Re: [Discuss-gnuradio] DDC and Polyphase Channelizer

[Jimmy Richardson]

Hi, Tom:

Please see my comments below.

Thanks

On 1/4/2011 11:44 PM, Tom Rondeau wrote:
> On Mon, Jan 3, 2011 at 11:25 PM, Jimmy Richardson<address@hidden>  wrote:
> > > Indeed. Very strange. My guess is that there is a misconception
> > > somewhere in the code about sample rate. I can't quite see it in my
> > > head, but I'm guessing that the channel spacing isn't exactly the
> > > channel spacing you think it is.
> > You mean the oversample rate O = N/i is wrong? As far as I can see, that's
> > the only number that could go wrong, since pfb_channelizer_ccf only takes 3
> > parameters, # of channels, taps and oversample rate, and it doesn't look
> > like the other two can be wrong.
> >
> > I checked the calculation of O again, but couldn't find the problem. In the
> > Matlab code, harris uses loops "for nn=1:28:5600-28" to do the processing,
> > so it does seems a 28-to-1 downsampling in 40 channels.
> No, not the oversample rate, just the sample rate. I'm thinking that
> there is some misunderstanding somewhere about the actual sample rate
> and the therefore the channel bandwidths such that the channels you
> are pulling out are not covering the same frequencies that you think
> they are.

Sorry, maybe I didn't get how the algorithm works, but it looks to me 
the actual sample rate does not enter into the equation, except in the 
filter generation part? If we use the same filter on both methods, then 
the actual sample rate should not cause the difference?

> On the other hand, it could be a miscalculation in the
> pfb_channelizer, although all of my tests with it came out fine.

Is the test cases included in the GNURadio source code? Also if I want 
to understand the logic inside pfb_channelizer implementation, are there 
any book or paper I should review besides harris' paper and book?

> > > Have you looked at the examples in gnuradio-examples/python/pfb?
> > > Specifically, channelize.py, chirp_channelize.py, and synth_to_chan.py.
> > I checked the first two samples when I wrote the channelizer, although it
> > seems they didn't use the oversample rate parameter. I couldn't find the
> > last one (synth_to_chan.py) though.
> >
> > > Tom
> The synth_to_chan.py is not in the tarball release. It was introduced
> a little while ago when I added the synthesis filterbank. You'd have
> to get it from the git repo.

Ok, I'll take a look.

> Tom