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From: ROB L on 10 Sep 2009 13:44
On 9 Sep, 08:34, Jon Kirwan <j...(a)infinitefactors.org> wrote:
> On Tue, 8 Sep 2009 21:36:19 -0700 (PDT), "m...(a)sushi.com"
>
>
>
>
>
> <m...(a)sushi.com> wrote:
> >On Sep 8, 5:10 pm, Bobby Joe <bobbyjoe23...(a)gmail.com> wrote:
> >> Is anyone familiar with driving large RGB led grids. Such as 32x32
> >> using cascaded LED drivers. Actually my specific grid is 24x19(each
> >> point is one led and not an rgb). I have seen 24-ch led drivers along
> >> with 16-ch x 8-com(for 128 total led's).
>
> >> Think of the grid as a led matrix display panel as essentially it is
> >> what it is. If I use 24-ch drivers then it requires 19 IC's. Some
> >> chips have built in PWM, dot correction, and other nice features but
> >> at a premium. I do not need error checking but thermal overload
> >> shutdown would be nice.
>
> >> Using a matrix would be much cheaper as I could use 1 24-ch driver and
> >> 19 fets, one for each row. The main issue I am worry about here is the
> >> duty cycle required for each led row and power requirements for the
> >> driver(which I can split the rows up to reduce the power consumption).
>
> >> If I require a nominal 10mA per led then this is 4.5A and
> >> approximately 20W's total dissipation. I'm not quite sure how to
> >> calculate the power dissipated by the IC. I would like to increase the
> >> nominal current to 20mA if possible just for headroom in case it is
> >> eventually required.
>
> >> The only problem here is that it requires a duty cycle of 1/19 which
> >> bumps up the peak current to approximately 200mA. Does this seem
> >> pretty extreme? The peak current at 1/10 @ 1Khz is R=60mA, G=B=100mA.
> >> So this seems to be pushing it assuming I can extrapolate linearly.
>
> >> If it's too much I can split the grid into two or three but I'd like
> >> to do it all at once.
>
> >> What kinda of effect does using PWM have on the led optics? Does the
> >> intensity and color end up changing or can I expect a fairly
> >> consistent output over a wide range of duty cycles?
>
> >> Are there issues with low current? I've heard of pre-charged fets but
> >> not sure exactly what they do. I would like to operate the driving
> >> chips for grayscale.
>
> >> I guess the real question I'm asking is if running a 24x19 grid is
> >> easily done off one or two drivers. My original thought was to use as
> >> many drivers as needed and take advantage of the features they have
> >> except it seems awful expensive just to drive the grid.
>
> >You really sound like you are biting off more than you can chew. I
> >designed the MAX7219, but it doesn't sound applicable for your
> >application.
>
> >Regarding PWM, there are two schools of thought, both which have been
> >discussed on SED. Some claim the eye retains the peak value, so as you
> >PWM, it it does not look linear. Others say the eye averages
> >perfectly. Who knows. An old HP app note claims the eye maintains the
> >peak value.
> ><snip>
>
> Broadly speaking, the eye averages when the rate is fast.  I've tested
> this and I have no question about it, anymore.  (When it isn't fast,
> other obvious things come into play -- namely, you can see the flicker
> which pretty much changes the ball game, anyway.)
>
> Anyone can purchase a copy of HP's "Optoelectronics: Fiber-Optics
> Applications Manual," 2nd edition, through alibris or some other
> bookseller outlet, and take a look at the quote in the last paragraph
> on page 5.25, "The human eye is a time average detector..."  That
> quote is also from HP.  In any case, it's clear enough through
> experiment, too.
>
> ...
>
> That said, the effect of pulsing is not entirely net-zero.  There is a
> suggestive curve on page 5.20 of the same book above, Figure 5.2.4-1,
> "Relative Luminous Efficiency (Luminous Intensity Per Unit Current) vs
> Peak Current Per Segment for a High-Efficiency Red Display."  The
> curve shows increased luminous efficiency when pulsing vs DC, using
> the time-averaged current as the standard (until the LED junction
> nears saturation, which it will do at some point.)
>
> Their example note that pulsing a high-efficiency red LED with 50mA at
> a 10% duty cycle (5mA time-averaged) yields a luminous intensity about
> 1.6 times as great as running the same LED at a constant 5mA.  The
> curve for this red LED basically flattens out at 1.6, so higher pulse
> currents aren't helpful in this case.
>
> Keep in mind that pulsing the LED with 50mA requires a higher drive
> voltage than if the same LED were run with a DC current equal to the
> time-averaged equivalent.  For example, rather than 1.9V(a)5mA/100% it
> might be 2.6V(a)50mA/10%; which is 9.5mW and 13mW average, respectively.
> So although it may be 1.6 times brighter, it's also about 1.4 times
> the power.  Some gain, but nothing to write home about and certainly
> not like some 10X brightness that a 'peak' response theory would
> suggest.
>
> (Higher temperatures also lower output, on the order of 1%/C,
> roughly.)
>
> On the other hand, if you have to drop voltage to control the current
> anyway, you might as well hand that over to the LED and make something
> out of it than just toss it all away in the regulation (or resistor)
> if you can afford the reduced overhead.
>
> Anyway, my experience is consistent with the comments from HP's book.
>
> Since you designed the MAX7219, you must have seen enough of all this
> on your own, by now.  How is it that you remain ambivalent about the
> question?
>
> Jon- Hide quoted text -
>
> - Show quoted text -

