Incorrect samplerate from RTL-SDR

Jiří Pinkava j-pi at seznam.cz
Fri Jan 10 00:50:51 UTC 2014


You cannot fit internal FIR filter in SDR to such narrow bandwidth, it
does not have enough coefficients.
You can adapt it only to wider band width (some small improvement can be
done for
sample rates above 2048 kHz). Select proper values is a bit tricky and
specialised software
is almost unavoidable. You can get good hint from FIR filter designer in
GNU Radio, but
this software is not designed for this task and the work is cumbersome.

The 8 / 12 bit means the coefficients  can be only in range form -128 to
127 / -2048 to 2047.
The usual FIR coefficients absolute value decreases from center to the
side eg:

1, -3, 5, 10, 5, -3, 1,

so this just reduces complexity of the device.

For lower bandwidth you need more cofficients (more samples because the
wave is wider).

JP


Dne 9.1.2014 23:53, Richard Koch napsal(a):
> Hi Steve,
>
> In regards to your post about the anti-aliasing filter
> you referred to, it appears that it is getting set in
> librtlsdr.c, rtlsdr_init_baseband()
>
> If one wanted to change that to optimize the anti-aliasing
> to a lower bandwidth, say 1500 Khz instead of the 2048 Khz
>
> How would you change the fir_coeff[] table entries to achieve
> this? The 8 bit entries and 12 bit entries are confusing to me.
>
>
> ==== orig post ====
>
>> I have seen other reports on the mailing list, saying that sample rates
>> between 300 kS/s and 900 kS/s don't work. If this applies to all RTL
>> devices, then maybe it should be documented and the library can return
>> an error code for such sample rates. Otherwise people will keep walking
>> into this trap.
> We could return some sort of error in those cases, but such low rates
> (< 1MS/s) aren't recommended in general. First of all, the
> anti-aliasing filter we're using has a fixed bandwidth of 2 MHz, and
> although the coefficients can be changed, I doubt you can get a nice
> filter for lower bandwidths with such a low order. And then the ADCs
> only have 8 bits resolution, so you want to improve that by decimating,
> and also profit from decimation gain.
> Even Realtek uses 2.048 MS/s for FM reception in their original Windows
> software. 		 	   		  





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