gr-analog : improve the filter quality of wfm_tx block

The resulting filter has much more stop band attenuation.
This filter will reduce the interpolation replicas.

Plus, the filter complexity is less (approx 2 times less coefficients).

The benefits can be seen on the example fm_tx.grc:
- the sprectrum adajacent channels are cleaner
- the output fm signal in time is smoother.

Signed-off-by: Adrien Michel <adriengit@users.noreply.github.com>

gr-analog/python/analog/wfm_tx.py

@@ -51,13 +51,10 @@ def __init__(self, audio_rate, quad_rate, tau=75e-6, max_dev=75e3, fh=-1.0):
 
         if do_interp:
             interp_factor = quad_rate // audio_rate
-            interp_taps = filter.optfir.low_pass(interp_factor,   # gain
-                                                 quad_rate,       # Fs
-                                                 16000,           # passband cutoff
-                                                 18000,           # stopband cutoff
-                                                 0.1,             # passband ripple dB
-                                                 40)              # stopband atten dB
-
+            interp_taps = filter.firdes.low_pass(interp_factor, # gain
+                                                 quad_rate,     # Fs
+                                                 19000,         # cutoff_freq
+                                                 4000)          # transition_width
             print("len(interp_taps) =", len(interp_taps))
             self.interpolator = filter.interp_fir_filter_fff(
                 interp_factor, interp_taps)