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'[EE]: Video matching transformer'
2002\07\17@174659 by Tal Bejerano - AMC

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hi All

Can anyone help finding 1:1 75:75ohm video matching transformer.
deep search in google and some companies search didn't gave any result.
what I'm looking for is to distribute one video signal to some amplifiers
inputs.



Regards

Tal Bejerano
AMC - ISRAEL

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2002\07\17@230949 by Jim

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Video (30 Hz to 4 or 5 MHz)

or RF (50 MHz ... 700 MHz) transformers?

(I *think* you may be looking for a 'video
distribution amplifier'.)

Jim

{Original Message removed}

2002\07\18@041743 by Alan B. Pearce

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>Video (30 Hz to 4 or 5 MHz)

>or RF (50 MHz ... 700 MHz) transformers?

>(I *think* you may be looking for a 'video
>distribution amplifier'.)

>Jim


>> Can anyone help finding 1:1 75:75ohm video matching transformer.
>> deep search in google and some companies search didn't gave any result.
>> what I'm looking for is to distribute one video signal to some amplifiers
>> inputs.

That was my thought too Jim.

Tal, a transformer will not work at video frequencies because there is an
inherent DC component to the signal, and TV set designers go to some lengths
to have what is known as a DC restorer circuit after AC coupled portions of
the circuitry. For this reason if you are really trying to distribute video
you will need a distribution amplifier.

If it really is RF that you are trying to deal with then your local TV shop
should have an aerial splitter/combiner that may do what you want. This
consists of a small ferrite based transformer with multiple windings on it
to split the signal out to multiple loads.

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2002\07\18@051551 by Tal Bejerano - AMC

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Alan And Jim

I have built a video distributor that made of some video amplifiers ic's.
my problem is how to connect the video amplifiers inputs pins all together
from one source without having any interferes.
do I need any resistor network? any transformers? capacitors? or just wire
them all together?
the video source is the video cassette AV output. audio is taking care with
other audio amplifiers.

Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\07\18@053928 by David Duffy

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At 12:12 PM 18/07/02 +0200, you wrote:
>Alan And Jim
>
>I have built a video distributor that made of some video amplifiers ic's.
>my problem is how to connect the video amplifiers inputs pins all together
>from one source without having any interferes.
>do I need any resistor network? any transformers? capacitors? or just wire
>them all together?
>the video source is the video cassette AV output. audio is taking care with
>other audio amplifiers.

Wiring the non inverting inputs together via series 51R resistors works well.
Just remember that you only need one 75 ohm termination resistor on the input.

>Video (30 Hz to 4 or 5 MHz)

{Quote hidden}

No, you really can use transformers for composite video. That's how "braid
breakers"
work. Mind you, the transformers are not your average type. (much more
expensive)
Tal doesn't need one in his case. You can buy transformers to send video
down CAT5
cabling. They work surprising well too. Not broadcast standard but not bad
either.
Regards...

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2002\07\18@054603 by Tal Bejerano - AMC

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David

Wire each input with 51R?

Regards

Tal Bejerano
AMC - ISRAEL


-----Original Message-----
From: pic microcontroller discussion list [spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU]On
Behalf Of David Duffy
Sent: Thursday, July 18, 2002 11:57 AM
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: Re: [EE]: Video matching transformer


At 12:12 PM 18/07/02 +0200, you wrote:
>Alan And Jim
>
>I have built a video distributor that made of some video amplifiers ic's.
>my problem is how to connect the video amplifiers inputs pins all together
>from one source without having any interferes.
>do I need any resistor network? any transformers? capacitors? or just wire
>them all together?
>the video source is the video cassette AV output. audio is taking care with
>other audio amplifiers.

Wiring the non inverting inputs together via series 51R resistors works
well.
Just remember that you only need one 75 ohm termination resistor on the
input.

>Video (30 Hz to 4 or 5 MHz)

{Quote hidden}

amplifiers
> >> inputs.
>
>That was my thought too Jim.
>
>Tal, a transformer will not work at video frequencies because there is an
>inherent DC component to the signal, and TV set designers go to some
lengths
>to have what is known as a DC restorer circuit after AC coupled portions of
>the circuitry. For this reason if you are really trying to distribute video
>you will need a distribution amplifier.

