Television interference (TVI) is a particular case of electromagnetic interference which affects

television Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...

reception. Many natural and man-made phenomena can disrupt the reception of television signals. These include naturally occurring and artificial spark discharges, and effects due to the operation of radio transmitters. Analog television broadcasts display different effects due to different kinds of interference. Digital television reception generally gives a good quality picture until the interference is so large that it can no longer be eliminated by the error checking systems in the receiver, at which point the video display becomes pixelated, distorts, or goes blank.

Co-channel and multipath (ghost)

During unusual atmospheric conditions, a distant station normally undectable at a particular location may provide a much stronger signal than usual. The analog television picture may display the sum of the two signals, producing an image from the strong local signal with traces or "ghosts" from the distant, weaker signal. Television broadcast stations are located and assigned to channels so that such events are rare. Readjustment of the receiving antenna may allow more of the distant signal to be rejected, improving image quality. A local signal may travel by more than one path from the transmitter to receiving antenna. "Multipath" reception is visible as multiple impressions of the same image, slightly shifted along the width of the screen due to the varying transmission path. Some multipath reception is momentary due to road vehicles or aircraft passing; other multipath problems may persist due to reflection off tall buildings or other landscape features. Strong multipath can cause the analog picture to "tear" or momentarily lose synchronization, causing it to roll or flip.Joma Hyppia, ''8 Ways to Beat TV Interference'', ''Popular Mechanics'', June 1980 pages 88-89

Static electricity and sparks

The sparks generated by static electricity can generate interference. Many systems where radio frequency interface is caused by sparking can be modeled as the following circuit. The source of energy charges C1 via a resistance, and when the spark gap breaks down, the electricity passes through L and excites the resonant LC circuit. The energy in the LC circuit is then radiated through the aerial. Sparktrans2 vers UK vectorised

As an example, when a person walks over a

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carpet, the rubbing of shoes on carpet performs the role of a battery and resistor, while the person acts as a

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

(C1 and C2), and the air between a hand and a door knob is a

spark gap A spark gap consists of an arrangement of two Conductor (material), conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential differenc ...

. Stray inductance acts as L.

Sparks and allied phenomena

Horizontal lines randomly arranged on a television screen may be caused by sparking in a malfunctioning electrical device. Electric railways can also be a strong source of this type of interference. Other possible sources of such interference include thermostats, fridges, freezers, fish tank heaters, central heating systems. These can create sparks as they turn on or off; as they age they can become worse. In some rare cases they can create non-stop interference through sparking. Electric motors that have a commutator can suffer from sparking at the

brushes A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped during u ...

. Ignition systems on cars and motorbikes.

Devices which switch at powerline frequency

Power line hardware can generate sparks at either a 100 or 120 Hz rate Light dimmers and other solid state power control devices can generate interference.

Thyristor A thyristor (, from a combination of Greek language ''θύρα'', meaning "door" or "valve", and ''transistor'' ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage ...

and

TRIAC A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor) is a three-terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericized trad ...

regulators without proper

chokes Choking, also known as foreign body airway obstruction (FBAO), is a phenomenon that occurs when breathing is impeded by a blockage inside of the respiratory tract. An obstruction that prevents oxygen from entering the lungs results in oxygen d ...

are a common source of

EMI EMI Group Limited (formerly EMI Group plc until 2007; originally an initialism for Electric and Musical Industries, also referred to as EMI Records or simply EMI) was a British transnational conglomerate founded in March 1931 in London. At t ...

as well. It is likely that a

thyristor A thyristor (, from a combination of Greek language ''θύρα'', meaning "door" or "valve", and ''transistor'' ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage ...

(SCR) power controller using the variable phase angle method will generate harmonics of the mains supply, while the spark at a contact will be a very wide band source whose frequency is not related to the power supply frequency. In

control systems the potential for

problems can be minimised by using

zero crossing A zero-crossing is a point where the sign of a mathematical function changes (e.g. from positive to negative), represented by an intercept of the axis (zero value) in the graph of the function. It is a commonly used term in electronics, mathema ...

switching where the

is switched on at the moment of time when the AC voltage changes from one direction to the other. Lightdimmer

Devices which switch faster than 200 Hz

Computers and other digital electronic equipment containing rapidly switching circuits. These devices create and use signals which are switched on/off at great speed, approximately

square wave Square wave may refer to: *Square wave (waveform) A square wave is a non-sinusoidal waveform, non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same ...

s. Any repetitive signal can be reduced down to a

Fourier series A Fourier series () is an Series expansion, expansion of a periodic function into a sum of trigonometric functions. The Fourier series is an example of a trigonometric series. By expressing a function as a sum of sines and cosines, many problems ...

of sine waves. A perfect square wave with fundamental frequency is:

E(t; \omega) = E_0 \sin(\omega t) + \frac + \frac + \dots

The square wave contains harmonics of the fundamental (that is, sine waves with a frequency that is a multiple of the fundamental frequency ) which go on upwards in frequency for ever, although at a decreasing amplitude. These harmonics are responsible for much of the interference created by

computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...

s. A modern PC is a device which is operating in the VHF/

UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...

frequency range using square waves. As the cases on many computers are not perfect shields, some of this radio-frequency energy can leak out and cause interference to radio (and sometimes TV) reception. Switched-mode power supplies or packs can be a source of interference. These are used in consumer electronic products such as phone charges and in some lighting systems.

Strong TV signals

It is possible to also get a bad picture if the signal strength of the TV transmitter is too high. An attenuator inserted in the antenna lead-in wire may be used if the television receiver displays signs of overload in the

RF front end In a radio receiver circuit, the RF front end, short for ''radio frequency front end'', is a generic term for all the circuitry between a receiver's antenna input up to and including the mixer stage. It consists of all the components in the ...

. Strong out-of-band signals may also affect television reception and may require band-pass filters to reduce the level of the undesired signal at the receiver.

References

{{Analogue TV transmitter topics Radio electronics Television technology Electromagnetic compatibility