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'Moving message display'
1997\02\09@225401 by JeffScholz

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Greetings,
       I am building a moving message display. It is 7 rows x 80 columns.
Multiplexing will be accomplished by loading shift registers with column
data and the powering sequential rows for 1.64mS each. Therefore, each row
will be refreshed once every 11.5mS.
       Is the refresh rate frequent enough to eliminate flicker? Can I
refresh at a slower rate? I sure could use the extra time for other tasks,
but I don't want a flickery display!!

Thanks!                      Jeff

1997\02\09@235624 by optoeng

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JeffScholz wrote:
>
> Greetings,
>         I am building a moving message display. It is 7 rows x 80 columns.
> Multiplexing will be accomplished by loading shift registers with column
> data and the powering sequential rows for 1.64mS each. Therefore, each row
> will be refreshed once every 11.5mS.
>         Is the refresh rate frequent enough to eliminate flicker? Can I
> refresh at a slower rate? I sure could use the extra time for other tasks,
> but I don't want a flickery display!!
>
> Thanks!                      Jeff


Critical flicker fusion frequency varies with the amount of ambient
light.  It's highest outdoors, which is why you sometimes see
fluorescent lights and video monitors flickering from outside looking
into a building.  The range from very dim lighting to outdoors is
roughly 36 Hz to 120 Hertz required to eliminated flicker perception.
--
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Paul Mathews, consulting engineer
AEngineering Co.

email: spam_OUToptoengTakeThisOuTspamwhidbey.com
non-contact sensing and optoelectronics specialists

1997\02\10@011936 by Don McKenzie

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JeffScholz wrote:
>
> Greetings,
>         I am building a moving message display. It is 7 rows x 80 columns.
> Multiplexing will be accomplished by loading shift registers with column
> data and the powering sequential rows for 1.64mS each. Therefore, each row
> will be refreshed once every 11.5mS.
>         Is the refresh rate frequent enough to eliminate flicker? Can I
> refresh at a slower rate? I sure could use the extra time for other tasks,
> but I don't want a flickery display!!
>
> Thanks!                      Jeff

I built an LED Moving message board around 1989 driven with a Z80 and
used 74LS164s as shift registers. The timing was set to 2ms per row by 7
rows which worked out at 14ms per cycle.

The speed of the Micro has a lot to do with the timing. Several 'sweet
spots' can be found to achieve persistance of vision with LED displays.

I was using a 555 timer driven interrupt to set the row scan update, so
I was able to easily select these 'sweet spots' by varying the interrupt
timing.

You load up your registers, switch the row on, go do some housekeeping
such as getting new data and mapping it to your ram mirror of the
display, then go switch the current row off, load the next, and so on.

Don McKenzie  .....donKILLspamspam@spam@dontronics.com   http://www.dontronics.com

SLI, the serial LCD that auto detects baud rates from 100 to 125K bps.
SimmStick(tm) A PIC proto PCB the size of a 30 pin Simm Memory Module.
Covers all versions of the PIC16cxx family plus the Atmel AT89C2051.

1997\02\10@023340 by Tim Kerby

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Hi
The television uses a refresh of 25 frames per second.  Does your tv flicker?

Tim



At 17:17 10/02/97 -0800, you wrote:
{Quote hidden}

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1997\02\10@023755 by Jeff King
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At 07:34 AM 2/10/97 +0000, Tim Kerby wrote:
>Hi
>The television uses a refresh of 25 frames per second.  Does your tv flicker?
>
>Tim

But don't forget its field rate is 50hz (PAL). So its refresh rate is 50 hz not
25 hz.

Regards,

------------------------------------
| Jeff King      Aero Data Systems |
| .....jeffKILLspamspam.....mich.com  P.O. Box 510895   |
| (810)471-1787  Livonia, MI 48151 |
|F(810)471-0279  United States     |
------------------------------------

1997\02\10@024213 by Mike

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At 07:34 AM 10/02/97 +0000, you wrote:
>Hi
>The television uses a refresh of 25 frames per second.  Does your tv flicker?
>
>Tim

All TV's flicker.

