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| From: | david vanhorn |
| Subject: | Re: explaining i/q |
| Date: | Wed, 4 Nov 2020 12:56:13 -0700 |
Don,
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) :-(
73
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.
>>>
>>>
>>
>>
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