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Re: taps for a Raised Cosine function

From: Gary Schafer
Subject: Re: taps for a Raised Cosine function
Date: Tue, 10 Aug 2021 16:36:13 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0


That pulse shaping is *identical* to that for a RDS signal. I'm able to demodulate the 
RBDS signal using a RRC filter in a "Polyphase Clock Sync" block. Adjusting the 
parameters from the RDS signal to that of your BBC signal (1187.5 Hz bit rate to 25 Hz 
bit rate), you'd get the following parameters for the various blocks:

Polyphase Clock Sync:
Samples/symbol: 50
Loop bandwidth: 2*pi/100
Taps: rrc_taps_rx
Filter size: 50
Initial phase: 1
Maximum rate deviation: 1.5
Output SPS: 1

RRC Filter Taps:
Id: rrc_taps_rx
Gain: 1
Sample rate: 1250
Symbol rate: 25
Excess BW: 0.35
Num Taps: 550

At the very least, if my hypothesis is correct, that *should* get you a 
constellation that is roughly a donut (as opposed to a big ball of dots).

Hope this helps.



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:

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.

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.


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 

- use firdes.pm_remez() ?

Thanks in advance,


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