TV - Television Transmission
Basic TV transmitter
The basic television Broadcast transmitter block diagram is shown in figure (a).
The block diagram can be broadly divided into two separate section, viz., one that - Generates an electronic signal (called video signal) corresponding to the actual picture and then uses this video signal to modulate an R-F carrier so as to be applied to the transmitting antenna for transmission, other that generates an electronic signal (called audio signal) containing sound information and then uses this signal to modulate another RF carrier and then applied to the transmitting antenna for transmission.
However only one antenna is used for transmission of the video as well as audio signals. Thus these modulated signals have to be combined together in some appropriate network. In addition there are other accessories also. For instance, video as well as audio signals have to be amplified to the desired degree before they modulate their respective RF carriers.
This function is performed by video and audio amplifiers. The block picture signal transmitter and audio signal transmitter shown in figure (a) may consist of modulators as the essential component; Video signal transmitter employs an AM transmitter as amplitude-modulation is used for video signals whereas audio signal transmitter employs FM modulator as frequency modulation is used for sound information. Scanning circuits are used to mike the electron beam scan the actual picture to produce the corresponding video signal. The scanning by electron beam is in the receiver too. The beam scans the picture tube to reproduce the original picture from the video signal and this scanning at the receiver must be matched properly to the scanning at the transmitter. It is for this reason that synchronizing Circuits are used at the transmitter as well as receiver.
Complete TV transmitter Block Diagram
Figure (b) depicts the complete block diagram of a Television Broadcast Transmitter. The important block have already been discussed individually in the preceding sections. that makes understanding of the diagram shown here much more simple. A brief explanation is given ahead.
The block diagram can be broadly divided into two -sections, viz., an amplitude modulated transmitter and a frequency modulated transmitter. Former is used for video modulation whereas latter is used for audio modulation.
Master oscillator in both generates an RF carrier frequency. Generally, a master oscillator generates a sub multiple of carrier and then drives harmonic generators (frequency multipliers) to achieve correct value carrier. Harmonic generators are nothing but class C tuned amplifiers whose output tuned circuit is to tuned to some harmonic of the input signal. In actual practice, master oscillator and harmonic generator are s crated or isolated by a buffer stage to av214Joactrrig of the harmonic generator on the oscillator output. The carrier is then fed to an amplitude modulator in video transmitter and a frequency modulator in audio transmitter. Into- the modulator, the modulation signal is also fed with proper amplitude. Since low-level modulation is employed, the modulating signal is amplified by linear amplifiers up-to the desired degree required for transmission. Video and audio signals on separate carriers are then combined together so as to be fed to the transmitting antenna as on signal.
Summery
A. Television transmitter essentially consists of a video modulator and an audio modulator. In video modulator, the video signal amplitude modulates RF carrier whereas an audio signal frequency modulates another RF carrier in audio modulator.
Video signal is generated by a television camera viewing the scene to be televised whereas the audio signal is produced by a microphone. However, in actual practice, there may be a number of televisions. Cameras viewing the scene and a number of microphones picking up sound.
These video and audio signals are amplified in video and audio amplifiers respectively prior to modulation. The modulated signals are combined in a combining network before they are applied to common transmitting aerial system.
A master oscillator is any LC oscillator that gives a highly frequency stable operation.
Multi-cavity Klystron is the most commonly used device for final stage of amplification. A multi-cavity Klystron is capable of generating, powers of the order of 20 KW or so with only 2W to 5W input. An automatic frequency control (AFC) arrangement is used in conjunction with the master oscillators that generate the carriers to keep their output frequency extremely stable.
Reactance tube modulators are the commonly used FM modulators. Modern FM modulators, however, employ varactor diodes as the reactance producing devices. A sync pulse generator generates horizontal and vertical synchronizing pulses which serve the purpose of locking line and field generators at the receiver to correct frequency.