Re: taps for a Raised Cosine function

[Kristoff]

Hi Fabian,

Thanks, i'll try both options and compair them.


I remember from my books on DSP it is possible to combine multiple 
consecutive FIT filters into one, so I did a little bit of more research 
on this.


I found (*) that -as convolution as associative- this implies this:

(( f * h1) * h2) = f * (h1 * h2)



So I guess I just use firdes.root_raised_cosine to calculate the RRC and 
then "numpy.convolute()" it with itself, I should get the RC.

Feel free to correct me if I am wrong. :-)



(*) 
https://dsp.stackexchange.com/questions/20194/concept-of-combining-multiple-fir-filters-into-1-fir-filter


Kristoff



On 10.08.21 21:01, Fabian Schwartau wrote:
> Hi Kristoff,
> You can just put an additional RRC in front of it or you can calculate 
> the tabs by folding die tabs of the RRC with themself. There is 
> probably a numpy function to do that. Have not tested any of that, but 
> both ways should do the trick.
>
> Fabian
>
> Am 10. August 2021 20:44:05 MESZ schrieb Kristoff <kristoff@skypro.be>:
>
>     Hi all,
>
>
>     I am playing around with PSK demodulation, now doing a small project
>     trying to decode the slow-speed PSK data on the 198 KHz carrier of BBC
>     radio 4.
>
>     The specs are here:
>     http://downloads.bbc.co.uk/rd/pubs/reports/1984-19.pdf  
> <http://downloads.bbc.co.uk/rd/pubs/reports/1984-19.pdf>
>
>     According the document (page 2 of the document, page 8 of the PDF), the
>     system uses a shaping-filter of cos(pi * f * Td / 4).
>     (with Td = 50)
>
>     If I am correct, this is in fact a raised-cosine filter with beta = 1.
>     https://en.wikipedia.org/wiki/Raised-cosine_filter  
> <https://en.wikipedia.org/wiki/Raised-cosine_filter>
>
>     So the document seems to specify a raised-cosine filter, both on the
>     transmit and receive side; so not a ROOT raised-cosine as is used in
>     most PSK systems.
>
>     Question:|
>     How do I create the filter-taps for the Polyphase clock sync block for a
>     raised-cosine filter?
>
>     I found a function firdes.root_raised_cosine, but no firdes.raised_cosine?
>
>     Is there a way to easily convert the taps of a RRC filter to those of a
>     RC filter? (RC = RRC^2, so square the value of all taps?)
>
>
>
>     Any other ideas?
>     - put an additional RRC low-pass filter in front of the Polyphase
>     clock-sync block?
>     - use firdes.pm_remez() ?
>
>
>
>     Thanks in advance,
>
>
>     Kr.