Hello,

For a project which I'm working on, I have a 60Hz sinewave modulated with a
square wave at 0.5Hz, modulation is 2.72%

The signal comes sampled at 8kHz, I should downsample it to 200Hz, so M =
40. I do that in two stages, first with M1 = 10, then with M2 = 4. I
implemented polyphase structure in Simulink, and runs great. By "great" I
mean that the output signal, sampled at 200Hz, preserves the squarewave
shape perfectly.

Ths structure used in Simulink is the one in P.P. Vaidyanathan's book
"Multirate Systems and Filter Banks", figure 4.3-4 (a), page 124.

However, In my C implementation (on SHARC, BTW), the downsampled signal
looks weird. By "weird" I mean that the envelope of the output signal,
instead of being flat (square) as it is expected to be, seems more like a
low frequency sinusoid, still it is noticeable when the amplitude rises and
falls at the same frequency of the original modulating square wave.

Given this information, and the fact that Simulink implementation maps
directly to the figure in Vaidyanathan's book, I am pretty sure that I have
some mistakes in my C implementation, but I can't find them.

Could someone here please give me advice on what mistakes in implementing
the polyphase M-fold decimation filters lead to the sort of "weirdness" that
I got from my C implementation? That would be of much support in finding
what I did wrong.

If someone who desires to help me wants to have a look at the Simulink Model
that I implemented, to the output of my C (for SHARC) implementation, and/or
to my C implementation itself, please let me know, and I will be more than
pleased in sending it to you.

Kindest regards,

--
Jaime Andrés Aranguren Cardona
[email protected]
SanJaaC Electronics
Soluciones en DSP
www.sanjaac.com

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