Reply by July 17, 20182018-07-17
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
Reply by Eric Jacobsen July 16, 20182018-07-16
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. ;)
Reply by Tauno Voipio July 16, 20182018-07-16
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
Reply by Steve Pope July 15, 20182018-07-15
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
Reply by Eric Jacobsen July 15, 20182018-07-15
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.
Reply by RichD July 15, 20182018-07-15
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
Reply by Steve Pope July 14, 20182018-07-14
Les Cargill  <lcargill99@comcast.com> wrote:

>Steve Pope wrote:
>> RichD <r_delaney2001@yahoo.com> 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.
>So you ran out of Dirac deltas? :) > ><enjoy the veal, I'll be here all week>
Relatedly, Dirac did indeed come up with the idea of imaginary mass, in his Dirac Sea formulation. So there. Steve
Reply by Gene Filatov July 14, 20182018-07-14
On 14.07.2018 6:39, Rob Doyle wrote:
> On 7/13/2018 7:44 PM, Les Cargill wrote: >> Richard (Rick) Lyons wrote: >>> On Thursday, July 12, 2018 at 9:00:59 AM UTC-7, Eric Jacobsen wrote: >>> >>> >>>> Think of the hands on a clock. The second hand rotates around the >>>> clock face at 1/60 rotations per second, the minute hand at 1/360 >>>> rotations/sec, and the hour hand at 1/4320 per second. Since the >>>> clock hands rotate, you could imagine running the clock backwards so >>>> that the hand rotate counter-clockwise instead of clockwise. In >>>> order to distinguish the backwards, ccw, rotations of the clock at the >>>> same frequencies from the forward, clock rotations, it's easy to put a >>>> negative sign in front of them. This is consistent mathematically. >>> >>> [Snipped by Lyons] >>> >>> Hi Eric. >>> You made me think about my battery-powered electric drill. >>> (One of the all-time great applications of electric motors!) >>> It has a "Reverse" button that allows me to both "drive" screws in >>> (clockwise rotation) and remove screws (counter-clockwise rotation). >>> If we define positive rotations/minute (RPM) as clockwise rotation, >>> then we could conceive of (define) "negative RPM" as being >>> counter-clockwise rotation. >>> >>> However, the notion of negative physical quantities is an interesting >>> topic. For example, I cannot conceive of an old clock whose pendulum >>> oscillates at a rate of negative one oscillations per second. >>> >> >> A pendulum only has "positive oscillations" and the time >> base only goes one way. So it's a "real-only" oscillator. It's >> insufficiently dimensional to account for negatives. >> >> I have no idea of what use this is, but: >> >> 1) If we filmed the pendulum, then ran the film backwards, it >> would be totally the same as if it ran forwards. Since the order of >> events is reversed and the the timebase is reversed, the reverses cancel >> each other out. >> >> 2) If there were a "complex pendulum", you might have enough >> dimensional to get negative quantities. All I see of "complex >> pendulums" is double >> pendulums. So perhaps a "complex pendulum" is a non-sequitur. But E/M >> waves in space are not. >> > > Foucaults's Pendulum? > > https://www.fi.edu/exhibit/foucault%E2%80%99s-pendulum > > It would definitely would be different if run backward - except if moved > to the southern hemisphere. > > Rob. >
In physics, that's called CPT symmetry. https://en.wikipedia.org/wiki/CPT_symmetry Gene
Reply by Rob Doyle July 14, 20182018-07-14
On 7/13/2018 7:44 PM, Les Cargill wrote:
> Richard (Rick) Lyons wrote: >> On Thursday, July 12, 2018 at 9:00:59 AM UTC-7, Eric Jacobsen wrote: >> >> >>> Think of the hands on a clock.&nbsp;&nbsp; The second hand rotates around the >>> clock face at 1/60 rotations per second, the minute hand at 1/360 >>> rotations/sec, and the hour hand at 1/4320 per second.&nbsp;&nbsp;&nbsp; Since the >>> clock hands rotate, you could imagine running the clock backwards so >>> that the hand rotate counter-clockwise instead of clockwise.&nbsp;&nbsp; In >>> order to distinguish the backwards, ccw, rotations of the clock at the >>> same frequencies from the forward, clock rotations, it's easy to put a >>> negative sign in front of them.&nbsp;&nbsp; This is consistent mathematically. >> >> [Snipped by Lyons] >> >> Hi Eric. >> You made me think about my battery-powered electric drill. >> (One of the all-time great applications of electric motors!) >> It has a "Reverse" button that allows me to both "drive" screws in >> (clockwise rotation) and remove screws (counter-clockwise rotation). >> If we define positive rotations/minute (RPM) as clockwise rotation, >> then we could conceive of (define) "negative RPM" as being >> counter-clockwise rotation. >> >> However, the notion of negative physical quantities is an interesting >> topic. For example, I cannot conceive of an old clock whose pendulum >> oscillates at a rate of negative one oscillations per second. >> > > A pendulum only has "positive oscillations" and the time > base only goes one way. So it's a "real-only" oscillator. It's > insufficiently dimensional to account for negatives. > > I have no idea of what use this is, but: > > 1) If we filmed the pendulum, then ran the film backwards, it > would be totally the same as if it ran forwards. Since the order of > events is reversed and the the timebase is reversed, the reverses cancel > each other out. > > 2) If there were a "complex pendulum", you might have enough dimensional > to get negative quantities. All I see of "complex pendulums" is double > pendulums. So perhaps a "complex pendulum" is a non-sequitur. But E/M > waves in space are not. >
Foucaults's Pendulum? https://www.fi.edu/exhibit/foucault%E2%80%99s-pendulum It would definitely would be different if run backward - except if moved to the southern hemisphere. Rob.
Reply by Les Cargill July 13, 20182018-07-13
Steve Pope wrote:
> RichD <r_delaney2001@yahoo.com> 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. > > Steve >
So you ran out of Dirac deltas? :) <enjoy the veal, I'll be here all week> -- Les Cargill