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.
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.
-
Jiří Pinkava -
Richard Koch