Re: taps for a Raised Cosine function

Hi gary, Fabian, all,


Small note:


The first tests seems to indicate the the rrc-filter works! Yes!

- I can decode large chunks of data with valid Manchester-encoding.
However, from time to time, I do get large blocks of data (10 to 20
seconds) with errors.
- Changing the value of alpha does not appears to have that much of an
influence.


I'll record a large audio-file of a couple of hours and so that I can do
a multiple tests all based on the same "reference" signal before making
a final statement about this.


I did notice that the costas-loop (in front of the polyphase clock-sync)
acts strange.

In this system, the phase-shift of the carrier is not shifted 180
degrees (as most PSK systems) but only 45 degrees (i.e., +- 22.5 degrees
compared a central reference). It looks like the costas-loops has issues
with that.
Sometimes, the points on the constellation-plot are on the right side of
the graph (i.e. above and below 0 degrees); and somethings they hover
above and below the 180 degrees point (on left side of the constellation
graph).
And this changes randomly.

I also have the impression that the polyphase clock-sync block is able
to do a better job when the constellation-points are around 180 degrees
then when they hover around 0 degrees as I see a lot more biterrors when
the points on the constellation-graph are on the right side of the graph.


I think we've got some more experimentation to do! :-)



Thanks for the help!


Kristoff


On 10.08.21 22:36, Gary Schafer wrote:
> Kr,
>
> 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.
>
> Gary
>
> **********************************************************************
>
> 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
>
> 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
>
> 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.
>
>
>
>