A television transmitter is a transmitter that is used for terrestrialtelevision broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized audio channel, which is received by television receivers belonging to a public audience, which display the image on a screen. A television transmitter, together with the broadcast studio which originates the content, is called a television station. Television transmitters must be licensed by governments, and are restricted to a certain frequency channel and power level. They transmit on frequencychannels in the VHF and 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 analog signalsmodulated 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 systems. These transmit pictures in a new format called HDTV which has higher resolution and a wider screen aspect ratio than analog. DTV makes more efficient use of scarce radio 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 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.
An international plan by ITU on broadcast standards which is usually known as Stockholm plan 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.
Input stage of a transmitter
The audio input is usually a signal with 15 kHz maximum bandwidth and 0 dBm maximum level. Preemphasistime constant is 50 μs. The signal after passing buffer stages is applied to a modulator, where it modulates an intermediate frequency carrier. The modulation technique is usually frequency modulation with a typical maximum deviation of 50 kHz. The video input is a composite video signal of maximum 1 volt on 75 Ω impedance. After buffer and 1 V clipping circuits, the signal is applied to the modulator where it modulates an intermediate frequency signal The modulator is an amplitude 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. However, the modulator is followed by a special filter known as Vestigal sideband filter. This filter is used to suppress a portion of one side band, thus bandwidth is reduced. 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 mixer. Another input to the mixer which is usually produced in a crystal oven oscillator is known as subcarrier. The two outputs of the mixer are the sum and difference of two signals. Unwanted signal is filtered out and the remaining signal is the radio frequency 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, LDMOS power transistors 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 using Envelope Tracking, which in the broadcast industry is often referred to as 'drain modulation'.
Combining aural and visual signals
There are two methods:
Split sound system: Actually there are two parallel transmitters one for aural and one for visual signal. The two signals are combined at the output via a high power combiner. In addition to a combiner, this system requires separate mixer and amplifiers for aural and visual signals. This is the system used in most high power applications.
Intercarrier system : There are two input stages one for AF and one for VF. But the two signals are combined in low powerIF circuits The mixer and the amplifiers are common to both signals and the system needs no high power combiners. So both the price of the transmitter and the power consumption is considerably lower than that of split sound system of the same power level. But two signals passing through amplifiers produce some intermodulation products. So intercarrier system is not suitable for high power applications and even at lower power transmitters a notch filter to reject the cross modulation products must be used at the output.
The output power
The output power of the transmitter is defined as the power during sync pulse. But the output power of the transmitting equipment and the output power of the antenna are two different quantities. The output power of the antenna is known as ERP which is actually the transmitter power times the antenna gain.
