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NBFM Tx Block High quad rate results in filter error

From: Solomon Tan
Subject: NBFM Tx Block High quad rate results in filter error
Date: Mon, 21 Jun 2021 20:30:29 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

Hi all,

Could someone share why a high quadrature rate in the NBFM Transmit
Block results in a "Runtime Error: insufficient extremals"? I dont get
it. In my example, I wanted to NBFM modulate some audio. I resampled the
audio to 80kHz from 48kHz. These are the parameters in the NBFM Transmit
1. Audio Rate: int(80e3)
2. Quadrature Rate: int(8e6)
3. Tau: 75e-6
4. Max Deviation: 5e3
5. Preemphasis High Corner Freq: -1.0

When I run the flowgraph, I get the following error:
gr::log :ERROR: pm_remez - insufficient extremals -- cannot continue
Traceback (most recent call last):
  File "/home/solomonbstoner/learn_tx_hackrf.py", line 183, in <module>
  File "/home/solomonbstoner/learn_tx_hackrf.py", line 161, in main
    tb = top_block_cls()
  File "/home/ladmin/learn_tx_hackrf.py", line 98, in init
    self.analog_nbfm_tx_0 = analog.nbfm_tx(
"/usr/local/lib/python3/dist-packages/gnuradio/analog/nbfm_tx.py", line
52, in init
    interp_taps = filter.optfir.low_pass(interp_factor,   # gain
  File "/usr/local/lib/python3/dist-packages/gnuradio/filter/optfir.py",
line 45, in low_pass
    taps = filter.pm_remez (n + nextra_taps, fo, ao, w, "bandpass")
RuntimeError: insufficient extremals -- cannot continue

But when I change the Quadrature Rate to a lower value, int(80e3), the
flowgraph runs. Why? What does quad rate have to do with "extremals",
whatever that is?


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