RF complex mixing

On July 13, Phil Martel wrote:
>>> Watch a film of spoked wagon wheels.  At a certain 
>>> speed, the wheels appear to rotate backwards.  It's a 
>>> consequence of aliasing, due to the low sample rate of the 
>>> human eye.
>>> That's a  negative frequency.
> 
>> I'm going to disagree with this analogy, as it is an aliasing
>> effect and the original question applies to continuous time
>> signals.
> 
> The aliasing is due to the sampling of the visual signal by 
> the camera 

Quite.   

However, the aliasing effect (of the wheels) has also been demonstrated with the naked eye, via visual perception experiments.       

Anyhow, the point is that a negative frequency can 
be sensible, in some contexts.  Although it's subjective, 
as the negative freq. (the wheel apparently spins 
backwards) is identical to a positive frequency, another 
artifact of the aliasing.

--
Rich

On Sat, 14 Jul 2018 23:14:22 -0700 (PDT), RichD
<r_delaney2001@yahoo.com> wrote:

>On July 13, Phil Martel wrote:
>>>> Watch a film of spoked wagon wheels.  At a certain 
>>>> speed, the wheels appear to rotate backwards.  It's a 
>>>> consequence of aliasing, due to the low sample rate of the 
>>>> human eye.
>>>> That's a  negative frequency.
>> 
>>> I'm going to disagree with this analogy, as it is an aliasing
>>> effect and the original question applies to continuous time
>>> signals.
>> 
>> The aliasing is due to the sampling of the visual signal by 
>> the camera 
>
>Quite.   
>
>However, the aliasing effect (of the wheels) has also been demonstrated with the naked eye, via visual perception experiments.       
>
>Anyhow, the point is that a negative frequency can 
>be sensible, in some contexts.  Although it's subjective, 
>as the negative freq. (the wheel apparently spins 
>backwards) is identical to a positive frequency, another 
>artifact of the aliasing.
>
>--
>Rich

Actual aliasing isn't subjective, because it is completely and
consistently predictable if the actual rotation rate and the sampling
rate are known.   Negative frequencies will always make sense with
rotational systems where rotation can be (or appear to be) reversed.

Visual aliasing is pretty common at night under artificial lighting
where the lights provide the sampling.   Incandescent, fluorescent,
and many other lighting technologies turn off 120 times/sec and
provide the sampling effect.

Eric Jacobsen <theman@ericjacobsen.org> wrote:

>Visual aliasing is pretty common at night under artificial lighting
>where the lights provide the sampling.   Incandescent, fluorescent,
>and many other lighting technologies turn off 120 times/sec and
>provide the sampling effect.

How important is this effect for incandescent lights?  I would 
think the thermal mass of the filament and the thermal resistance
from the filament to its surroundings would, in some cases,
prevent it from cooling enough mid-cycle to cause flickering.

But I must say I've never calculated this.

Hi, BTW.

Steve

On 16.7.18 05:49, Steve Pope wrote:
> Eric Jacobsen <theman@ericjacobsen.org> wrote:
> 
>> Visual aliasing is pretty common at night under artificial lighting
>> where the lights provide the sampling.   Incandescent, fluorescent,
>> and many other lighting technologies turn off 120 times/sec and
>> provide the sampling effect.
> 
> How important is this effect for incandescent lights?  I would
> think the thermal mass of the filament and the thermal resistance
> from the filament to its surroundings would, in some cases,
> prevent it from cooling enough mid-cycle to cause flickering.
> 
> But I must say I've never calculated this.
> 
> Hi, BTW.
> 
> Steve


The incandescents do flicker, but only slightly.

About 60 years ago (probably in 1960) I built a crude telephone
by modulating the power feed to a flashlight bulb and listening
to it with a scraped-off OC71 transistor. A pair of automobile
headlight reflectors provided some antenna gain.

-- 

-TV

On Mon, 16 Jul 2018 02:49:54 +0000 (UTC), spope384@gmail.com (Steve
Pope) wrote:

>Eric Jacobsen <theman@ericjacobsen.org> wrote:
>
>>Visual aliasing is pretty common at night under artificial lighting
>>where the lights provide the sampling.   Incandescent, fluorescent,
>>and many other lighting technologies turn off 120 times/sec and
>>provide the sampling effect.
>
>How important is this effect for incandescent lights?  I would 
>think the thermal mass of the filament and the thermal resistance
>from the filament to its surroundings would, in some cases,
>prevent it from cooling enough mid-cycle to cause flickering.
>
>But I must say I've never calculated this.
>
>Hi, BTW.
>
>Steve

Yes, hi!

A simple way to evaluate artificial lighting for this effect is with a
little photovoltaic cell or photosensor circuit connected to an
oscilloscope.   You can see the voltage drop out at the zero
crossings, which indicates the lights going out.   Incandescents do
it, too.   LED lighting may do it at a frequency other than the line
frequency, depending on how the drive circuits are made.

It's kinda weird, but, then, also not.  ;)

On Monday, July 16, 2018 at 1:57:35 PM UTC-4, Eric Jacobsen wrote:
> On Mon, 16 Jul 2018 02:49:54 +0000 (UTC), spope384@gmail.com (Steve
> Pope) wrote:
> 
> >Eric Jacobsen <theman@ericjacobsen.org> wrote:
> >
> >>Visual aliasing is pretty common at night under artificial lighting
> >>where the lights provide the sampling.   Incandescent, fluorescent,
> >>and many other lighting technologies turn off 120 times/sec and
> >>provide the sampling effect.
> >
> >How important is this effect for incandescent lights?  I would 
> >think the thermal mass of the filament and the thermal resistance
> >from the filament to its surroundings would, in some cases,
> >prevent it from cooling enough mid-cycle to cause flickering.
> >
> >But I must say I've never calculated this.
> >
> >Hi, BTW.
> >
> >Steve
> 
> Yes, hi!
> 
> A simple way to evaluate artificial lighting for this effect is with a
> little photovoltaic cell or photosensor circuit connected to an
> oscilloscope.   You can see the voltage drop out at the zero
> crossings, which indicates the lights going out.   Incandescents do
> it, too.   LED lighting may do it at a frequency other than the line
> frequency, depending on how the drive circuits are made.
> 
> It's kinda weird, but, then, also not.  ;)

in the old days :-)
there were strobe discs used to check the the speed of turntables.

They worked best with  a small neon light, ok with a fluorescent  and with an incandescent you could barely see the strobe effect. but it was there.

mark