It is amazing how many people are desperate to believe that pulsating
a light source produces some majical effect in the eye and perception
is enhanced.
The effects of 'flicker' are well known and have been documented for a
very long time now - having been rigorously studied in a scientific
manner by several well known authorities.

Miso is a major proponent of the magic of modulating a lightsource to
improve its perseption in SED - I suspect he did something using the
magic principal and has been desperate to gain some credence for it
ever since.

Just have a look for the work of Brocker, Sulza, Talbot, etc - *any*
decent website dealing in vision will reveal all.

From: Jon Kirwan on 10 Sep 2009 15:11
On Thu, 10 Sep 2009 10:44:33 -0700 (PDT), ROB L
<h.lewis(a)connect-2.co.uk> wrote:

>On 9 Sep, 08:34, Jon Kirwan <j...(a)infinitefactors.org> wrote:
>> On Tue, 8 Sep 2009 21:36:19 -0700 (PDT), "m...(a)sushi.com"
>>
>> <m...(a)sushi.com> wrote:
>> >On Sep 8, 5:10�pm, Bobby Joe <bobbyjoe23...(a)gmail.com> wrote:
>> >> Is anyone familiar with driving large RGB led grids. Such as 32x32
>> >> using cascaded LED drivers. Actually my specific grid is 24x19(each
>> >> point is one led and not an rgb). I have seen 24-ch led drivers along
>> >> with 16-ch x 8-com(for 128 total led's).
>>
>> >> Think of the grid as a led matrix display panel as essentially it is
>> >> what it is. If I use 24-ch drivers then it requires 19 IC's. Some
>> >> chips have built in PWM, dot correction, and other nice features but
>> >> at a premium. I do not need error checking but thermal overload
>> >> shutdown would be nice.
>>
>> >> Using a matrix would be much cheaper as I could use 1 24-ch driver and
>> >> 19 fets, one for each row. The main issue I am worry about here is the
>> >> duty cycle required for each led row and power requirements for the
>> >> driver(which I can split the rows up to reduce the power consumption).
>>
>> >> If I require a nominal 10mA per led then this is 4.5A and
>> >> approximately 20W's total dissipation. I'm not quite sure how to
>> >> calculate the power dissipated by the IC. I would like to increase the
>> >> nominal current to 20mA if possible just for headroom in case it is
>> >> eventually required.
>>
>> >> The only problem here is that it requires a duty cycle of 1/19 which
>> >> bumps up the peak current to approximately 200mA. Does this seem
>> >> pretty extreme? The peak current at 1/10 @ 1Khz is R=60mA, G=B=100mA.
>> >> So this seems to be pushing it assuming I can extrapolate linearly.
>>
>> >> If it's too much I can split the grid into two or three but I'd like
>> >> to do it all at once.
>>
>> >> What kinda of effect does using PWM have on the led optics? Does the
>> >> intensity and color end up changing or can I expect a fairly
>> >> consistent output over a wide range of duty cycles?
>>
>> >> Are there issues with low current? I've heard of pre-charged fets but
>> >> not sure exactly what they do. I would like to operate the driving
>> >> chips for grayscale.
>>
>> >> I guess the real question I'm asking is if running a 24x19 grid is
>> >> easily done off one or two drivers. My original thought was to use as
>> >> many drivers as needed and take advantage of the features they have
>> >> except it seems awful expensive just to drive the grid.