Some people notice this flicker - others do not. Depends on a whole range of
factors. I believe every persons brain has a built in clock rate of around
20 to 40 cycles per second. There is a basic minimum response time which
is considered to be a certain multiple of the human clock rate. Thats one
reason that sprinters (in the olympics) are considered to have a false
start if there response rate is less than 75mS AFTER the gun !

There seems to be some correlation between metabolic rate and a persons
likelihood to notice flicker. My girlfriend was diagnosed with a thyroid
condition that has kept her metabolic rate slow for years - since she
has been on medication she notices flicker on some SVGA monitors where she
would'nt notice it before...

Personally I can't stand anything below about 60Hz on a SVGA but put up with
a TV at 25 since its a smallish screen and the intensity is downand I think
the phosphor persistence is a bit longer ?

Rgds

Mike

1997\02\10@024830 by Mike

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At 02:36 AM 10/02/97 -0500, you wrote:
>At 07:34 AM 2/10/97 +0000, Tim Kerby wrote:
>>Hi
>>The television uses a refresh of 25 frames per second.  Does your tv flicker?
>>
>>Tim
>
>But don't forget its field rate is 50hz (PAL). So its refresh rate is 50 hz not
>25 hz.

Its interlaced though isn't it so a different picture each 25th of a second not
a different picture each 50th of a second - eh ?

So its 25Hz isnt it...

Rgds

Mike

1997\02\10@025249 by Andrew Warren

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Tim Kerby <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU> wrote:

> The television uses a refresh of 25 frames per second.  Does your tv
> flicker?

Tim:

As you say, televisions (in countries with 50 Hz systems) refresh
the entire frame 25 times per second.  They do it, however, by
interlacing the odd and even lines (the odd and even "fields") and
refreshing each field FIFTY times per second.  This interlacing
scheme was adopted specifically to combat flicker.

Additionally, TV screens have much higher persistence than LEDs. That
is, once an image is drawn on a TV screen, it takes a while for it to
fade.  LEDs, on the other hand, have essentially zero persistence.

Finally, most people watch TV in fairly dim ambient light.  This
helps to reduce apparent flicker as well.

Even so, however, televisions DO flicker; a regular (60 Hz in the
USA) television looks absolutely AWFUL when it's compared
side-by-side to a frequency-doubled 120-Hz TV with a non-interlaced
display fed from a big RAM.

-Andy

=== Andrew Warren - fastfwdspamspam_OUTix.netcom.com                 ===
=== Fast Forward Engineering - Vista, California          ===
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1997\02\10@025909 by Jeff King

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At 03:42 PM 2/10/97 +0800, Mike wrote:
>At 02:36 AM 10/02/97 -0500, you wrote:
>>At 07:34 AM 2/10/97 +0000, Tim Kerby wrote:
>>>Hi
>>>The television uses a refresh of 25 frames per second.  Does your tv flicker?
>>>
>>>Tim
>>
>>But don't forget its field rate is 50hz (PAL). So its refresh rate is 50
hz not
{Quote hidden}

Two TV fields make a frame. Not sure on PAL but on NTSC its 262.5 lines per
field (525 lines total). This is interlaced video. The refresh rate and
persistence of the phosphers determine the flicker. Yes, a new image
every 1/25 of a second but the screen is updated (refreshed) every 1/50
of a second.

Regards,

------------------------------------
| Jeff King      Aero Data Systems |
| @spam@jeffKILLspamspammich.com  P.O. Box 510895   |
| (810)471-1787  Livonia, MI 48151 |
|F(810)471-0279  United States     |
------------------------------------

1997\02\10@040322 by Kalle Pihlajasaari

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Hi Tim,

> The television uses a refresh of 25 frames per second.  Does your tv flicker?

It has Phosphor persistance to add to your eye persistance so not a
fair comparison.

The worst flicker used to be 22 Hz cinema, I seem to recall that
it is higher these days.

