[Discuss-gnuradio] Interpolation before USRP sink

[Alberto Trentadue]

Hello to the list

I am trying to build a simple SSB transmitter using GRC - GR-3.2.2.
I use the 32Khz sampled audio source and bandpass it to reduce bandwidth to 
minimum.
In order to match USRP usable interpolation, I have to set an interpolation by 
10 to the bandpass, so that 32K * 10 * 
400 = 128MSPS.

I've actually managed to get the RF signal out of the USRP, but it isn't really 
satisfactory.
The 10-interpolated spectrum contains my baseband signal's spectral replicas 
spaced by 32kHz, as it always is the 
case when interpolate.
My problem is that if I put a lowpass filter between the bandpass and USRP 
sink, working at 320kHz, the CPU cannot 
keep up and I get uUuUaOaO like raining.

I use an AMD-3X Phenom with 3x2.3GHz cores and Ubuntu 9.04.
Actually I see that 1 of the cores jumps to full 2.3G speed and gets stuck at 
100% usage.

>From the theory I understand that we should lowpass after interpolating.
Is there any more efficient way to get rid of the spectral replicas, working at 
320kSPS?

Thanks in advance for any suggestion.

Alberto


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