>>
>> >You really sound like you are biting off more than you can chew. I
>> >designed the MAX7219, but it doesn't sound applicable for your
>> >application.
>>
>> >Regarding PWM, there are two schools of thought, both which have been
>> >discussed on SED. Some claim the eye retains the peak value, so as you
>> >PWM, it it does not look linear. Others say the eye averages
>> >perfectly. Who knows. An old HP app note claims the eye maintains the
>> >peak value.
>> ><snip>
>>
>> Broadly speaking, the eye averages when the rate is fast. �I've tested
>> this and I have no question about it, anymore. �(When it isn't fast,
>> other obvious things come into play -- namely, you can see the flicker
>> which pretty much changes the ball game, anyway.)
>>
>> Anyone can purchase a copy of HP's "Optoelectronics: Fiber-Optics
>> Applications Manual," 2nd edition, through alibris or some other
>> bookseller outlet, and take a look at the quote in the last paragraph
>> on page 5.25, "The human eye is a time average detector..." �That
>> quote is also from HP. �In any case, it's clear enough through
>> experiment, too.
>>
>> ...
>>
>> That said, the effect of pulsing is not entirely net-zero. �There is a
>> suggestive curve on page 5.20 of the same book above, Figure 5.2.4-1,
>> "Relative Luminous Efficiency (Luminous Intensity Per Unit Current) vs
>> Peak Current Per Segment for a High-Efficiency Red Display." �The
>> curve shows increased luminous efficiency when pulsing vs DC, using
>> the time-averaged current as the standard (until the LED junction
>> nears saturation, which it will do at some point.)
>>
>> Their example note that pulsing a high-efficiency red LED with 50mA at
>> a 10% duty cycle (5mA time-averaged) yields a luminous intensity about
>> 1.6 times as great as running the same LED at a constant 5mA. �The
>> curve for this red LED basically flattens out at 1.6, so higher pulse
>> currents aren't helpful in this case.
>>
>> Keep in mind that pulsing the LED with 50mA requires a higher drive
>> voltage than if the same LED were run with a DC current equal to the
>> time-averaged equivalent. �For example, rather than 1.9V(a)5mA/100% it
>> might be 2.6V(a)50mA/10%; which is 9.5mW and 13mW average, respectively.
>> So although it may be 1.6 times brighter, it's also about 1.4 times
>> the power. �Some gain, but nothing to write home about and certainly
>> not like some 10X brightness that a 'peak' response theory would
>> suggest.
>>
>> (Higher temperatures also lower output, on the order of 1%/C,
>> roughly.)
>>
>> On the other hand, if you have to drop voltage to control the current
>> anyway, you might as well hand that over to the LED and make something
>> out of it than just toss it all away in the regulation (or resistor)
>> if you can afford the reduced overhead.
>>
>> Anyway, my experience is consistent with the comments from HP's book.
>>
>> Since you designed the MAX7219, you must have seen enough of all this
>> on your own, by now. �How is it that you remain ambivalent about the
>> question?
>>
>> Jon- Hide quoted text -
>>
>> - Show quoted text -
>
>It is amazing how many people are desperate to believe that pulsating
>a light source produces some majical effect in the eye and perception
>is enhanced.