> >> data and the powering sequential rows for 1.64mS each. Therefore, each row
> >> will be refreshed once every 11.5mS.
> >
> >used 74LS164s as shift registers. The timing was set to 2ms per row by 7
> >rows which worked out at 14ms per cycle.

Cheers
--
Kalle Pihlajasaari   KILLspamkalleKILLspamspamip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer

1997\02\10@081533 by Byron A Jeff

face picon face
>
> Greetings,
>         I am building a moving message display. It is 7 rows x 80 columns.
> Multiplexing will be accomplished by loading shift registers with column
> data and the powering sequential rows for 1.64mS each. Therefore, each row
> will be refreshed once every 11.5mS.

Would you mind telling us how you're going to sink the current for each
row? even at a nominal 20ma per LED you can have a max sink current of
1.6A per row. That's a whole bunch of current!

Oh and the perceived brightness is more a function of peak current through
the LED rather than average brightness. So typically an LED pulsed at
100ma at a low duty cycle will have a higher perceied brightness than a
20ma at a higher duty cycle.

That's why I usually drive columns instead of rows at a much higher frequency.
Using drivers like the Allegro UCN5801 sink driver at a peak 500ma per, each
LED can be pulsed at 70ma each. And with loading taking only 10us or so at
20 Mhz on a pic (Read column, write to latch, shift column driver) it leaves
almost 600 uS between columns to do other stuff. Works great interrupt driven.


>         Is the refresh rate frequent enough to eliminate flicker?

86 Hz? probably not. May work if you can get the LED's bright enough. Do
you have a 5-8A sink for each row available (60-100ma current per LED)?

>Can I
> refresh at a slower rate? I sure could use the extra time for other tasks,
> but I don't want a flickery display!!

This is easily determined by testint. Just wire up one 5x7 display and vary
the refresh rate. It becomes very apparent when flicker occurs.

BTW how did you pick your display numbers above?

Your best bet is to pump up the current and drop the duty cycle, the
perceived brightness will go up and you'll get some of the persistence effect
from the user's eye meaning that you can go longer before flicker is perceived.

BAJ

1997\02\10@102206 by Miller, Steve

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In my experience this these displays, flicker becomes a problem only when
the refresh rate drops down close to 60 HZ.   You are proposing a refresh
rate of 87HZ, so I think you will be fine.  You could drop refresh
interval to 14 ms and still be fine.  However, the faster you refresh the
better.  Slower refresh rates appear jumpy if you blink your eyes
rapidly.  My former boss once built a system that refreshed at 8 mS.
This display was rock solid, and he always berated my stuff in
comparison.  (He could refresh so fast because he did not do special
effects.)
---- Steve
----------
From:  pic microcontroller discussion [SMTP:RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU]
Sent:  Sunday, February 09, 1997 12:38 AM
To:  PICLIST
Subject:  Moving message display


Greetings,
       I am building a moving message display. It is 7 rows x 80
columns.
Multiplexing will be accomplished by loading shift registers with column
data and the powering sequential rows for 1.64mS each. Therefore, each
row
will be refreshed once every 11.5mS.
       Is the refresh rate frequent enough to eliminate flicker? Can I
refresh at a slower rate? I sure could use the extra time for other
tasks,
but I don't want a flickery display!!

Thanks!                      Jeff

1997\02\10@131320 by Tim Kerby

picon face
Its not the brain that is slow.  It is retinal persistence that causes
images to merge.  I recently made a light listner converting light to
sound.  The ear can hear 50Hz mains hum for example and much above.  I have
a 50Hz monitor and it flickers if I do not directly view it.  Does the
retina respond slower in the center?


