Television transmitter

Source: Wikipedia, the free encyclopedia.

A television transmitter is a

UHF bands. Since radio waves of these frequencies travel by line of sight
, they are limited by the horizon to reception distances of 40–60 miles depending on the height of transmitter station.

Television transmitters use one of two different technologies: analog, in which the picture and sound are transmitted by

modulated onto the radio carrier wave, and digital in which the picture and sound are transmitted by digital signals. The original television technology, analog television, began to be replaced in a transition beginning in 2006 in many countries with digital television (DTV) systems. These transmit pictures in a new format called HDTV (high-definition television) which has higher resolution and a wider screen aspect ratio than analog. DTV makes more efficient use of scarce radio spectrum bandwidth, as several DTV channels can be transmitted in the same bandwidth as a single analog channel. In both analog and digital television, different countries use several incompatible modulation standards to add the video and audio signals to the radio carrier wave
.

The principles of primarily analog systems are summarized as they are typically more complex than digital transmitters due to the multiplexing of VSB and FM modulation stages.

Types of transmitters

There are many types of transmitters depending on

  • The system standard
  • Output power
  • Back up facility, usually the Modulator, Multiplexer and Power Amplifier
  • dual sound
    ) facility, for analogue TV systems
  • Aural and visual power combining principal, for analogue TV systems
  • Active circuit element in the final amplifier stage

The system standard

Main article: Broadcast television systems
A typical television transmitter and antenna at Karadağ, Karaman, Turkey

An international plan by ITU (International Telecommunication Union) on broadcast standards which is usually known as Stockholm plan (1961) defines standards used in broadcasting. In this plan, most important figures for transmitters are radio frequency, frequency separation between aural and visual carriers and band width. [1]

Input stage of a transmitter

The

modulator, where it modulates an intermediate frequency carrier (IF). The modulation technique is usually frequency modulation (FM) with a typical maximum deviation
of 50 kHz (for 1 kHz. input at 0 dBm level).

The

Vestigal sideband
(VSB) filter. This filter is used to suppress a portion of one side band, thus bandwidth is reduced. (Since both side bands contain identical information, this suppression doesn't cause a loss in information.) Although the suppression causes phase delay problems the VSB stage also includes correction circuits to equalise the phase.

Output stages

The modulated signal is applied to a

oscillator is known as subcarrier. The two outputs of the mixer are the sum and difference of two signals. Unwanted signal (usually the sum) is filtered out and the remaining signal is the radio frequency (RF) signal. Then the signal is applied to the amplifier stages. The number of series amplifiers depends on the required output power. The final stage is usually an amplifier consisting of many parallel power transistors. But in older transmitters tetrodes or klystrons
are also utilized.

In modern solid-state VHF and UHF transmitters,

Envelope Tracking
, which in the broadcast industry is often referred to as 'drain modulation'.

Combining aural and visual signals

There are two methods:

  • combiner
    . This is the system used in most high power applications.

Block diagram of a TV transmitter (intercarrier method).

  • Intercarrier system: There are two input stages, one for AF and one for VF respectively. The two signals are combined in low power IF circuits (i.e., after modulators). Because the mixer and amplifiers are common to both signals, the system needs no high power combiners and therefore the price and power consumption is considerably lower than that of split sound system of the same operational level. An adverse effect of the two signals passing through amplifiers is intermodulation products, so the intercarrier system is not suitable for high power applications. In the case of lower power transmitters, a notch filter to reject the cross modulation products must be used at the output.

Output power

See also: Output power of an analog TV transmitter

The output power of the transmitter is defined as the power during

sync pulse (Real output power is variable depending on the content). The quantifiable power from the transmitting equipment and antenna are different from one another. The output power of the antenna is known as ERP
, which is represented by the formula

where Po represents output power, and Ga represents antenna gain.

See also

References

  1. ^ Analogue TV Broadcast Systems by Paul Schlyter

Further reading

  • Bernard Grob,Charles E.Herndon: Television and video systems, Glencoe McGraw-Hill
  • Reference data for Radio Engineers, Chapter 30, Howard W.Sams Co Inc., Indianapolis,1977,
    ISBN 0-672-21218-8
  • FARWAY IRFC, TV and Radio Transmission , Radio Data System Encoders , Broadcasting Technologies
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