Well, probably because there is some truth mixed into the lie. Human
perception has many remarkable features.

The sense of color perception is an example where studies made in the
early 1900's lead to the 1931 CIE color standards (revised in 1964.)
Edwin Land's remarkable studies in the late 1970's and early 1980's
brought whole new aspects to the question, demonstrating (in part) a
few interesting ideas: for example, that although there is a dramatic
shift in the distribution of emitted wavelengths by a tungsten
filament as it is dimmed and brightened as a light source, a stretched
canvas containing myriad color swatches had their colors easily
discerned by observers despite the radical changes in lighting... but
that if a drape with a hole in it were placed over the surrounding
colors that perception was lost and behaved far more as one would
expect from the reflected wavelengths. In other words, color
perception includes information from surrounding areas, as well, in
order to develop the fuller sense.

And of course most everyone is aware of some of the many "optical
illusions" that abound.

So it's easy to accept the idea about 'peak response,' otherwise
ignorant. I don't fault anyone for getting it wrong. It's just, as
you say later, that there is abundant evidence to the contrary if one
either reads the work of others or else performs a few simple
experiments of their own.

On that latter point, I'm reminded of Galileo's comment within The
Assayer. It is well worth reading from beginning to end, but here is
a snippet from Stillman Drake's translation. In it, Galileo is
responding to Grassi (writing under the pseudonym of 'Sarsi') and the
adherence to the idea that the Earth is surrounded by a sphere of fire
and that when objects rise up into the sky they get close to this
sphere and can burst into fire in response:

"... But it is wrong to say, as Sarsi does, that Guiducci and I
would laugh and joke at the experiences adduced by Aristotle.
We merely do not believe that a cold arrow shot from a bow can
take fire in the air; rather, we think that if an arrow were
shot when afire, it would cool down more quickly than it would
if it were held still. This is not derision; it is simply the
statement of our opinion.

"Sarsi goes on to say that since this experience of Aristotle's
has failed to convince us, many other great men also have
written things of the same sort. To this I reply that if in
order to refute Aristotle's statement we are obliged to
represent that no other men have believed it, then nobody on
earth can ever refute it, since nothing can make those who
have believed it not believe it. But it is news to me that
any man would actually put the testimony of writers ahead of
what experience shows him. To adduce more witnesses serves
no purpose, Sarsi, for we have never denied that such things
have been written and believed. We did say they are false,
but so far as authority is concerned yours alone is as
effective as an army's in rendering the events true or false.
You take your stand on the authority of many poets against
our experiments. I reply that if those poets could be present
at our experiments they would change their views, and without
disgrace they could say they had been writing hyperbolically
-- or even admit they had been wrong.

"Well, if we cannot have the presence of your poets (who, as I
say, would yield to experience), we do have at hand archers
and catapultists, and you may see for yourself whether citing
your authorities to them can strengthen their arms to such an
extent that the arrows they shoot and the lead balls they hurl
will take fire and melt in the air. In that way you will be
able to find out just how much force human authority has upon
the facts of Nature, which remains deaf and inexorable to our
wishes. You say there is no longer an Acestes or a Mezentius
or other mighty paladin? I shall be content to have you shoot
an arrow not with a simple longbow, but with the stoutest
steel crossbow, or use a catapult drawn by levers and
windlasses that could not be managed by thirty of your ancient
heroes. Shoot ten arrows, or a hundred, and if it ever
happens that on one of them the feathers so much as slightly
tan -- let alone its shaft taking fire or its steel tip
melting -- I shall not only concede the argument but forfeit
your respect, which I regard so highly..."

Nature doesn't care a whit about human opinion, wish, fantasy, or
authority -- it does what it does, consistently, and without recourse
to human desire about it. But also, without disgrace, we can admit
our errors in the face of contrary experiment. In other words, it's
not a disgrace to lose ignorance and become informed by experimental
result.