Tim

At 15:41 10/02/97 +0800, you wrote:
>At 07:34 AM 10/02/97 +0000, you wrote:
>>Hi
>>The television uses a refresh of 25 frames per second.  Does your tv
flicker?
{Quote hidden}

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If you can read this, it is the end of the message!
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1997\02\10@145004 by o*ptoeng

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Tim Kerby wrote:
>
> Its not the brain that is slow.  It is retinal persistence that causes
> images to merge.  I recently made a light listner converting light to
> sound.  The ear can hear 50Hz mains hum for example and much above.  I have
> a 50Hz monitor and it flickers if I do not directly view it.  Does the
> retina respond slower in the center?
>

Yes.  Center (fovea) is mostly cone cells, color vision, high acuity,
lower sensitivity, lower critical flicker fusion freq.
--
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Paul Mathews, consulting engineer
AEngineering Co.

email: spamBeGoneoptoengspamBeGonespamwhidbey.com
non-contact sensing and optoelectronics specialists

1997\02\10@171138 by Martin McCormick

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       If you want a comparison that is pretty standard all over the world,
sound movie film is shown at 24 frames per second, but you actually see
72 images per second as most film projectors have a three-bladed shutter
that makes one revolution per frame.  The one difference you might run in to
would be the special projectors used in T.V. film chains.  They have a
different shutter which helps synchronize the framing speed with video.

Martin McCormick

1997\02\10@175637 by William Chops Westfield

face picon face
   Does the retina respond slower in the center?

Yes.  The center of the eye has a high concentration of "cones" (color
receptors), which are relatively slow.  As you get into the peripheral
areas, the concentration of "rods" (B&W receptors) goes up.  These are
faster and more sensitive to absolute light levels.  This is also why you
can see dim stars/commets/etc better when you're not looking directly at
them.

BillW

1997\02\10@193305 by Steve Hardy

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> From: Martin McCormick <TakeThisOuTmartinEraseMEspamspam_OUTDC.CIS.OKSTATE.EDU>
>
>         If you want a comparison that is pretty standard all over the world,
> sound movie film is shown at 24 frames per second, but you actually see
> 72 images per second as most film projectors have a three-bladed shutter
> that makes one revolution per frame.  The one difference you might run in to
> would be the special projectors used in T.V. film chains.  They have a
> different shutter which helps synchronize the framing speed with video.
>
> Martin McCormick
>

IMAX and OMNIMAX uses a two-bladed shutter giving 48Hz frame rate.
Having been to the OMNIMAX theatre in Portland Oregon I didn't notice
any flicker at 48Hz.  I don't notice the 50Hz flicker of our PAL TVs
unless walking past a TV shop and I catch a glimpse of all those TVs
out of the corner of my eye.

My only criticism of OMNIMAX is that rapid motion of the scene (e.g.
panning) is a bit jerky.  This is especially noticeable because of the
extreme width of the image, and the fact that the 'real' frame rate is
only 24Hz.  Apart from that, it's a wonderful (Aussie) invention.

Getting back to the original query (relating to LED displays), the
refresh should be done row-by-row (i.e. the largest number of LEDs at a
time).  Why?  Well, for a particular brightness of the overall display,
the average current for each LED must be the same.  If row-by-row
MUXing is performed, the peak current per LED is less (assuming
rows < cols).  Generally, the drivers will need to be designed for
peak current.  If you work out the economics, it is cheaper to design
row MUXing.

This is borne out for actual displays.  I have never seen a 'ticker
tape' display with column MUXing.  In fact, they all refresh by
row, starting with the bottom row and moving up.  This gives a natural
rightward slope to characters that are moving rightward across the
display (which is the usual case).

The best economy of drivers is obtained for square displays.  Of
course you could arrange for your long thin display to be electrically
'square' by partitioning it into the appropriate sized segments
however moving displays may look a bit fragmented.

Regards,
SJH
Canberra, Australia

1997\02\10@201053 by Clyde Smith-Stubbs

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Thus spake Steve Hardy (RemoveMEhardyspamTakeThisOuTSWENG.STORTEK.COM):

> time).  Why?  Well, for a particular brightness of the overall display,
> the average current for each LED must be the same.  If row-by-row

Not true. The perceived brightness is related more to peak brightness
than average - i.e. for the same average current the LED will appear
brighter with a low duty cycle (and therefore high peak current)
than with a high duty cycle. Driver considerations are another matter,
but for maximum brightness you should aim to drive the LED at its
rated peak current, and adjust the duty cycle so as not to exceed the
rated average current.

