friend of the author recently purchased a good secondhand
colour television at a bargain price. The reason for the
bargain price became apparent some weeks later - the set would
not give a stable picture when playing a video tape that has
copy-protection, such as those hired from a video
It was at this point that the author became
involved. The television manufacturer's UK agent was
aware of the problem, but could not offer a modification to
resolve it! The author did not feel inclined to delve
into the innards of the television without information, and
his friend didn't really want to buy another set. The
only remaining option was to design a device to remove the
copy-protection from the signal before it reached the
TV. The result is the Video Stabiliser described
The principle of copy protection is to
alter the video signal such that it can be displayed normally
on a TV or monitor, but not recorded by a video
recorder. By examining the signal with an oscilloscope,
the author found two areas where it is altered.
the video level in the top section of the picture, normally
reserved for teletext information, is varied from black to
peak white. Many video recorders use this part of the
signal to set their automatic recording level, resulting in a
signal with wildly varying amplitude that a television cannot
Secondly the colour burst has a
DC offset in the last few lines of the picture. This is
visible on some televisions as a dark section at the bottom of
the screen. Many video recorders will confuse this with
the frame sync signal, and lock onto it instead of the true
The Video Stabiliser presented here will
remove both of the above signal alterations. The
prototype enabled the author's friend to enjoy his new
toy! Removal of the colour burst offset is optional and
selected by a front panel switch. A second switch
enables the unit to be bypassed when not required.
prototype unit was connected between the video output
connector on the video recorder and the video input connector
on the television. If it is being used with a television
that does not have a composite video input socket, the signal
must be modulated before being fed to the aerial socket.
This can be achieved by connecting the unit between two video
How It Works
circuit diagram is shown in fig ?.
The video enters the
unit via PL1. SW1 selects whether the circuit is enabled
or bypassed. R8 terminates the input.
around TR1 is a sync separator, the output across R4 pulses
between 0V and 6V approx.
TR2 is fed by an integrator
circuit to produce the frame sync. D5, R13 and C9 extend
this over the teletext area at the top of the screen.
This is squared up by the logic gate to form the Blank Enable
signal, which is high during this part of the
If SW2 is closed, D6, C10 and R14 extend the
line sync pulse over the colour burst. The non-extended
sync is gated with this to produce the Burst Enable signal,
which is high during the colour burst.
Video Enable is
high whenever the other two enable lines are low, and covers
the normal video and sync part of the picture.
signal is inverted by IC1 and R6. This is then
attenuated and set to a suitable DC level by R7, R9, R12, R13
and C5, and forms the blanked video signal.
signal is held with its base line at around 0V by C7, R10, and
D2. This is the normal video. Note that D2 must be
a germanium device.
C6, R16 and R17 remove all DC from
the video signal and leave the chrominance and colour burst,
biased a little above the 0V rail. This forms the
These three signals are fed into
analogue switches controlled by the enable lines. Only
one switch is closed at a time, and consequently the required
signal is assembled. The result is buffered by the
output amplifier (TR3, TR4 and surrounding components).
C13 adds a little HF boost to compensate for losses earlier in
A supply of 9V (+/- 1V) at 35mA is
required, this does not need to be regulated since the current
consumption is reasonably constant. A low cost unit
built into a large 13 Amp plug may be suitable - select one
that can be left on indefinitely. If the voltage is
selectable it may have to be set to 6V since the load is
Any ripple on the power supply output is removed
by C12. D5 protects the unit against reverse
polarity. The supply to the sync separator is further
decoupled by R19 and C9.
Note that IC1 and IC2 are
operated from a supply of about 6V (due to R14, R24 and
C14). This is because the sync separator output only
goes up to about 6V and a gate powered from the 9V rail would
not respond to it.
circuit is constructed on a single sided PCB, 84mm x
92mm. The component overlay is shown in fig ?.
Construction is reasonably
straightforward. Start with the six wire links, and then
continue in the usual size order. IC1 and IC2 are both
CMOS devices and IC sockets are recommended. D2 is a
germanium diode, take care to ensure that it does not get too
hot. Veropins or 0.1" header strip are recommended for
the off-board connection.
Any case that is large enough
to hold the PCB, sockets and switches would be suitable.
A metal case may be preferable for screening but no problems
were experiences with the prototype in its plastic
The interwiring is shown in fig ?. Screened
cable may be used for the video connections, but this is not
really necessary providing the wire is short.
switches can be considered as optional - if one or both are
not required link wires should be fitted in the PCB as
Providing the unit is
carefully constructed there is no reason why it shouldn't work
first time. If an oscilloscope is available, the output
can be viewed when playing a copy-protected tape to verify
that the offending signals have been removed. Otherwise
simply connect the unit to your equipment and try
Measure the voltage across C12, which must be 9V
+/-1V. If the voltage is selectable on the PSU, try
other settings to bring the voltage within
Alternatively, if the voltage is a little too
high it may be possible to reduce it by connecting one or more
1N4001 diodes in series, in the positive connection to the
PCB. Each diode will drop about 0.6V.
cases a dark section may be visible at the top of the
screen. This is caused by the delay produced by R15 and
C8 being longer than required. It appears that this
delay differs slightly with different makes of 4001 IC,
however the colour burst delay due to C10 and R18 is
If this problem occurs there are two
options. If you have a stock of IC's try some other
4001's to find one that's suitable. Otherwise try
reducing the value of R15 to 27K or 22K, find the highest
value where the dark section is not visible.
For normal viewing it may be preferable to set
SW1 to bypass, to eliminate the inevitable slight signal
degrading that this unit causes. When viewing a
troublesome video film, the unit should be switched in.
SW2 should only be closed if it proves necessary, since this
may cause slight colour variation problems.
not use this unit for any other purpose!
Resistors (All 0.25W 5% or
BNC Socket (or as
2.1mm Power Socket (or as
DPDT Miniature Toggle
SPST Miniature Toggle Switch
PCB, Case, Power Supply
(see text), Veropins, Wire, PCB Mounting Hardware, Video Cable
and Connectors as Required.
All components are readily available
and no buying problems are envisaged.
capacitors should be rated at 16V or greater, and be radial
types. The other capacitors should have a lead pitch of
0.2" (5mm), other types may fit if the leads are
Before purchasing a power supply, check the
latest Greenweld bargain list. They often list suitable
types for about two or three pounds.