The Assayer is interesting to read, in hindsight.

>The effects of 'flicker' are well known and have been documented for a
>very long time now - having been rigorously studied in a scientific
>manner by several well known authorities.
>
>Miso is a major proponent of the magic of modulating a lightsource to
>improve its perseption in SED - I suspect he did something using the
>magic principal and has been desperate to gain some credence for it
>ever since.

I must have missed that.

>Just have a look for the work of Brocker, Sulza, Talbot, etc - *any*
>decent website dealing in vision will reveal all.

Yes, the Talbot-Plateau law. But there are some 'effects' at low
blinking rates, below the critical fusion frequency (which varies by
area of stimulus as well as whether scotopic or photopic dominates.)

The one thing I remember was the value of 70Hz as the photopic figure.
In my own experiments here, with a small green LED, I found that about
45Hz fused it (age: 50 years) if I didn't move at all, but that for
modest ranges of motion I needed closer to 60Hz to fuse it well. All
this goes to the point that 70Hz is probably a good top end benchmark
for still objects. Under vibration, I'd insist on a still higher
modulation frequency.

Jon
From: Jan Panteltje on 10 Sep 2009 17:57
On a sunny day (Thu, 10 Sep 2009 19:11:44 GMT) it happened Jon Kirwan
<jonk(a)infinitefactors.org> wrote in
<0igia5tclsglj8c68fu46jhj30clsss0t7(a)4ax.com>:

>
> "... But it is wrong to say, as Sarsi does, that Guiducci and I
> would laugh and joke at the experiences adduced by Aristotle.
> We merely do not believe that a cold arrow shot from a bow can
> take fire in the air; rather, we think that if an arrow were
> shot when afire, it would cool down more quickly than it would
> if it were held still. This is not derision; it is simply the
> statement of our opinion.

All is relative, if you fired an arrow with enough speed,
then the air friction would set it on fire.
Maybe Aristotle observed some meteorites.

From: Jon Kirwan on 10 Sep 2009 21:45
On Thu, 10 Sep 2009 21:57:48 GMT, Jan Panteltje
<pNaonStpealmtje(a)yahoo.com> wrote:

>On a sunny day (Thu, 10 Sep 2009 19:11:44 GMT) it happened Jon Kirwan
><jonk(a)infinitefactors.org> wrote in
><0igia5tclsglj8c68fu46jhj30clsss0t7(a)4ax.com>:
>
>> "... But it is wrong to say, as Sarsi does, that Guiducci and I
>> would laugh and joke at the experiences adduced by Aristotle.
>> We merely do not believe that a cold arrow shot from a bow can
>> take fire in the air; rather, we think that if an arrow were
>> shot when afire, it would cool down more quickly than it would
>> if it were held still. This is not derision; it is simply the
>> statement of our opinion.
>
>All is relative, if you fired an arrow with enough speed,
>then the air friction would set it on fire.

Galileo was well aware of friction heating.

>Maybe Aristotle observed some meteorites.

Well, yes. It's possible that an observation like that might have
stimulated some thoughts about fire. However, Aristotle goes at great
length in Book I about his ideas. So we really don't need to guess.
He struggles with a question he poses about "the intervals" in the
"upper region" and writes,

"If the intervals were full of fire and the bodies consisted
of fire every one of the other elements would long ago have
vanished. However, they cannot simply be said to be full
of air either; for even if there were two elements to fill
the space between the earth and the heavens, the air would
far exceed the quantitu required to maintain its proper
proportion to the other elements."

He begins his conclusion like this,

"But whenever a particle of air grows heavy, the warmth in
it is squeezed out into the upper region and it sinks, and
other particles in turn are carried up together with the
fiery exhalation. Thus the one region is always full of
air and the other of fire, and each of them is perpetually
in a state of change."

Here, he is clearly NOT arguing from the perspective of meteors. In
fact, he doesn't even bring them up. Though I suppose they may have
played a role inspiring his direction of thought.

Jon
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