rx_fm tuning not accurate

[rrobinett]

In addition to the noisy audio problem I describe in an earlier issue, I am unable to accurately set the tuning frequency of my RTL-SDR V3 when using rx_fm.

Using CubicSDR I can tune to the pilot carrier of a local ATSCTV broadcast station at 210.310 Mhz by tuning to USB at 210.309Mhz and hear a clean (approximately) 1 KHz tone. As I expect that audio tone reliably changes frequency when I change the tuning frequency is 1 Hz steps. That suggests that CubicSDR communicates a very accurate tuning frequency to the RTL-SDR through the Soapy API.

In contrast, when using rtl_fx and rx_fm tuned to 210.309Mhz the audio tone differs from the 1000 Hz heard with Cubic. Worse than that, the audio tone changes in jumps when I tune up and down from 210.309 Mhx in 1, 10, 100 and even 1000 Hz steps. That suggests to me that both rtl_fm and rx_fm are truncating or otherwise mangling the translation of the tuning frequency in its translation into the value programmed into the RTL-SDR chipset.

As a first step in debugging this, I wonder if rx_fm could use the Soapy drivers supplied in CubicSDR.

Thanks for any guidance you can supply