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Re: explaining i/q

From: Fons Adriaensen
Subject: Re: explaining i/q
Date: Wed, 4 Nov 2020 23:26:09 +0100

On Wed, Nov 04, 2020 at 05:14:00PM +0100, Kristoff wrote:
> For us, "even if we would be able to look at a rotating object up-front and
> from a 90 degrees angle at the same time, if the object would be frozen in
> time we would still not be able to determine if the doll rotates left of
> right".

The way complex samples are treated in theory means that the I and Q
parts refer to the same time, They are considered to be a single
sample having a complex value.

But you can never detect rotation (or a frequency in DSP terms) from
a single sample, be it real or complex. You always need a sequence.

A sequence of real valued samples (either I or Q separately) allows
you to detect the rotation, but the sense remains ambiguous. Having
both the I and Q sequences resolves that ambiguity. 

You could object that using complex samples requires twice the 
memory for the same sample rate, since every sample consists of
two numerical values. But since you can now separate positive
and negative frequencies, you get twice the bandwidth as well. 
So there is no penalty for using complex signals.


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