--
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clydeEraseMEspam.....htsoft.com      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
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1997\02\10@215256 by Mike

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At 06:13 PM 10/02/97 +0000, you wrote:
>Its not the brain that is slow.  It is retinal persistence that causes
>images to merge.  I recently made a light listner converting light to
>sound.  The ear can hear 50Hz mains hum for example and much above.  I have
>a 50Hz monitor and it flickers if I do not directly view it.  Does the
>retina respond slower in the center?

You missed my point, I think, if there is less retinal persistence then
you are more likely to notice flicker. The brain response is there as well
and is not independent - for example in the human eye there is less
structure for detecting moving edges than say in the frogs eye. The whole
issue is actually quite complex - and yes the retinas response is highly
non-linear - there are structures that respond better to moving edges
outside the center of vision.

Rgds

Mike

1997\02\10@234101 by tjaart

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Tim Kerby wrote:
>
> Its not the brain that is slow.  It is retinal persistence that causes
> images to merge.  I recently made a light listner converting light to
> sound.  The ear can hear 50Hz mains hum for example and much above.  I have
> a 50Hz monitor and it flickers if I do not directly view it.  Does the
> retina respond slower in the center?
>

Yep. You see black&white better around the centre, and these cells
respond faster than the colour ones in the centre. Try looking *just*
past an object at night, and you'll notice how you actually see better.

--
Friendly Regards

Tjaart van der Walt
EraseMEtjaartspamwasp.co.za
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1997\02\11@031128 by John Dammeyer

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At 11:39 AM 11/02/1997 EST, you wrote:
>IMAX and OMNIMAX uses a two-bladed shutter giving 48Hz frame rate.
>Having been to the OMNIMAX theatre in Portland Oregon I didn't notice
>any flicker at 48Hz.  I don't notice the 50Hz flicker of our PAL TVs
>unless walking past a TV shop and I catch a glimpse of all those TVs
>out of the corner of my eye.
>
>My only criticism of OMNIMAX is that rapid motion of the scene (e.g.
>panning) is a bit jerky.  This is especially noticeable because of the
>extreme width of the image, and the fact that the 'real' frame rate is
>only 24Hz.  Apart from that, it's a wonderful (Aussie) invention.

I beg to differ.  Canadians invented and pateneted the IMAX system.

Pioneers are the ones, face down in the mud,
with arrows in their backs.
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1997\02\11@090600 by JeffScholz

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At 01:14 PM 2/10/97 +0000, Byron A Jeff wrote:
{Quote hidden}

tasks, but I don't want a flickery display!!
{Quote hidden}

I agree. A bunch of current. I plan on using TIP120 darlingtons. A robust power
supply may be one of my smaller challenges as this is only my second pic
project.
The display numbers were picked based on a compromise between wanting a larger
display and the available ram in a 16c63.

Thanks to all who have responded with comments!  Jeff

1997\02\11@133246 by Leon Heller

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picon face
In message <RemoveME3.0.16.19970210180913.3f576cd6EraseMEspamEraseMEpop-3.ukonline.co.uk>, Tim
Kerby <RemoveMEtim.kerbyspam_OUTspamKILLspamUKONLINE.CO.UK> writes
>Its not the brain that is slow.  It is retinal persistence that causes
>images to merge.  I recently made a light listner converting light to
>sound.  The ear can hear 50Hz mains hum for example and much above.  I have
>a 50Hz monitor and it flickers if I do not directly view it.  Does the
>retina respond slower in the center?

The peripheral visual system is *much* more sensitive to movement
(including flicker) than at the centre. This is very noticeable with
flickering fluorescent lamps. Presumably we have evolved this way to
detect predators creeping up on us from behind.

