Am Sonntag, den 15.07.2012, 01:22 +0200 schrieb Francesco Gugliuzza:
2012/7/14 Michael Karcher osmosdr@mkarcher.dialup.fu-berlin.de:
I furthermore found out that this is not the only filtering going on in the RTL2832 chip in SDR mode. There seems to be a second anti-aliasing filter before the DAC, making signals disappear that are at frequencies above 0.7 times the sample rate (in the baseband), even if the programmable FIR filter lets them pass.
That's probably an effect of the E4000 output filter. You can see the filter bandwidth being set in the tuner initalization code.
Good point, but actually, I am sure it is not an effect of the tuner output filter. My USB stick contains a fc0012 tuner, which is always set to 6MHz bandwidth. The bandwidth of the suspected anti-alias filter changes clearly when I change the sample rate, though, on the other hand, if I manipulate the FIR coefficients to contain sharp dips in the range of hundred kHz, the dip position does not change when I change the sample rate. This makes me cofident that the suspected anti-alias filter has to be located in or after the resampling unit.
Regards, Michael Karcher