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Re: [Discuss-gnuradio] DDC and Polyphase Channelizer

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] DDC and Polyphase Channelizer
Date: Thu, 13 Jan 2011 18:03:58 -0500

On Wed, Jan 5, 2011 at 4:14 AM, Jimmy Richardson <address@hidden> wrote:
> 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?

No, the sample rate as a real number doesn't enter into the code, but
it does abstractly in where the signals exist, what the channel
bandwidths mean, etc. I'm not sure what else to say here, really, just
to think closely about the signals and the channelizer. Again, it
could be that there is a bug in the code that only worked for my

>> 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?

No, they aren't. It shouldn't be difficult to modify the example codes
that are there, though, to see what happens and make sure it makes


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