No, you really can use transformers for composite video. That's how "braid
breakers"
work. Mind you, the transformers are not your average type. (much more
expensive)
Tal doesn't need one in his case. You can buy transformers to send video
down CAT5
cabling. They work surprising well too. Not broadcast standard but not bad
either.
Regards...

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2002\07\18@061016 by David Duffy

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At 12:42 PM 18/07/02 +0200, you wrote:
>David
>
>Wire each input with 51R?

Use a 51R resistor between your input socket and each video
opamp non-inverting input pin. (4 opamps - use 4 resistors)
I'm taking a bit of a wind guess on your circuit mind you!
Send it to me & I will help you out if I can.  :-)
Regards...

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2002\07\18@215036 by Doug Butler

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There certainly are video transformers.  We use them occasionally at work
when we need to DC isolate a video signal.  It is just a toroidial
transformer carefully wound to give a good frequency responce from about
30Hz to 5MHz.  The DC component is lost but I have never noticed that to
make a visable difference in the picture.  I think modern monitors di their
own level shifting.  I will try to find out who makes them tomorrow.

Doug Butler
Sherpa Engineering


> {Original Message removed}

2002\07\19@031023 by Alan B. Pearce

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>There certainly are video transformers.  We use them occasionally
>at work when we need to DC isolate a video signal.  It is just a
>toroidial transformer carefully wound to give a good frequency
>responce from about 30Hz to 5MHz.  The DC component is lost but I
>have never noticed that to make a visable difference in the picture.
>I think modern monitors di their own level shifting.  I will try
>to find out who makes them tomorrow.

OK it may be that the expectation is that the monitor will do the DC restore
function anyway, and hence there is no attempt to do it with the
transformer.

IIRC the DC restoration is done by clamping the signal to ground during the
sync pulse, so the worst case is that the transformer has to maintain a
reasonably constant level signal for one line period. My TV & video theory
is getting pretty rusty, but that is how I remember it.

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2002\07\19@032321 by Roman Black

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Alan B. Pearce wrote:
>
> >There certainly are video transformers.  We use them occasionally
> >at work when we need to DC isolate a video signal.

> OK it may be that the expectation is that the monitor will do the DC restore
> function anyway, and hence there is no attempt to do it with the
> transformer.
>
> IIRC the DC restoration is done by clamping the signal to ground during the
> sync pulse, so the worst case is that the transformer has to maintain a
> reasonably constant level signal for one line period. My TV & video theory
> is getting pretty rusty, but that is how I remember it.


It may be possible to just couple the video signal
through a series capacitor, eliminating the need for
the video transformer.
-Roman

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2002\07\19@083734 by Olin Lathrop

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> IIRC the DC restoration is done by clamping the signal to ground during
the
> sync pulse,

The DC level for the scan line data is relative to the "front porch", not
the sync tip.  The amplitude of the sync tips are *not* constant from one
video source to another.


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2002\07\19@094418 by Peter L. Peres

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On Fri, 19 Jul 2002, Alan B. Pearce wrote:

>>There certainly are video transformers.  We use them occasionally
>>at work when we need to DC isolate a video signal.  It is just a
>>toroidial transformer carefully wound to give a good frequency
>>responce from about 30Hz to 5MHz.  The DC component is lost but I
>>have never noticed that to make a visable difference in the picture.
>>I think modern monitors di their own level shifting.  I will try
>>to find out who makes them tomorrow.
>
>OK it may be that the expectation is that the monitor will do the DC restore
>function anyway, and hence there is no attempt to do it with the
>transformer.
>
>IIRC the DC restoration is done by clamping the signal to ground during the
>sync pulse, so the worst case is that the transformer has to maintain a
>reasonably constant level signal for one line period. My TV & video theory
>is getting pretty rusty, but that is how I remember it.

You are right, hovewer all video and monitor equipment currently in use is
internally ac coupled, even though the lines are resistively matched at 75
ohms. So each and every unit restores the dc level using more or less
elaborate clamping.

Peter

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