Re: explaining i/q

[Christophe Seguinot]

Hi

I do agree with Jeff Long.

You will have to distinguish Sampling and IQ:

• Complex signal are not related to sampling, even if sampled signals in GR are generally complex
  
• Complex signal come from the baseband equivalent representation of  passband signal (modulated). Explaining that with at least some background in complex number, phase of a modulated signal, and/or ID modulator and demodulator.
  
• Sampling is an operation which  aims at giving an approximate representation of time continuous signals. It is applied to real signal as well as complex signals. Continuous signal can't be represented in a computer.
  

Regards, Christophe

On 04/11/2020 13:50, Kristoff wrote:

> Jef,
>       
>       
>       
>       Concerning the term "slope". Well, I also have my doubts about it.
>       I think that for a lot of people, this would create the assumption
>       that the signal then goes from the 'i' value to the 'q' value in a
>       straight line, which is -as we know- not the case.
>       
>       
>       Sometimes it helps to -at first- give a very basic mental image of
>       something, and -at the end, when people understand the topic-
>       "correct" that image with a more correct one, or just point them
>       to some youtube video that explains the topic in more detail.
>       
>       
>       
>       Anycase,this is indeed all an interesting exercise in braking down
>       concepts into very small steps.
>       
>       
>       The amateur-radio community is a bit strange as most people do
>       have a technical background, but for a large number of hams, that
>       is mainly based on assumptions or "that's what they said in the
>       ham-radio courses", without understanding the full technical
>       details, especially topics that are highly based on math.
>       
>       For most hams, "SDR" is just "that piece of software you install
>       on your computer to look at  waterfall graphs".
>       
>       
>       So we have a very long way to go. :-)
>       
>       
>       
>       73
>       
>       kristoff - ON1ARF
>       
>       
>       
>       
>       On 4/11/2020 02:21, Jeff Long wrote:
>       
> > It's more important to give people some
> >         mental picture than to make sure it's completely correct. But, I
> >         would not use the "slope" terminology. The important things are,
> >         as you've said, (1) with the complex type, you can have a signal
> >         at baseband that is not symmetric, and (2) the price for this is
> >         doubling the amount of data needed. The signal you deal with at
> >         baseband is the same signal that is seen centered on the RF
> >         carrier.
> >         
> >         
> >         I don't see a great way to talk about "phase" without going into
> >         the math. It is important to get into "phase" when you talk
> >         about any modulation fancier than slow FSK.
> >         
> >         
> >         Good luck. Hope you find the right balance between useful,
> >         digestible, and correct.
> >         
> >         
> >         On Tue, Nov 3, 2020 at 7:20 PM David Hagood
> >         <david.hagood@gmail.com
> >         <mailto:david.hagood@gmail.com>> wrote:
> >         
> >         
> >             I am sorrowful that you have decided you are going to stick
> >         with an
> >         
> >             explanation that is fundamentally incorrect. I know how
> >         direct
> >         
> >             conversion systems work - I design the software for them for
> >         a
> >         
> >             living.
> >         
> >             What you are basing your mental model on is an optimization
> >         for
> >         
> >             the case
> >         
> >             where the system is both sub-sampling the signal and going
> >         digital in
> >         
> >             the same operation. However, in many extremely high sample
> >         rate
> >         
> >             systems,
> >         
> >             the signal is brought down to baseband by mixing it with
> >         analog
> >         
> >             quadrature signals - that's the place where I and Q come
> >         from - and I
> >         
> >             assure you the only "delay by 90 degrees" is in the creation
> >         of the
> >         
> >             quadrature LO signals, not in the sampling of the actual
> >         data. But
> >         
> >             I've
> >         
> >             been around the Sun enough times to know that since you have
> >         decided
> >         
> >             upon this course and don't seem to want to change, there's
> >         no
> >         
> >             point in
> >         
> >             continuing to try to help.
> >         
> >         
> >         
> >       
>       
>       
>