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Re: explaining i/q

From: david vanhorn
Subject: Re: explaining i/q
Date: Wed, 4 Nov 2020 12:56:13 -0700

You can lead a horse to water...

Then there's hams like this: https://g3rbj.co.uk/

On Wed, Nov 4, 2020 at 12:04 PM Kristoff <kristoff@skypro.be> wrote:

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 .
> https://m.youtube.com/watch?list=PLvOgjCaG0WzDAF1Um894vv95mrcyortOB&v=h_7d-m1ehoY
>> On Nov 4, 2020, at 7:52 AM, Kristoff <kristoff@skypro.be> 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.

K1FZY (WA4TPW) SK  9/29/37-4/13/15

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