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'[EE]:Multi-video to RF converter'
2002\03\08@233930 by dtth

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I’m trying to cascade multiple video to RF converter and send all these
RF signal to TV antenna input so that I can see multiple CCD camera
images on my TV. So far I’m trying to get video from two CCD cams but
problem arise when I turn on the power of the second converter. If the
second converter is of, I can see video from CCD cam 1 clearly but when
I switch on the second converter, the video is lost.  Comments are welcome :-)
Regards,
Donny Tan

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2002\03\09@022041 by sambuddy

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What your problem probably is,  is that the RF converters (modulators) are on the same channel (frequency). The only real way around this I can think of is to buy ones
that can opperate on different channels.

Regards
Stuart


9/03/02 11:35:39 PM, dtth <spam_OUTdtthTakeThisOuTspamTM.NET.MY> wrote:

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2002\03\09@073611 by Peter L. Peres

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Instead of cascading the RF in/outs use only the outputs and a cable
junction/distribution box (4-way or such). The pictures may get grainy.
You may have to retune the modulators so they do not sit on the same
frequency.

Peter

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2002\03\09@090118 by Roman Black

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Stuart O'Reilly wrote:
{Quote hidden}

Maybe a re-think of the total system and what you
NEED it to do? When a solution is difficult there is
probably an easier way to do it? :o)
-Roman

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2002\03\09@104110 by M. Adam Davis

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I'm assuming the RF converters are on seperate channels. If any are on
the same channel, then obviously you will have problems.

The not so nice thing about the american standard for TV is that the
bandwidth set aside for each channel was set aside based on the black
and white signals being sent at the time, and the thought that there
really wouldn't be that many stations broadcasting in one area at one
time. What this means is that a signal transmitting on a TV channel (say
3) will cause interferrence with a signal transmitting on an adjacent
channel (say 4) since the bandwith for a color signal is much larger
than a single channel. (This is NOT true for cable signals, which have a
larger bandwidth for each channel)

Lastly, I've noticed that cheap rf modulators create a small amount of
interference on many other channels, though they provide a good picture
for their own signal. Sometimes I think the people who make them assume
that when the power is on to the modulator, the only thing being watched
is their signal.

A real video distribution system has expensive modulators (well over $50
per channel), or a switch box of some kind.

Good luck.

-Adam

dtth wrote:

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2002\03\09@125955 by Robert Rolf

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"M. Adam Davis" wrote:
>
> I'm assuming the RF converters are on seperate channels. If any are on
> the same channel, then obviously you will have problems.
>
> The not so nice thing about the american standard for TV is that the
> bandwidth set aside for each channel was set aside based on the black
> and white signals being sent at the time, and the thought that there
> really wouldn't be that many stations broadcasting in one area at one
> time. What this means is that a signal transmitting on a TV channel (say
> 3) will cause interferrence with a signal transmitting on an adjacent
> channel (say 4) since the bandwith for a color signal is much larger
> than a single channel.

NO, it it NOT! It contains more information but it does NOT use more
bandwidth.

The NTSC committee was very clever in the way it
interleaved the extra information required for color, within the
existing B&W signal. That is why we have such bizzare values for
our scan rates. The harmonics of all the primary timings do not
interfere with each other because of a careful choice for those
frequencies. 14.31818 Mhz/4 = 3.479545 color subcarrier/227.5
=15.734263Khz H rate/262.5 =59.94 Hz V rate. The division by a
1/2 integer causes alternate fields to have 180 degree phase
reversals, resulting in visual cancelation of the color subcarrier
in Pre-color B&W sets. This also causes the odd harmonics to
fall into the gaps in the spectrum of other harmonics.
Modern color receivers/VCRs use this characteristic of the encoding
in their' 'comb filters' to get really good color extraction.

NTSC color was an amazing feat of engineering for it's time.
PAL uses similar frequency interleaving (different S/C (4.43...Mhz
and divisors), but with a 50Hz final V rate.

> (This is NOT true for cable signals, which have a
> larger bandwidth for each channel)

NO, cable signals have EXACTLY the same bandwidth constraints as
'off-air' signals, otherwise we'd all need new TV tuners to use basic
cable. Cable systems reduce the co-channel interferance by reducing
the amplitude of the audio carrier by some 20db, and by using
'HRC' harmonically related carriers (Phase locking if you will)
so that any intermodulation (beat) products between signals have
a stable phase, and are less visible as a result.


> Lastly, I've noticed that cheap rf modulators create a small amount of
> interference on many other channels, though they provide a good picture
> for their own signal. Sometimes I think the people who make them assume
> that when the power is on to the modulator, the only thing being watched
> is their signal.

