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'[PICLIST] [OT] Time Division Multiplexed'
2001\05\31@123843 by ckchan

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Greetings,

pls point me to site that have this information about time division
multiplexed for LED display. i'm having hard time understanding how they
scan out a character. thanks.


regards,
ckchan

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2001\05\31@133935 by Mark Newland

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I don't know of any site but this is basically what it is doing.

The human eye has a scan rate of about 50 frames per second.  That means
that if you flash something less than 50 times per second (lets pick 20),
you will see the pulses or flashing.  Above 50 times per second (lets say
70), the human eye will see it as being constantly on. Knowing this, we can
flash out LED display at a rate faster than 50 times per second and won't
notice a thing.

Time division multiplexing means that we only display 1 digit at a time.
First we display the one's digit, then we display the 10's digit, then we
display the 100's digit, then we display the 1000's digit, etc.  If we can
display all the digits (one at a time) more than 50 times per second, you
shouldn't notice any flicking.

You will have two sets of control lines to handle this.  One will be the
digit information and these lines will all be tied together at each
display.  You will have a seperate set of control lines telling which digit
we are displaying at any given time.  Only one of these lines will be active
at any one time.  This 2nd set of control lines connect to either the common
anode or common cathode depending on what type of LED display your useing.

ckchan wrote:

{Quote hidden}

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'[PICLIST] [OT] Time Division Multiplexed'
2001\06\01@024530 by ckchan
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Greetings,


{Quote hidden}

Thanks for responding. Well, i've read up some app note, it seem that they will
scan each row for predetermined time, and the data shifted out will be
displayed. what is the algorithm to pump out data into the shift register ? tia.



regards,
ckchan

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2001\06\01@072901 by michael brown

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> I don't know of any site but this is basically what it is doing.
>
> The human eye has a scan rate of about 50 frames per second.  That means
> that if you flash something less than 50 times per second (lets pick 20),
> you will see the pulses or flashing.  Above 50 times per second (lets say
> 70), the human eye will see it as being constantly on. Knowing this, we
can
> flash out LED display at a rate faster than 50 times per second and won't
> notice a thing.

I think its more like 10 images per second.  But that does depend on the
person.  Thats why movies appear to move at only 20 or so frames per second.

>
> Time division multiplexing means that we only display 1 digit at a time.
> First we display the one's digit, then we display the 10's digit, then we
> display the 100's digit, then we display the 1000's digit, etc.  If we can
> display all the digits (one at a time) more than 50 times per second, you
> shouldn't notice any flicking.

I'm not sure why this is, but if you stand facing a mirror and hold a
digital clock with the display facing the mirror (so you can see its
reflection) and move it around quickly, often you can see the individual
numbers broken apart.

>
> You will have two sets of control lines to handle this.  One will be the
> digit information and these lines will all be tied together at each
> display.  You will have a seperate set of control lines telling which
digit
> we are displaying at any given time.  Only one of these lines will be
active
> at any one time.  This 2nd set of control lines connect to either the
common
{Quote hidden}

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2001\06\01@083040 by Bob Ammerman
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US (NTSC) Television: approximately 30 frames per second with 2 fields per
frame.

Euro (PAL) TV: approximately 25 frames per second with 2 fields per frame.

Movies: 24 frames per second.

You local game of Quake III: Arena: depends on your video card and monitor
refresh rate :-)

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

2001\06\01@092351 by Alan B. Pearce

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>I'm not sure why this is, but if you stand facing a mirror and hold a
>digital clock with the display facing the mirror (so you can see its
>reflection) and move it around quickly, often you can see the individual
>numbers broken apart.

It does not need to be in a mirror. If you have a calculator or digital
watch with LED display and wave it about in front of you while looking at a
fixed point you can see this effect.

Another effect that I have noticed is that a TV screen will show flicker
when seen out the corner of your eye. Peripheral vision is much more
sensitive to these effects, and does not have the "afterglow" effect of
straight ahead vision. The other effect I have noticed is when there is an
oscilloscope screen off centre of vision, it is sometimes possible to get
the effect of the spot scan being stopped mid screen when you move your
head. This is somewhat dependant on the scope scan speed and the direction
of movement of your eyeball relative to the trace scan direction.

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2001\06\01@103130 by rottosen

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"Alan B. Pearce" wrote:
{Quote hidden}

I was once testing a circuit that generated pulses at a slow rate. The
scope took a good fraction of a second to sweep across the CRT when
viewing the pulses. I noticed that the scope sweep was nonlinear -- the
pulses were bunched at one edge of the screen. I spent some time trying
to find what was funny about the scope.

It turned out that it was an illusion. My eyes fixed at the left side of
the screen waiting for the sweep to start and then took a while to catch
up with the slow sweep of the beam across the screen. This made the
pulses look closer together at the start of the sweep if I remember
correctly.


-- Rich



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2001\06\01@104809 by John Walshe

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AFAIK the reason you see the broken image is very similar to that of an
object viewed under a strobe light. Although we cannot see it ourselves, our
eyes are in constant motion but most of that motion is centred about
direction in which you are viewing(i.e. centre of attention). But because
the eye is taking many "snapshots" in any fixed direction, any object moving
into and out of that space at fast enough a speed will appear and disappear.

John

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