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Re: explaining i/q

From: Fons Adriaensen
Subject: Re: explaining i/q
Date: Wed, 4 Nov 2020 12:25:48 +0100

On Wed, Nov 04, 2020 at 12:38:27AM +0100, Kristoff wrote:
> So, what is iq-sampling?
> IQ-sampling is like sampling a normal ("real") signal -i.e. what most people
> are familiar with-, ... except that you sample the data twice for each
> period: once at timer "t" and a second time 1/4 sampling period later. (*)

This is fundamentally wrong, and you will do your audience
a very bad service.

What are you going the answer if anyone in your audience
asks the obvious question:

   So, it's the same as having four times the sample rate,
   and then throwing away half of the samples ? Why would
   I want to do that ?

The 'spinning dancer' is really the right intuitive model.

You look at a rotating object from two points, the second at
90 degrees from the first. This allows you to find out in 
which sense it rotates.

For example it allows you to know if some frequency is in
the upper or lower sideband, even if the signal shifted to
zero carrier frequency.

There is another good reason to use complex-valued signals:
it really simplifies all DSP theory. A lot of strange factors
of 2, corner cases at or around DC,  and other anomalies that
occur with real-valued signals just disappear.  

Whenever I have to explain some DSP principles, I start with
the complex valued version, and then showing the real-valued
one as a special case.


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