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[Discuss-gnuradio] Re: gr_pfb_clock_sync_ccf.cc question related to work

From: John Andrews
Subject: [Discuss-gnuradio] Re: gr_pfb_clock_sync_ccf.cc question related to work function
Date: Mon, 22 Nov 2010 03:01:10 -0800

I got it. The polyphase filter is implemented by having a single stage filter and choosing the right taps from the set of 'nfilts' taps rather than having nfilts filter stages. This of course is computationally more efficient.

On Sat, Nov 20, 2010 at 9:47 PM, John Andrews <address@hidden> wrote:
I was trying to understand the code of the new clock sync block gr_pfb_clock_sync_ccf.cc and although I understand most of it there is one particular thing that confuses me. This is what I understood so far

1. There are two filter banks present. A) For RRC filtering B) the differential filter bank
2. These filters are implemented as polyphase filters thus, each of these filters has nfilts=32 sub-filters
3. The theory behind this block is found in section 13.3.2 of fred harris' book on multi-rate filters so I understand the concept behind this block pretty well.

What confuses me is the coding.
In line 265 of gr_pfb_clock_sync_ccf.cc

out[i] = d_filters[d_filtnum]->filter(&in[count]);

the input to the block is passed only to one sub-filter of each filter bank. Why? Looking at the section in harris' book i thought it was an input to all the sub-filters and we choose one of the outputs as the sync'd sample.


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