Yep, that is what they assume, since they are meant for 'closed circuit'
operation. They don't bother with any filtering of the output, so there
are lots of spurious harmonics in their output.

Broadcast signals use "VSB" vestigial side band transmission.
In essense they are a SINGLE side band signal with 1Mhz? of the complimentary
side band also transmitted to improve the strength of the sync signals.
Cheap RF modulators don't use a VSB filter (they're not cheap),
and so are double side band (normal AM).
As a result occupy a 10Mhz channel, instead of the 6Mz of a real
'broadcast' signal.


> A real video distribution system has expensive modulators (well over $50
> per channel), or a switch box of some kind.

And the expensive modulators usually DO have the required VSB filter.


> Good luck.

One can purchase 'cable compatible' tunable RF modulators, that
will do the job. Look in any good 'home automation' magazine for
sources. Radio Shack used to sell a "4 channel UHF modulator" that
basically took channel 2-5 input on each of 4 ports and upconverted
them all to the UHF band 66-83 for cable distribution. It worked OK,
but was not frequency stable so modern digital tuners had problems
with it.

You can probably find some old VCR RF modulators (ch 3-4) to get
you a cheap solution for two cameras. Some had Inductors for tuning,
so you could push them up to ch 5 & 6 or down to ch2, or you could change the
crystal. You will have to use every other channel to use them without
interference.

Robert

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2002\03\09@180751 by Olin Lathrop

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> The not so nice thing about the american standard for TV is that the
> bandwidth set aside for each channel was set aside based on the black
> and white signals being sent at the time, and the thought that there
> really wouldn't be that many stations broadcasting in one area at one
> time. What this means is that a signal transmitting on a TV channel (say
> 3) will cause interferrence with a signal transmitting on an adjacent
> channel (say 4) since the bandwith for a color signal is much larger
> than a single channel.

Where do you get this stuff from!?  Yes, the 6MHz bandwidth was decided
based on the black+white and vacuum tube technology of the time.  However,
the color standard was deliberately made to fit in the same 6MHz bandwidth
by giving away a little bit of horizontal spacial resolution.  This was
actually a rather clever bit of engineering and an excellent lesson in
applied signal processing.  There's lots of information out there
describing the color broadcast TV standard, so I won't waste time
explaining it again here.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

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2002\03\10@044107 by M. Adam Davis

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Ah.  I read it somewhere, but do not remember where.  Apparently not
only were they wrong, but I've been spreading incorrect information...

At least I know now.  Thanks!

-Adam

Olin Lathrop wrote:

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2002\03\10@055315 by Lee Mason

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If the modulators are not on the same channel then I think the problem is a
feature of the modulator, it is a "off through" type. This type of modulator
will only allow rf to pass through when the power is removed, this is to
prevent interference from external RF on the
same channel.

Two ways to fix this
1 Find the pin diodes that are shorting the input when the power is applied
and remove them

2 Get a "mixed boost" modulator.


Regards




Lee Mason  <EraseMELeespam_OUTspamTakeThisOuTreflow.co.uk>
{Original Message removed}

2002\03\10@114913 by peter cousens
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3 use a simple resistive mixer. The values should not be critical unless
you're running the signal long distances after mixing 33 ohms from each
modulator and direct from the junction to the output should work ok



Video modulators need a far greater bandwidth between channels as the
sideband/s are not limited, you will NOT be able to use 6Meg, 10Meg
between channels is probably a good figure.

{Original Message removed}

2002\03\10@134730 by Robert Rolf

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peter cousens wrote:
>
> 3 use a simple resistive mixer. The values should not be critical unless

Or use a simple RF splitter, in reverse. That way your losses are much
lower since it is transformer based summation.

> you're running the signal long distances after mixing 33 ohms from each
> modulator and direct from the junction to the output should work ok
>
> Video modulators need a far greater bandwidth between channels as the
> sideband/s are not limited, you will NOT be able to use 6Meg, 10Meg
> between channels is probably a good figure.

Except that the Tuner he is likely to use won't see a signal that far
off channel (NA spacing is 6Mhz. Europe is mostly 8Mhz).
Every ALTERNATE channel works. E.G. OK 2,4,6. I've done it many
times with VCR RF modulators (from UMatics, which have cleaner outputs).

If you don't mind a little bit of interferance you can do 2,3,4,5,6,
depending on the quality of the modulator (most are crappy).
There are commercial, frequency selective RF combiners used for CATV
head ends that mix in off air signals, but they are quite expensive.

If he is in fact 'cascading' the cheap VCR modulator outputs (series chain),
then of course only the last modulator's output will be seen.


> {Original Message removed}

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