Re: FIR Hilbert transformers

Hi!

> The signal consists of frequencies that are all close to the carrier.
> Those terms in the computation that involve the product of two of those
> frequencies yield frequencies approximately double the carrier's, and
> possibly DC. Those are easily filtered out. What remains is what you want.

Ah.. Very narrow band signal. That is not what I had.
Now I understand :)

So one method for envelope detection would be:
1.) bandpass filter
2.) abs(analytical signal).

And the bandpass filter would probably be a pair
of two filters multiplied one with cosine and the other
with sine of FS/4 so that the resulting two time series
would be exactly 90 degrees a part. All done without
a hilbert transform and probably fairly wide bandpass
filter.

But that still does not explain why using abs(real signal)
would not be good enough, if the envelope has a low
enough frequency. Although the bandpass design really
does seem to give a very clean result without the need for
very sharp lowpass filters (optionally implemented with a pair of
decimation/interpolation.)

> The envelope of a single-sideband signal is not its modulation. (That's
> why an ordinary AM (envelope) detector won't demodulate it.

I dont see why. If the envelope is there, it is there... (looking only
at values bigger than zero).

>> What is polarity? (not an electrical engineer).
>
> The envelope has both negative and positive parts; an AM envelope is
> symmetrical about the time axis -- there are two curves. The demodulated
> signal is either the all-positive or all-negative part.

Humm... But abs(analytical signal) gives only one part.
So that implies that you would really have to follow the peaks
to see the difference in the envelope between all-positive and
all-negative part. Where in real world do you need to have
info like that? Is upper and lower envelope related to upper
and lower sideband?

Thanks!
Atmapuri



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