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Atelevision transmitter is atransmitter that is used forterrestrial (over-the-air)television broadcasting. It is an electronic device that radiatesradio waves that carry avideo signal representing moving images, along with a synchronizedaudio channel, which is received bytelevision receivers ('televisions' or 'TVs') belonging to a public audience, which display the image on a screen. A television transmitter, together with thebroadcast studio which originates the content, is called atelevision station. Television transmitters must be licensed by governments, and are restricted to a certainfrequency channel and power level. They transmit on frequencychannels in theVHF andUHF bands. Since radio waves of these frequencies travel byline 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 byanalog signalsmodulated onto the radiocarrier wave, anddigital in which the picture and sound are transmitted bydigital signals. The original television technology,analog television, began to be replaced in atransition beginning in 2006 in many countries withdigital television (DTV) systems. These transmit pictures in a new format called HDTV (high-definition television) which has higher resolution and a wider screenaspect ratio than analog. DTV makes more efficient use of scarceradio spectrumbandwidth, 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 incompatiblemodulation standards to add the video and audio signals to the radiocarrier 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.
There are many types of transmitters depending on
An international plan byITU (International Telecommunication Union) on broadcast standards which is usually known asStockholm plan (1961) defines standards used inbroadcasting. In this plan, most important figures for transmitters areradio frequency, frequency separation between aural and visual carriers andband width.[1]
Theaudio (AF) input (or inputs in case ofstereophonic broadcasting) is usually a signal with 15 kHz maximum bandwidth and 0 dBm maximum level.Preemphasis time constant is 50 μs. The signal after passingbuffer stages is applied to amodulator, where it modulates anintermediate frequency carrier (IF). The modulation technique is usuallyfrequency modulation (FM) with a typical maximumdeviation of 50 kHz (for 1 kHz. input at 0 dBm level).
Thevideo (VF) input is acomposite video signal (video information withsync) of maximum 1 volt on 75 Ω impedance. (1 V limit is forluminance signal. Some operators may accept superimposedcolor signals slightly over 1 V.) After buffer and 1 Vclipping circuits, the signal is applied to the modulator where it modulates an intermediate frequency signal (which is different from the one used for aural signal.) The modulator is anamplitude modulator which modulates the IF signal in a manner where 1 V VF corresponds to low level IF and 0 volt VF corresponds to high level IF. AM modulator produces two symmetrical side bands in the modulated signals. Thus, IF band width is two times the video band width. (i.e. if the VF bandwidth is 4.2 MHz, the IF bandwidth is 8.4 MHz.) However, the modulator is followed by a special filter known asVestigal 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.
The modulated signal is applied to amixer (also known asfrequency converter). Another input to the mixer which is usually produced in a crystal ovenoscillator is known assubcarrier. 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 theamplifier stages. The number of series amplifiers depends on the required outputpower. The final stage is usually an amplifier consisting of many parallel powertransistors. But in older transmitterstetrodes orklystrons are also utilized.
In modern solid-state VHF and UHF transmitters,LDMOS powertransistors are the device of choice for the output stage, with the latest products employing 50V LDMOS devices for higher efficiency and power density. Even higher energy efficiency is possible usingEnvelope Tracking, which in the broadcast industry is often referred to as 'drain modulation'.
There are two methods:
The output power of the transmitter is defined as the power duringsync pulse (Real output power is variable depending on the content). The quantifiable power from the transmitting equipment andantenna are different from one another. The output power of the antenna is known asERP, which is represented by the formula
where Po represents output power, and Ga represents antenna gain.
