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Re: [Discuss-gnuradio] atsc_cpll finally works

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] atsc_cpll finally works
Date: Fri, 23 May 2008 11:07:50 -0700
User-agent: Mutt/1.5.17 (2007-11-01)

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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