Leon
--
Leon Heller, G1HSM
RemoveMEleonTakeThisOuTspamspamlfheller.demon.co.uk
Tel: +44 (0) 118 947 1424 (home)
    +44 (0) 1344 385556 (work)

1997\02\11@232119 by John Payson

picon face
>         I am building a moving message display. It is 7 rows x 80 columns.
> Multiplexing will be accomplished by loading shift registers with column
> data and the powering sequential rows for 1.64mS each. Therefore, each row
> will be refreshed once every 11.5mS.
>         Is the refresh rate frequent enough to eliminate flicker? Can I
> refresh at a slower rate? I sure could use the extra time for other tasks,
> but I don't want a flickery display!!

* EVERYTHING YOU EVER WANTED TO KNOW ABOUT TV'S, LED'S, REFRESH RATES,
 AND FLICKER (but were afraid to ask) *

[plus lots of things you didn't]


When trying to design a display to show moving images, there are two important
and related properties to be considered:

-1- Refresh rate
-2- Update rate

The first factor, refresh rate, indicates how many times per second each part
of the display will "flash".  For computer screens, this is typically 50-90Hz.
For televion sets, it is typically 50-60Hz (for objects with low detail) or
25-30Hz (for objects with high detail).  For movie screens it is typically
48Hz (North America) or 50HZ (Europe).  For LED message displays, it is typi-
cally around 60Hz, though some LED displays have a driver for each LED and thus
illuminate continuously.

Some people might wonder how a television set can have a different refresh rate
for high- and low- detail objects.  While this may seem impossible, it is a
natural result of a process called interlacing.  The electron beam in the set
sweeps top-to-bottom 50 or 60 times a second, but each time it only scans the
even or odd lines.  An object which is large enough to appear on several con-
secutive scan lines will thus be drawn on every scan, while a small object
that only appears on one scan line will only be drawn on every other scan.
Note that the worst flicker occurs on objects which are drawn equally brightly
on two consecutive scan lines without appearing on the line above; such obj-
ects will appear to "bounce" between the two locations 25 or 30 times per sec-
ond; flicker may be mitigated, however, by ensuring that the overall bright-
ness and apparent center of any object is the same in both fields.  For exam-
ple, if there is a bright line on row 26 and dim lines on rows 25 and 27, the
overal effect will be a non-flickering, somewhat blurry line on row 26.

The second major factor to consider for moving displays is update rate.  This
is the number of times per second that the information on the display is act-
ually changed.  For movies, this is typically 24 or 25Hz; for televion it may
either be equal to the "field rate" (50 or 60Hz) or the "frame rate" (25 or
30Hz).  Typically, moving subjects will appear best when the update rate is
once per field, but updating the image every field may cause fine details in
the picture to flicker (if a small object's position happens to be on an even
scan line when the display is scanning the odd ones, and vice versa, that ob-
ject will not appear) and a VCR which attempts a "freeze frame" will capture
the even scan lines of one image and the odd scan lines of another (producing
a double-exposure effect).

While having an update rate slower than the refresh rate may produce jerky
motion, it may also have another nasty effect: objects which are moving
around the screen may appear as double- or triple- images (if the update
rate is 1/2 or 1/3 the refresh rate).  This effect may be demonstrated if
you write a program to scroll a text screen at precisely half the monitor's
refresh rate; if you watch the text as it scrolls up, you will see "phantom"
lines of text between the real ones.

To understand where the phantom images are "coming from", you need to remember
that a person's eye will move as he/she watches a moving object.  If a 7-row
LED display is scanned at a rate of 100Hz (700 lines/second) and moves left by
one dot every other frame, then the user's eye will travel left at a rate of
50 dot/second (i.e. 1/2 dot per frame, or 1/14 dot per scan line).  Thus, the
second time a frame is drawn it will appear 1/2 dot to the right of the first
time (since the eye--and the retinal image of the first dot--will have shifted
left 1/2 dot) and so the user will see two dots even though in reality there
is only one.  Note as well that even as the display moves from one scan line to
the next the user's eye will move left, so the display may appear slightly
skewed.

