RE: [Discuss-gnuradio] fractional_interpolator_cc?

[Bob McGwier]

Whenever you do an efficient implementation of the rational ratio resampler,
you pick out one of the polyphase arms to do the actually evaluation of the
filter taps and skip all of the implied zeros in the upsampled signal (for
the obvious computational advantage).  BUT, you can pick which "phase" or
step to start evaluating the filter.  This is the genesis of the term
"polyphase".

HOWEVER, the fractional interpolator is NOT the rational resampler which is

http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/lib/filter
/gr_rational_resampler_base_XXX.cc.t

and the fractional interpolator is intended for use with resampling ratios
that would require HUGE filters to be partitioned in order to achieve the
rational resampler.  Since all digital computers are by their nature,
rational number machines,  it would be possible, but onerous.



Bob


ARRL SDR Working Group Chair
Member: ARRL, AMSAT, AMSAT-DL, TAPR, Packrats,
NJQRP, QRP ARCI, QCWA, FRC.
"Trample the slow ....  Hurdle the dead"


-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Brian Padalino
Sent: Tuesday, October 21, 2008 10:34 AM
To: Clark Pope
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] fractional_interpolator_cc?

On Tue, Oct 21, 2008 at 10:15 AM, Clark Pope <address@hidden> wrote:
>
> I'm not able to get this function to work correctly. I have it just
connected between a snapshot file and an output file. I set the
interpolation factor to 2.0 and the output file ends up half the size of the
input file. It's like it is a decimation block instead of interpolation?

How does the rational resampler work out - or can you not represent
the ratio as a rational number?

Are you calling it correctly?  It seems as if there is a phase shift
as well as an interpolation ratio being passed in.

 
http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/lib/filter
/gr_fractional_interpolator_cc.cc#L38
 
http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/lib/filter
/gr_fractional_interpolator_cc.cc#L67

There may be an issue here:

 
http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/lib/filter
/gr_fractional_interpolator_cc.cc#L49

Might be upside down?

Brian


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