How to connect two
TV sets to one VCR
In September, we ran a project that could
extend a video recorder's IR remote control
to allow operation from another room. It was
a great little project but it didn't explain just
how the remote TV set was also connected to
the VCR. Here's how to do it.
By JIM LAWLER
So how does the VCR in the
lounge room feed its picture to a second TV set in the bedroom? Well,
it's quite easy to arrange a simple
two-TV set-up in any home of
average size.
The first thing to understand is
that the RF signals at the output of
your VCR are at a very high frequency and very low voltage. The
signals are measured in microvolts
which means that special care must
be taken with distribution if they
are not .going to be lost along the
way.
First and foremost, it is necessary to understand that high fre-
quency transmission lines are not
like ordinary power cables. They
have to be properly matched to the
VCR and the TV if the signal is not
to be dissipated in the wiring. Second, if a cable is not properly terminated, it can set up reflections
along the line which show up as
ghosts on the picture.
To avoid signal losses and ghosts,
it is important to match the impedance of the transmission line to
the output of the VCR and to the input of the TV. For convenience, this
impedance has been standardised
at 75 ohms and all domestic equipment is designed for this value. So,
provided that you select cable and
hardware suited to this application,
no problems will be encountered.
To avoid complex mathematics in
signal level calculations, engineers
and technicians have adopted the
use of decibels as a convenient
measure of signal level. For TV
signals, most antenna installers use
dBµV, or decibels above 1 microvolt.
By using dBµ V values, it is only
necessary to add together all the
losses in the system and subtract
them from the signal level available to arrive at the level that will
be available at the TV. Or working
the other way, one adds the
calculated losses to the required
signal level at the TV, to arrive at
the necessary input level from the
VCR or other source (eg, an antenna). If that sounds confusing, don't
worry. We'll give you a few examples to show how it's done,
For the rest of this article, we
will talk about dBµV on the
understanding that older TVs need
60dB of signal to give a good picture. Modern sets need only around
These TV signal
splitters are made by
HPM Industries Pty Ltd
and are available from
large hardware stores
(eg, BBC Hardware &
Mitre-10). The unit at
left is a 2-way splitter
box (Cat. 406/2S/TV)
while at right is
boosted 4-way splitter
(Cat. 406/B4S/TV) which
has an overall gain of
8dB at each outlet.
6
SILICON CHIP
L.....______.1
VCR
VCR
TWO-WAY
SPLITTER
i-----
□
LOCAL TV
(b)
------.0
Dc::::::::::l
REMOTE TV
VCR
Ct:::=::J
LOCAL TV
(c)
1 2 3
TO REMOTE
RECEIVERS
Fig.1: here's how to split the output from your VCR to feed two or four
TV sets. In the arrangement shown in (b), a simple 2-way splitter is
used to split the output from the VCR, while at (c) the output from the
VCR is amplified before being fed to a 4-way splitter. The amplifier &
splitter may be separate units or combined in a single box.
55dB for the same result but it's
wise to use the higher level to leave
a good margin of safety.
For difficult or critical applications, it is often necessary to
measure the available signal with a
signal strength meter. However, for
domestic applications, an approximation of signal strength will
usually give satisfactory results.
Most video recorders have an
output level of about 65dB. This
represents the level of signal that
would reach a TV set fed from an
average antenna in an average
reception area. Thus, if the VCR
output is fed to a single TV at this
level, the picture should be just
about perfect. Even allowing for
poor contacts in the plugs and
sockets, enough signal should still
be left over to give an excellent
picture.
Splitting the signal
If one wants to split the signal
between two TV sets, certain
precautions must be taken. As
already mentioned, the impedance
must be matched and maintained
throughout the system. This means
that the leads to the TV sets cannot
simply be cut and twisted together,
as this would result in a gross
mismatch of impedances. If this
were done, you would get ghosts
and a very noisy (or snowy) picture.
Instead, a splitter umt must be
used which allows accurate matching and a minimum of signal loss
at the split. Fortunately, splitters
are readily available from electronics retailers and even from
most hardware stores.
Splitters come in many styles,
some square and boxy and others
similar in appearance to the common mains power outlet. Depending
on your application, you can get
2-way splitters (to feed two TV
sets), 3-way splitters and 4-way
splitters. In-line splitters that connect to the cable with common coax
plugs are also available but these
are not the best choice because the
plugs and sockets can work loose
after awhile. The wall mounted,
screw fixing type are much more
reliable.
