Re: [Discuss-gnuradio] isolate channels from wideband

That would be extremely awesome. Especially for people with some DSP
filter experience, the Polyphase filterbanks are somewhat hard to
understand, and I think Tom's article / presentation (which I can't seem
to find right now) on the PFBs can only profit from a real-world example
accompanying their message.

--Marcus

On 21.07.2015 20:56, Chris Kuethe wrote:
> Maybe I'll do up an illustrated example on this using NOAA weather
> radio, or the pager band
>
> On Tue, Jul 21, 2015 at 11:42 AM, <mleech@ripnet.com> wrote:
>> I just use the built-in firdes stuff, rather than using an external
>> designer.
>>
>>
>>
>>
>>
>>
>>
>> On 2015-07-21 14:38, Marcus Müller wrote:
>>
>> Hi Rich, hello Markus,
>>
>> On 21.07.2015 19:51, Richard Bell wrote:
>>
>> GNU Radio has channelizers built-in, but I've not used them yet, so I don't
>> know how far they take you into this kind of task.
>>
>> the Polyphase channelizer is actually an implementation derived from that
>> school of thought, and it works amazingly well.
>> In fact, in preparation of a presentation at a certain ham conference, I
>> tried using it to get 20 PMR/LPD channels out of a 1MS/s signal in real
>> time, and then just shuffle them around, before feeding them back into the
>> inverse synthesizer PFB.
>>
>> It's pretty easy:
>> Design a single low pass filter, as if you just wanted to filter out the
>> channel which is centered exactly at your RF center frequency, i.e. 0Hz,
>> with the full sampling rate [2], using the gr_filter_design tool. Play
>> around with the different window types[1], and bear in mind that the
>> suppression outside your desired passband needs to be high enough so that
>> the sum of the energy in all other channels don't hurt your channel too
>> much, but don't overdo it (60dB suppression should be enough).
>> Now you get a long filter. Copy and paste the filter coefficients from
>> gr_filter_design to your PFB filter taps property.
>> Set your channelizers number of channels according to your plans -- 40, if
>> you want to get all the 40 25kHz channels in 2MHz. You get a block with 40
>> outputs!
>> Explaining things like channel mapping is best done by pointing you at the
>> official documentation: [3]
>>
>>
>> Greetings!
>> Marcus
>>
>> [1] Hamming is not always the best choice, I'd try that, Blackman-harris,
>> and Kaiser. I personally like harris in this case -- we want to get a full
>> channel, two adjacent channels are usually not occupied, and as soon as we
>> pass the stopband frequency, we're basically at -100dB.
>> [2] assuming you want to use 2MS/s for your 2MHz wide band, 2MHz sampling
>> rate, and assuming 25kHz wide channels, 12.5kHz cut off frequency, 25kHz
>> start of stoppband. I get something like 440 taps.
>> [3]
>> https://gnuradio.org/doc/doxygen/classgr_1_1filter_1_1pfb__channelizer__ccf.html
>>
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