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Re: explaining i/q

From: Kristoff
Subject: Re: explaining i/q
Date: Wed, 4 Nov 2020 19:46:11 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0


A small (slightly) remark about this video, and about hams.

When I gave my first video-presentation for the Belgian SDR Meetup (in September), I have a presentation on GR (an example of an RTTY decoder). But, to keep the presentation on topic, I first posted a "list of interesting things to view so you can better understand the presentation" (the video you mentioned, three of the videos by Michael Ossmann, ...).

When I asked the audience during the presentation who had taken the time to actually do this, I did not get any positive answers.

You know,, ...last time when we did a workshop in a hackerspace on a certain topic and asked the people to do some preparation, I think that more then 3/4 did do that.
The same when I organise a workshop at work.

Yeah ... Hams .. (sigh) :-(

kristoff - ON1ARF

On 4/11/2020 15:22, Don Wade wrote:
Here’s a YouTube video that’s got a bit of pencil math (so it doesn’t drone on) and oscilloscopes (for the ham guys), so it’s got a bit for everyone .


On Nov 4, 2020, at 7:52 AM, Kristoff <kristoff@skypro.be> wrote:


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. :-)

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
   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
   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
   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.

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