The double-vision effect is the primary reason why most LED displays are scan-
ned and scrolled at the same (approximately 60Hz) rate.  By using the same
rates for both, each dot will be made to appear precisely once on the display
(if you watch, e.g. the top of an "E" it will appear as though all five dots
are all moving, even though most of them are just staying put from one frame
to the next).  In addition, by scanning the display bottom-to-top, the skew of
the text during a normal right-left scroll will be about a 14% lean to the
right, which will appear fairly attractive.

The primary difficulty in producing a nice scrolling LED display is making the
thing big enough to allow the unit to be scanned fast enough to avoid flicker
while maintaining a 1:1 scan/update ratio to avoid "double-vision".

BTW, if you are designing a display primarily for use in scrolling mode, you
may visually "compress" the display horizontally at some expense in refresh
rate.  If your display is wired for "row-scanning", your choices are limitted
to 1:1, 2:1, 3:1, etc. with anything beyond 2:1 being rather dicey (and even
2:1 being iffy).  If your display is wired for horizontal scanning (e.g. you
use 5x7 display modules turned on their sides) then you may use a wide var-
iety of compression ratios (this works best if the row modulus is prime).  I
have, for example, constructed a small "nametag" with a 28x5 display which
can scroll messages quite nicely (it gets about 8-10 characters legibly on
the screen at once, with an apparent screen width of 48 pixels).  Really
cool trick...

Anyway, stay tuned for when I write the "everything ELSE you wanted to know..."
article.

1997\02\14@132741 by rpragana

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Hi Jeff and Tim,

 There is another matter worth talking about: the screen persistence.
LEDs don't have the same glow as screen phosphors, so the "turn-off"
faster. That explain why we should use a faster frame-ratio with LED
displays.

regards,
Rildo Pragana <EraseMErpraganaspamspamspamBeGoneacm.org>  from the Brazilian's Venice (Recife, PE)


On Mon, 10 Feb 1997, Jeff King wrote:

> At 07:34 AM 2/10/97 +0000, Tim Kerby wrote:
> >Hi
> >The television uses a refresh of 25 frames per second.  Does your tv flicker?
> >
> >Tim
>
> But don't forget its field rate is 50hz (PAL). So its refresh rate is 50 hz
not
> 25 hz.
>

1997\02\23@050925 by Zemin Liu

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It is frequent enough.

Slowest rate > 7 X 50 = 350/sec.

Zemin Liu


{Quote hidden}


'Moving Message Display'
1998\06\09@190003 by Thomas McGahee
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Pedro,
How long does your message have to be? You are limited
in the amount of RAM available on the PIC, but you can
augment that by storing the stuff in shift registers
or even standard RAM chips. I have built LED displays
that use a PIC to shift column data to the left. Each
PIC controls three banks of data that consist of 7
rows and 6 columns. Each bank thus accounts
for a 5x7 character plus inter-character spacing.
Altogether each PIC controls 7 rows by 24 columns.
Data is displayed 3 columns at a time, so duty cycle
is close to 1/8. The number of columns is limited by the
available RAM in the PIC. I needed a few registers
to hold system variables.  :)

One of the tricks that I use is to latch
two of the columns and the row select bits and then
put the last column out direct (not latched).
That saves me one set of latches.
I use 74373 style tristate latches. I drive
standard size LEDs.

Data is received over a single pin serial line and
transmitted over a second single pin serial line. This
allows me to connect the "output" of one PIC driver module to the
"input" of the next. Currently I use a portable PC to
input the data to the first (rightmost) Pic driver module.

Input data is received as 8 bit data and stored in a
file register. Every time new data comes in, the data
is "bumped" or moved to the file register to the "left".
When data is bumped out of the "leftmost" register,
I send it in serial format to the PIC module to the
left of the current pic module. I use a single system
clock that drives all the PIC modules.

Modules can be added pretty much indefinitely as long
as you buffer the clock occassionally.

