Interference effects on TV Interference mechanisms
Interfering signals can arrive at the television,
radio or other domestic entertainment equipment
by one or more of the following paths:
Via the aerial and the
co-ax feeder downlead
The signals are fed directly from the aerial via the
downlead to the receiver RF stages. Dependent on
the received signal strength, frequency and other
characteristics of the signal. together with
characteristics of the receiver PP stage, various
effects such as overloading, intermodulation, or
spurious responses can occur See Check Chart 2.
A high-pass filter will remedy most problems.
Via the braid of the
downlead
The interfering signals are induced on the braid of
the aerial downlead. acting much as a long wire
aerial. The signals cause currents to flow through
the chassis or earthy parts of the receiven causing
radio frequency voltages to be developed across
parts which should be at equal potential. These
unwanted RF voltages have a direct eftect on the
correct functioning of various circuits to produce
interfering effects. In particular the colour decoder
circuits (colour sub-carrier on 4,433 MHz) are
affected by amateur radio transmissions on the 3.5
and 7 MHz bands.
See Check Chart 2. Fit a braid-breaker.
Via the IF lead from the
tuner
The intertering the IF and subsequent circuitry
to result in various effects including toss of colour,
poor synchronisation of the picture and
patterning.
See Check Chart 2. Internal receiver checks are
necessary.
As direct pick-up
The unwanted interfering signals, which may be at
high level from a nearby transmitter, are picked
up directly by various parts of the receiver
circuitry. In some cases they may be demodulated
and amplified by the audio circuits, resulting in
interference to the sound output.
See Check Chart 3, Internal modifications may be
necessary.
Via the mains lead
Interference signals on the mains supply can be
conducted via the mains lead to any section of the
receiver circuitry. Alternatively the interfering PF
signals can be induced directly into the mains
lead to have the same effect.
See Check Chart 2. Fit toroidal ferrite.
| Other leads
All interconnecting leads associated with the
system used eg. inter-unit connection leads and
leads to loudspeakers. can pick up unwanted
signals. The interference is conducted via these
leads to any part of the system circuitry.
Broad band (impulsive) interference is readily
recognised as bands of spots on the picture. Two
picture examples are given on page - of Part 1.
There is no recognisable pattern in the spots
whose size and brightness are completely
random. The picture interference is usually
accompanied by a continuous buzz on sound
where the interfering device is a commutator
moton or a short buzz or click where the cause is a
thermostat or other type of switch. A similar
continuous interference effect can, but rarely be
caused by a faulty insulator on high voltage
overhead power lines.
Narrow band radio transmitter type interference
can be displayed on the screen in various forms,
dependent on the transmitter mode of modulation.
Appendix 1 details radio spectrum occupancy
and related modes of modulation.
The various modes of modulation and their screen
effects are given below;
a. Amplitude Modulation (AM) The amplitude of
the modulated carrier varies in sympathy with the
intensity of the modulation, but the frequency of
the carrier remains constant. The interference
takes two forms:
- ) A beat pattern from the carrier.
- )Moving horizontal white bands in sympathy
with the transmitter modulation.
b. Frequency Modulation (FM) The amplitude of
the carrier remains constant but the frequency of
the carrier varies with the modulation.
The interference appears as a beat pattern
varying in sympathy with the transmitter
modulation. The effect is similar to the sound"
chroma beat seen when a receiver is detuned.
C. Single-sideband (SSB) As for an AM
transmission but wflh the carrier and one
sideband suppressed. As the carrier is suppressed,
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