diff options
Diffstat (limited to 'src/libemphasis/emphasis.c')
| -rw-r--r-- | src/libemphasis/emphasis.c | 19 |
1 files changed, 12 insertions, 7 deletions
diff --git a/src/libemphasis/emphasis.c b/src/libemphasis/emphasis.c index ed4e2ec..843bb66 100644 --- a/src/libemphasis/emphasis.c +++ b/src/libemphasis/emphasis.c @@ -27,9 +27,6 @@ #define PI M_PI -#define CUT_OFF_H 100.0 /* cut-off frequency for high-pass filter */ -#define CUT_OFF_L 4000.0 /* cut-off frequency for low-pass filter */ - static void gen_sine(sample_t *samples, int num, int samplerate, double freq) { int i; @@ -50,7 +47,13 @@ static double get_level(sample_t *samples, int num) return envelope; } -int init_emphasis(emphasis_t *state, int samplerate, double cut_off) +/* calculate cut off from time constant in uS */ +double timeconstant2cutoff(double time_constant_us) +{ + return 1.0 / (2.0 * PI * time_constant_us / 1e6); +} + +int init_emphasis(emphasis_t *state, int samplerate, double cut_off, double cut_off_h, double cut_off_l) { double factor; sample_t test_samples[samplerate / 10]; @@ -67,10 +70,12 @@ int init_emphasis(emphasis_t *state, int samplerate, double cut_off) state->d.amp = 1.0; /* do not de-emphasis below CUT_OFF_H */ - iir_highpass_init(&state->d.hp, CUT_OFF_H, samplerate, 1); + iir_highpass_init(&state->d.hp, cut_off_h, samplerate, 1); - /* do not pre-emphasis above CUT_OFF_L */ - iir_lowpass_init(&state->p.lp, CUT_OFF_L, samplerate, 1); + /* do not pre-emphasis above CUT_OFF_L + * Mobile network specifications want -18 dB per octave. + * With two interations we have 24 dB, - 6 dB (from emphasis). */ + iir_lowpass_init(&state->p.lp, cut_off_l, samplerate, 2); /* calibrate amplification to be neutral at 1000 Hz */ gen_sine(test_samples, sizeof(test_samples) / sizeof(test_samples[0]), samplerate, 1000.0); |