diff options
Diffstat (limited to 'gr-analog/python/analog/fm_emph.py')
| -rw-r--r-- | gr-analog/python/analog/fm_emph.py | 79 |
1 files changed, 31 insertions, 48 deletions
diff --git a/gr-analog/python/analog/fm_emph.py b/gr-analog/python/analog/fm_emph.py index 7637743d91..bfa4742ace 100644 --- a/gr-analog/python/analog/fm_emph.py +++ b/gr-analog/python/analog/fm_emph.py @@ -251,69 +251,52 @@ class fm_preemph(gr.hier_block2): """ FM Preemphasis IIR filter. """ - def __init__(self, fs, tau=75e-6, fh=0.0): + def __init__(self, fs, tau=75e-6, fh=-1.0): """ Args: fs: sampling frequency in Hz (float) tau: Time constant in seconds (75us in US, 50us in EUR) (float) - fh: High frequency at which to flatten out; 0.0 means none (float) + fh: High frequency at which to flatten out (< 0 means default of 0.925*fs/2.0) (float) """ gr.hier_block2.__init__(self, "fm_preemph", gr.io_signature(1, 1, gr.sizeof_float), # Input signature gr.io_signature(1, 1, gr.sizeof_float)) # Output signature - if fh > 0.0 and fh < (fs / 2.0): - # Digital corner frequencies - w_cl = 1.0 / tau - w_ch = 2.0 * math.pi * fh + # Set fh to something sensible, if needed. + # N.B. fh == fs/2.0 or fh == 0.0 results in a pole on the unit circle + # at z = -1.0 or z = 1.0 respectively. That makes the filter unstable + # and useless. + if fh <= 0.0 or fh >= fs/2.0: + fh = 0.925 * fs/2.0 - # Prewarped analog corner frequencies - w_cla = 2.0 * fs * math.tan(w_cl / (2.0 * fs)) - w_cha = 2.0 * fs * math.tan(w_ch / (2.0 * fs)) + # Digital corner frequencies + w_cl = 1.0 / tau + w_ch = 2.0 * math.pi * fh - # Resulting digital pole, zero, and gain term from the bilinear - # transformation of H(s) = (s + w_cla) / (s + w_cha) to - # H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1) - kl = -w_cla / (2.0 * fs) - kh = -w_cha / (2.0 * fs) - z1 = (1.0 + kl) / (1.0 - kl) - p1 = (1.0 + kh) / (1.0 - kh) - b0 = (1.0 - kl) / (1.0 - kh) + # Prewarped analog corner frequencies + w_cla = 2.0 * fs * math.tan(w_cl / (2.0 * fs)) + w_cha = 2.0 * fs * math.tan(w_ch / (2.0 * fs)) - # Since H(s = infinity) = 1.0, then H(z = -1) = 1.0 and - # this filter has 0 dB gain at fs/2.0. - # That isn't what users are going to expect, so adjust with a - # gain, g, so that H(z = 1) = 1.0 for 0 dB gain at DC. - w_0dB = 2.0 * math.pi * 0.0 - g = abs(1.0 - p1 * cmath.rect(1.0, -w_0dB)) \ - / (b0 * abs(1.0 - z1 * cmath.rect(1.0, -w_0dB))) + # Resulting digital pole, zero, and gain term from the bilinear + # transformation of H(s) = (s + w_cla) / (s + w_cha) to + # H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1) + kl = -w_cla / (2.0 * fs) + kh = -w_cha / (2.0 * fs) + z1 = (1.0 + kl) / (1.0 - kl) + p1 = (1.0 + kh) / (1.0 - kh) + b0 = (1.0 - kl) / (1.0 - kh) - btaps = [ g * b0 * 1.0, g * b0 * -z1 ] - ataps = [ 1.0, -p1 ] + # Since H(s = infinity) = 1.0, then H(z = -1) = 1.0 and + # this filter has 0 dB gain at fs/2.0. + # That isn't what users are going to expect, so adjust with a + # gain, g, so that H(z = 1) = 1.0 for 0 dB gain at DC. + w_0dB = 2.0 * math.pi * 0.0 + g = abs(1.0 - p1 * cmath.rect(1.0, -w_0dB)) \ + / (b0 * abs(1.0 - z1 * cmath.rect(1.0, -w_0dB))) - else: - # Just use H(s) = (s + 1/RC)/(1/RC) as the transfer function - - # Digital corner frequencies - w_cl = 1.0 / tau - - # Prewarped analog corner frequencies - w_cla = 2.0 * fs * math.tan(w_cl / (2.0 * fs)) - - # Resulting digital pole, zero, and gain term from the bilinear - # transformation of H(s) = (s + w_cl)/w_cl to - # H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1) - kl = -w_cla / (2.0 * fs) - z1 = (1.0 + kl) / (1.0 - kl) - p1 = -1.0 - b0 = (1.0 - kl) / -kl - - # Since H(s = 0) = 1.0, then H(z = 1) = 1.0 and - # has 0 dB gain at DC - - btaps = [ b0 * 1.0, b0 * -z1 ] - ataps = [ 1.0, -p1 ] + btaps = [ g * b0 * 1.0, g * b0 * -z1 ] + ataps = [ 1.0, -p1 ] if 0: print "btaps =", btaps |