[hpsdr] Is there a DSP or DttSP guru in the house?

[Bill Tracey]

Do we have any DSP or DttSP folks on the list?

Looking to clean up our support on the Janus prototype for mixed sampling 
rates.  Currently our system runs all of the channels (IQ in, mic in, 
received audio out,  IQ out) at the same sampling rate.   We want to change 
this such that we can run IQ in at 48, 96 or 192 khz and hold the other 
three channels at 48 khz sampling.   My understanding of the current DttSP 
and the PowerSDR code is that there is one and only one sampling rate for 
all channels, so I'm looking at doing this by upsampling/interpolating the 
mic in channel to the IQ in rate and then downsampling the IQ out and 
received out streams when IQ in is running at 96 or 192 khz.

I believe the output side downsampling is easy  - just throw away the 
appropriate number of samples.   The input side is not so clear to me.  If 
I zero stuff to upsample I believe I will get images of the spectrum at 
multiples of the original sampling rate.    This would suggest i need to 
follow my upsampling with some sort of low pass filter to remove the extra 
image.

Does anyone know if this is actually needed with the current DttSP/PowerSDR 
code?  I'm sort of hoping there's already an LPF in the mic input chain 
that will take care of the images I'm introducing by upsampling.  If not, 
anyone have any suggestions/code pointers to some code to use to do the 
needed filtering.

Regards,

Bill (kd5tfd)




 1147617298.0