The main necessity is that the
splitter be designed for 75-ohm
systems. It must also be a wideband
type if it is to handle UHF signals.
In the normal domestic VCR installation, a lead is taken straight
from the VCR's "RF Out" socket to
the TV's antenna socket (Fig.la).
For a two-TV system, we need to
split the single output cable from
the video using a 2-way splitter.
One output from the splitter is then
connected to the local TV while the
other is connected to the remote TV
in the other room (Fig.lb).
The cable to the remote TV
should be as short as possible
otherwise cable losses may become
a problem. If the cable length is
10-20 metres or so, the losses can
usually be ignored (except in difficult cases). But 40 or 50 metres of
cable will cause a loss that must be
allowed for in most installations.
The need to keep the lead short
means that it is best run under the
floor. This way, the cable can be
run in a straight line from a point
near the VCR to a spot immediately
under the outlet in the other room.
The best procedure is to pull the
carpet back, drill a 10mm hole for
the cable hard up against the skirting board, and then fit the splitter
(or outlet socket) just above the
hole. When the carpet is pushed
back, the cable will be almost invisible and the whole assembly is neat
and inconspicuous.
Calculating signal levels
The best place to start this kind
of installation is to fit an outlet
socket at the remote TV end of the
cable. To get a good picture, you
will need 60dB of signal at this
point. The next step is to measure
the cable length back to the VCR
and add the loss represented by
this length of cable. Let's say it was
2dB, which is an average figure for
a bout 15 metres of domestic grade
coax. This means that there must
be at least 62dB of signal fed into
the cable.
We now need to take into account the losses in the splitter. A
NOVEMBER 1990
7
4-way splitter. The losses in these
items can amount to 5, 7 or even
9dB, so they become quite important in the calculations.
Note that you can use a 4-way
splitter to feed three TV sets, although it will have somewhat
higher losses than a 3-way type. If
you do use this kind of setup, the
unused output should have a
75-ohm resistor connected across it
to ensure correct matching. Do not
leave the unused output open
circuit.
Distribution amplifiers
These splitter units are sold by Dick Smith Electronics but similar units are
also available from other retailers. The units shown are a 2-way splitter, a
4-way splitter & a 2-way plug-type splitter (bottom).
This photo shows the 2-way & 4-way splitters with their covers removed. All
you have to do is connect the lead from your VCR to the input terminal, then
run coaxial cable from the outputs to your TV sets.
good quality 2-way splitter will
have an insertion loss of 3dB on
each leg. This means that the input
to the splitter must be + 3dB above
its calculated output, or 65dB.
Notice how the losses are added all
the way from the output to the
input.
In this exercise, we have calculated that we want 65db into the
splitter and this is just about what
8
SILICON CHIP
is supplied by the average VCR. If
the cable loss was higher (as it
might be for a large house or if the
cable could not be run via a direct
route), then there could be too little
signal at the remote TV. In this
case, some kind of amplifier would
be needed.
Also, if you want to send your
VCR signals to more than two TV
sets, you will need to use a 3 or
For anything but the most compact systems, you will almost certainly need a small distribution
amplifier to compensate for the
higher losses in the multi-way splitter. Various manufacturers have
recognised this need and have provided a series of small distribution
amplifiers for domestic use. These
give from 10 to 25dB signal gain
and can compensate for the losses
in any reasonable home situation.
Mostly, they are powered from the
mains, usually via a 6-12 volt
plugpack supply.
The amplifier can be mounted
close to the back of the VCR or on
the skirting board close to the
power point. The "RF Out" from
the VCR goes to the amplifier input,
while the amplifier output is fed to
the splitter (Fig.le). Do not install
the amplifier after the splitter.
(Some years ago I made my own
distribution amplifier from a disused masthead amplifier. I modified
the input to suit 75-ohm coax and
arranged a 12-volt power feed from
within the VCR itself. This amplifier
provided 26dB of gain following the
VCR output and allowed me to
reticulate signal to the TV set in the
lounge, as well as to the kitchen, a
bedroom and to four outlets in my
workshop downstairs!)
The simple distribution system
described here will suit up to about
four TV sets. If more sets are needed on the system, or if long ea ble
runs are necessary, then another
system of distribution will be needed. This is the "Main Trunk"
system which is simple enough in
principle but somewhat beyond the
scope of this article. Maybe
another time.
~