Instead of using standard serial UART routines, I used a
self-clocking scheme that allows a large range of
baud rates. The PC communicates in serial mode, but using
a bit from the PCs parallel port. My reason was simply
so that I could control the self-clocking. One bit
of the parallel port was used as an input. This allowed
me to send clocking and other handshake info to the PC so it
could operate at maximum speed.

The PC would output serial data in self-clocking fashion
to the rightmost PIC. The MSB of the 8 bit data was used to
indicate that control rather than data was being sent.
This allowed me to send instructions such as Clear All LEDs
to the entire string of PIC modules. Each PIC would
simply relay these commands when they received them.
I wrote up a program in Foxpro that allows me to create
custom characters up to 7x8. This program then created
a memory map that allows me to decode any ascii character
to its visual components. To make the display capable of
handling more characters I use the MSB to indicate when
a character is finished. I usually include one blank
column as a separator for characters. Thus a character
like "i" will occupy less width than the character "W".
In other words, the technique allows proportional fonts.

I prepare the text that I want displayed in a text file.
Then I run the PC program that loads the desired font
set, decodes the characters, and sends the column data
to the first PIC. Scrolling speed is software adjustable,
and provision is made to use multiple font sets within a
single message. Once a message is completely sent I send
out a string of spaces and send the message out again and
again.

This all works fairly well, is easy to understand and
implement on a 16C84, and has a reasonable parts count.
The biggest expense is all the LEDs. If you use the newer
super-bright LEDs you will pay more, but you can move the
LEDs further apart and get a really large display that
is visible for reasonable distances.

Hope this helps.
Fr. Tom McGahee

----------
{Quote hidden}

1998\06\09@224309 by Regulus Berdin

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> From: Thomas McGahee <spamBeGonetom_mcgaheeSTOPspamspamEraseMESIGMAIS.COM>
> One of the tricks that I use is to latch
> two of the columns and the row select bits and then
> put the last column out direct (not latched).
> That saves me one set of latches.
> I use 74373 style tristate latches. I drive
> standard size LEDs.

I had built before a similar display but I had used 74164
which had no tristate.  What I did was to turn-off
momentarily the scanning column during shifting of data.
All 74164s are cascaded so only 2 pins (CLK and DATA)
are used for inputs and 7 for scanning.  I had to use
'164 because it is the only available at the local store.
It was made with a 6805 micro.


> Data is received over a single pin serial line and
> transmitted over a second single pin serial line. This
> allows me to connect the "output" of one PIC driver module to the
> "input" of the next. Currently I use a portable PC to
> input the data to the first (rightmost) Pic driver module.

Before I made the 6805 interface I used a PC to test the
display module using its parallel port. I made the test
program using qbasic using graphics mode for the character
generation.


> I wrote up a program in Foxpro that allows me to create
> custom characters up to 7x8.

I used Excel to map the characters by putting '1' on the
cells and making the column width smaller so it may look
like a 5x7 char.  Custom chars are easily made this way
and no other programming is needed only by just making the
right formula.


Rgds,
Reggie

1998\06\10@151736 by Pedro Barrios

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Tom,

>How long does your message have to be?

My message is pretty short, probably no more than 20 characters.


>Altogether each PIC controls 7 rows by 24 columns.

How did you manage to control 24 columns? Are you using a decoder here?

>One of the tricks that I use is to latch
>two of the columns and the row select bits and then
>put the last column out direct (not latched).
>That saves me one set of latches.
>I use 74373 style tristate latches. I drive
>standard size LEDs.

       ^
       |
How is this saving you a set of latches?


WOW ! impressive ! This is far too advanced for what I want
to do. I content myself with putting the message directly
in the source code; at least for now :)

Thanks,

Pedro


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1998\06\10@151741 by Pedro Barrios

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Hi Reggie,

I'm also using Excel the way you describe it. I, however,
don't understand how a formula can be used to generate
a message.

Would you please explain me a little bit more about this?

Thanks,

Pedro

{Quote hidden}

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