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jaac · Guest

Hello guys,

I've been struggling with algorithms for sample rate conversion, namely
decimation by a factor of 40, to convert from 8kHz sample rate to 200
Hz.

I've started with the block diagrams from P.P. Vaidyanathan's book
"Multirate Systems and Filter Banks" implementing them in Simulink, and
then writting my own C code. From the Simulink model everything was OK.
But my C code (running on a SHARC, BTW) gave me some sort of modulation
on the decimated signal.

I asked for support, people here guided me to the code available at
DSPGuru. I understood it, implemented it, and gave me exactly the same
results as my original C code.

Moreover, I did the decimation by brute force, which means a long
lowpass filter, then discarding 39 out of 40 samples, and again the
same results.

I've been wondering why does it work perfectly in Cimulink, and not in
my code. In my test, the signal to be downsampled is a carrier at 60Hz,
with the "information" modulating this carrier, AM. The strange thing
is that even if I use a pure 60Hz tone as input, I get a modulated
output with my code, not with Simulink.

Someone suggested slightly changing the carrier frequency to say 60.37
Hz, and voilà: even in Simulink it appears that the decimated signal
seems to be modulated.

The conclusion is: I can't do the sample rate conversion for my
application, because MY information is on the envelope of the carrier,
because it's AM modulation.

But my BIG question is: is it TRUE that sampling rate conversion CAN
change the envelope of the output signal? Isn't there a way to do
sample rate conversion which preservers the signal shape, regardless of
the relations between original sample rate, final sample rate and
upsampling/downsampling factors?

Thank you very much,

JaaC

Tim Wescott · Guest

jaac wrote:

Jerry Avins · Guest

jaac wrote:

jaac · Guest

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC

jaac · Guest

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC

jaac · Guest

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC

jaac · Guest

Hi Tim,

Thanks for prompt reply.

To be precise, my project is related to the measurement of the
phenomena named "flicker", which consists of amplitude variations on
the AC line, 50/60 Hz, 220 / 110 Vrms. That's why my information is
precisely in the envelope of the carrier, which here is 60 Hz (possibly
with fluctuations, of course).

As stated on the IEC standard, the demodulation is done by squaring,
then some filtering is performed, to eliminate DC component and
components higher than Fs/2. And then further filtering and analysis of
the data, and storage.

Your idea of upsampling before demodulation, excuse me, is not a good
one. For my case, it would be better to do all of the downsampling
after the demodulation. I know that it is the almost the same as you
suggested, it is just locating the sample rate conversion to the place
where it is more benefical. I bet that's what you had in your mind.

The idea of synchronous AM demodulation seems interesting. Could you
please elaborate on the subject, or point me to information about it?
Kindest regards,

JaaC

Tim Wescott · Guest

jaac wrote:

Philip Martel · Guest

"jaac" <jaime.aranguren@ieee.org> wrote in message
news:1104711708.340093.186300@f14g2000cwb.googlegroups.com...

jaac · Guest

Hi Phil,

Thanks for the reply.

You are right. I would get aliasing. Thank you for pointing this out.

To be honest the 200Hz Fs comes from originally having the carrier at
50 Hz in the IEC document, but this won't be my case. If I stay without
the sample rate conversion, and sampling directly at low frequency, I'd
better change it to 240Hz, and I'll have to redesign my filters (I'm
lazy to do that).

More interesting, I'll go for the decimation after squaring at 8kHz.
So, my bandpass filter, from 0.05Hz to 35Hz @ 200Hz will effectively
reject the aliased signals, I won't modify the original signal's shape
(important because my information is there), and more important, I'll
only need one ADC per channel, instead of two (one sampling at 8kHz for
spectral analysis, the other one sampling at 200Hz for waveshape
fluctuation analysis).

Thank you very much!

JaaC

Philip Martel · Guest

"jaac" <jaime.aranguren@ieee.org> wrote in message
news:1104773346.953976.218710@f14g2000cwb.googlegroups.com...

jaac · Guest

Philip Martel wrote:

jaac · Guest

Philip Martel wrote: