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Re: [Discuss-gnuradio] atsc_cpll finally works

From: Achilleas Anastasopoulos
Subject: Re: [Discuss-gnuradio] atsc_cpll finally works
Date: Sat, 24 May 2008 21:13:13 -0400
User-agent: Mozilla Thunderbird 1.0.2 (Windows/20050317)

Chuck, Eric,

I think there is a way to perform the cPLL at 8 complex Msps and upsample to 16Msps only at the very end when you want to get the Real signal out for further processing. I believe this works (i didn't see any point where there is a possibility for aliasing) and it can result in some savings in complexity without sacrifising performance.
Hopefully I didn't miss anything big...

Take a look at the block diagram here:



Eric Blossom wrote:
On Fri, May 23, 2008 at 09:00:31AM -0400, Achilleas Anastasopoulos wrote:

OK, I see.

1) Usually in fractional interpolators the trade-off is: more taps vs finer sampling. It might be possible to increase the number of taps and still be able to work with only a 12.8 (or 16) real-Msps. Having said that, I have to admit I haven't looked at exactly what this particular timing loop is doing.

The current resampler requires only 8 taps at each possible fractional
point (1/128).  It manages this by having a frequency response that is
unconstrained between Fs/4 and Fs/2.  That's the source of the
constraint on sample rate and occupied bandwidth.  The filter design
code is in gnuradio-core/src/gen_interpolator_taps.  See the README
there for additional info and references.

We could use a different fractional interpolator, perhaps the one by
Julius O. Smith:  http://ccrma-www.stanford.edu/~jos/resample.
With it, 12.8 real-MS/s or 16 real-MS/s should be possible.


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