"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

> dtth wrote:
>
> >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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