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'[EE]: Detecting Vertical Sync'
2002\09\16@143551 by -8859-1?Q?Hern=E1n_Freschi?=

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Hi there. I’m playing around with video and PICs, and I’d like to
overlay signals on the video. Question is: How do I detect the VSYNC and if possible HSYNC signals?

**** I don’t want to use ICs like LM1881 or EL4581, because I can’t get
them at my local electronics store, and I have to order them from
somewhere else (and wait!). ****

Is there a way of doing that with op-amps or something? All I want is a
pulse on the PIC port when VSYNC is present, so I can trigger the
software.

(BTW, the circuit would be for PAL-N).

Hernan Freschi

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2002\09\16@150705 by Olin Lathrop

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>>
Hi there. I'm playing around with video and PICs, and I'd like to
overlay signals on the video.
Question is: How do I detect the VSYNC and if possible HSYNC signals?

**** I don't want to use ICs like LM1881 or EL4581, because I can't get
them at my local electronics store, and I have to order them from
somewhere else (and wait!). ****

Is there a way of doing that with op-amps or something? All I want is a
pulse on the PIC port when VSYNC is present, so I can trigger the
software.
<<

This is like asking is it possible to heat food without a microwave oven.
Early TVs are an existance proof that this can be done with analog
electronics without any integrated circuits.

You first have to detect Hsync.  This is easy as it is the deliberately the
most negative point of the waveform.  Next low pass filter the Hsync pulses
and compare to a threshold.  That will be Vsync.  The Hsync pulses are
stretched during the vertical blanking interval.  If you don't want the lag
from a low pass filter, just look at the length of Vsync pulses.  Hsync
starts at the first long Vsync pulse.


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2002\09\16@154203 by Geo

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On 16 Sep 2002, at 15:05, Olin Lathrop wrote:

> This is like asking is it possible to heat food without a microwave oven.
> Early TVs are an existance proof that this can be done with analog
> electronics without any integrated circuits.
>
> You first have to detect Hsync.  This is easy as it is the deliberately the
> most negative point of the waveform.  Next low pass filter the Hsync pulses
> and compare to a threshold.  That will be Vsync.  The Hsync pulses are
> stretched during the vertical blanking interval.  If you don't want the lag
> from a low pass filter, just look at the length of Vsync pulses.  Hsync
> starts at the first long Vsync pulse.
>
Good reply. I have just had to do something similar with AVR. The pcb was
already designed! They just used a LM31x (I forget) comparator with a pot to
set the trip level in the centre of the sync pulse. I interrupted on each sync
pulse and checked for a timer overflow - the timer is reset for 40uS so you
can see if time between sync pulses was more or less than this..
The mark 2 pcb *WILL* use an LM1881 or Elantec equivalent to do the work
fom me and remove the preset pot adjustment (user twiddle) problem.


George Smith

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2002\09\16@160632 by Leandro

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Hi Hernan Freschi

If you don't want to use the 1881 (great ic!) you could grab a 12C508 and
program it for detect the pulses, I think that the best would be something
like the 16c621 (they have 2 comparators built in, but are more expensive)
the 12C508 are very cheap, and you could make it to detect the vertical
and horizontal sync and let you know to a bigger IC via another pin.

a good source to figure it out how to do it is looking for the tv decoding
cable circuits (a.k.a. "Decos" in my country ;-), those usually detect, using
operational amps, the supressed sync of the video encoded signal to
lock on, the code is very interesting to take a look, there are over there
a lot of that, but you could start looking for the "3chi" code Decos.
if they can lock on a supressed sync video signals, do that with a good
video signal it's far to easy :-).

BTW: if you use Pal-N, are you from Argentina?, if so, did you try to
find the LM1881 in GM Electronica or CIKA ?, I usually get this ic from
those stores.

If you can't find any 3chi code, i can look for it, contact me via private
and I'll send you the asm if you need it.

good luck

Leo


At 03:35 PM 9/16/02, you wrote:
{Quote hidden}

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2002\09\16@165550 by Francisco Ares

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Check the schematics on page 6 from this PDF

http://www.national.com/ds/LM/LM1881.pdf

It is the LM1881 datasheet

Francisco


Hernan Freschi wrote:

{Quote hidden}

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2002\09\17@034656 by Alan B. Pearce

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>Is there a way of doing that with op-amps or something? All I want is a
>pulse on the PIC port when VSYNC is present, so I can trigger the
>software.

Well you should be able to do it in software without too much trouble. You
are going to need the HSYNC signal in the PIC anyway, and all you need to do
is look for when the pulse has the inverted width to know when you are in
the VSYNC time.

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2002\09\17@123243 by Harold M Hallikainen

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On Mon, 16 Sep 2002 20:41:27 +0100 Geo <.....lintechKILLspamspam@spam@BLUEYONDER.CO.UK> writes:
> On 16 Sep 2002, at 15:05, Olin Lathrop wrote:
>
> > This is like asking is it possible to heat food without a
> microwave oven.
> > Early TVs are an existance proof that this can be done with analog
> > electronics without any integrated circuits.
> >

       But, you've gotta use vacuum tubes!

Harold


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2002\09\17@124357 by -8859-1?Q?Hern=E1n_Freschi?=
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Well, considering that the microwave IS a vacuum tube (have you ever
heard the word MAGNETRON?)

-----Mensaje original-----
De: pic microcontroller discussion list [.....PICLISTKILLspamspam.....MITVMA.MIT.EDU]
En nombre de Harold M Hallikainen
Enviado el: Martes, 17 de Septiembre de 2002 01:22 p.m.
Para: EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU
Asunto: Re: [EE]: Detecting Vertical Sync

On Mon, 16 Sep 2002 20:41:27 +0100 Geo <lintechspamspam_OUTBLUEYONDER.CO.UK>
writes:
> On 16 Sep 2002, at 15:05, Olin Lathrop wrote:
>
> > This is like asking is it possible to heat food without a
> microwave oven.
> > Early TVs are an existance proof that this can be done with analog
> > electronics without any integrated circuits.
> >

       But, you've gotta use vacuum tubes!

Harold


FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at
http://www.dovesystems.com

Reach broadcasters, engineers, manufacturers, compliance labs, and
attorneys.
Advertise at http://www.hallikainen.com/FccRules/ .


________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
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2002\09\17@133432 by Peter L. Peres

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On Mon, 16 Sep 2002, Geo wrote:

*>Good reply. I have just had to do something similar with AVR. The pcb was
*>already designed! They just used a LM31x (I forget) comparator with a pot to
*>set the trip level in the centre of the sync pulse. I interrupted on each sync

Come on, everyone knows about clamping and using the peak rectified
voltage from clamping to feed the other side of the comparator. Or amplify
the clamped video 8-10 times and feed directly to digital input (clamped
0.3Vpp sync pulses become 0.3*8 = 2.4Vpp pulses and the rest of the
picture only raises the voltage more). The amplifier does not need to have
video bandwidth. You can use the clamping circuit I sent to the list in a
previous thread (using bipolar current mirror as clamp).

Peter

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2002\09\17@143113 by Geo

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On 17 Sep 2002, at 1:13, Peter L. Peres wrote:

> On Mon, 16 Sep 2002, Geo wrote:
>
> * The pcb was
> *already designed! They just used a LM31x (I forget) comparator with a pot to
> *set the trip level in the centre of the sync pulse.
>
> Come on, everyone knows about clamping and using the peak rectified
> voltage from clamping to feed the other side of the comparator.

As I said - the pcb was already designed - I was presented with a loaded
board to get going.



George Smith

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