I am trying to demodulate an AM signal in GNU Radio, and I am having problems with the sampling rates. I will start off with an example where the flow graph was working. The sampling rate is 256kHz, and it works just fine:
Next, I tried a sampling rate of 192kHz, and then it didn't work. Is there a problem with this flow graph?
Next, I modified the 2nd flow graph (changed the sampling rate, but didn't need to), and it worked almost perfectly, other than that the voice sounded very slightly too low pitch/slow. This sampling rate suffered from the same problem as that of figure 2, however I tweaked the rational resampler block until the voices sounded normal.
As some background, I am running an RTL SDR (DVB-T) dongle, on an Odroid U3 (ARM architecture). I have a "Ham it up" upconverter, so that the AM (810Kkz) is shifted up by 125MHz, into the VHF band that the dongle can actually receive.
UPDATE: There were two issues with the flow graph:
The filter cutoff frequency needed to be calculated using normalized frequency (see MBaz's detailed answer).
The R820T chip does indeed, as MBaz ascertained in the comment section, have only a limited set of sampling rates that it supports.
In order for the flow graph to output natural sounding audio, it must use one of the acceptable sampling rates, and also have the proper filter setting. The 2nd and 3rd flow graphs didn't work because they had the wrong sample rates.