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Simplify the video deemphasis filter generation #479

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Apr 9, 2020
Merged

Simplify the video deemphasis filter generation #479

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20 changes: 9 additions & 11 deletions lddecode/core.py
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Original file line number Diff line number Diff line change
Expand Up @@ -141,7 +141,7 @@ def calclinelen(SP, mult, mhz):
'audio_notchwidth': 350000,
'audio_notchorder': 2,

'video_deemp': (120*.32, 320*.32),
'video_deemp': (120e-9, 320e-9),

# This BPF is similar but not *quite* identical to what Pioneer did
'video_bpf_low': 3400000,
Expand Down Expand Up @@ -172,7 +172,7 @@ def calclinelen(SP, mult, mhz):
'audio_notchwidth': 350000,
'audio_notchorder': 2,

'video_deemp': (120*.32, 320*.32),
'video_deemp': (120e-9, 320e-9),

'video_bpf_low': 3800000,
'video_bpf_high': 12500000,
Expand All @@ -199,7 +199,7 @@ def calclinelen(SP, mult, mhz):
'audio_notchwidth': 200000,
'audio_notchorder': 2,

'video_deemp': (100*.30, 400*.30),
'video_deemp': (100e-9, 400e-9),

# XXX: guessing here!
'video_bpf_low': 2700000,
Expand Down Expand Up @@ -227,7 +227,7 @@ def calclinelen(SP, mult, mhz):
'audio_notchwidth': 200000,
'audio_notchorder': 2,

'video_deemp': (100*.30, 400*.30),
'video_deemp': (100e-9, 400e-9),

# XXX: guessing here!
'video_bpf_low': 3200000,
Expand Down Expand Up @@ -452,15 +452,13 @@ def computevideofilters(self):
video_hpf = sps.butter(DP['video_hpf_order'], DP['video_hpf_freq']/self.freq_hz_half, 'high')
SF['Fvideo_hpf'] = filtfft(video_hpf, self.blocklen)

# The deemphasis filter. This math is probably still quite wrong, but with the right values it works
deemp0, deemp1 = DP['video_deemp']
[tf_b, tf_a] = sps.zpk2tf([-deemp1*(10**-10)], [-deemp0*(10**-10)], deemp0 / deemp1)
SF['Fdeemp'] = filtfft(sps.bilinear(tf_b, tf_a, 1.0/self.freq_hz_half), self.blocklen)
# The deemphasis filter
deemp1, deemp2 = DP['video_deemp']
SF['Fdeemp'] = filtfft(emphasis_iir(deemp1, deemp2, self.freq_hz), self.blocklen)

# The direct opposite of the above, used in test signal generation
[tf_b, tf_a] = sps.zpk2tf([-deemp0*(10**-10)], [-deemp1*(10**-10)], deemp1 / deemp0)
SF['Femp'] = filtfft(sps.bilinear(tf_b, tf_a, 1.0/self.freq_hz_half), self.blocklen)

SF['Femp'] = filtfft(emphasis_iir(deemp2, deemp1, self.freq_hz), self.blocklen)

# Post processing: lowpass filter + deemp
SF['FVideo'] = SF['Fvideo_lpf'] * SF['Fdeemp']
#SF['FVideo'] = SF['Fdeemp']
Expand Down
12 changes: 12 additions & 0 deletions lddecode/utils.py
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Expand Up @@ -521,6 +521,18 @@ def __call__(self, infile, sample, readlen):
# This can be complex multiplied with the raw RF to do a good chunk of real demoduation work
#fft_hilbert = np.fft.fft(hilbert_filter, blocklen)

def emphasis_iir(t1, t2, fs):
"""Generate an IIR filter for 6dB/octave pre-emphasis (t1 > t2) or
de-emphasis (t1 < t2), given time constants for the two corners."""

# Convert time constants to frequencies, and pre-warp for bilinear transform
w1 = 2 * fs * np.tan((1 / t1) / (2 * fs))
w2 = 2 * fs * np.tan((1 / t2) / (2 * fs))

# Zero at t1, pole at t2
tf_b, tf_a = sps.zpk2tf([-w1], [-w2], w2 / w1)
return sps.bilinear(tf_b, tf_a, fs)

# This converts a regular B, A filter to an FFT of our selected block length
def filtfft(filt, blocklen):
return sps.freqz(filt[0], filt[1], blocklen, whole=1)[1]
Expand Down