,i+si)) * taps[ti]; //@fir_interpolate_cc: q loop
 			iof(output,oi)=acci;
 			qof(output,oi)=accq;
 			oi++;