Block diagram of television 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.
Block diagram of television receiver
A radio receiver designed to amplify and convert the video and audio radio- frequency signals of a television broadcast that have been picked up by a television antenna; the receiver reproduces the visual image broadcast and the accompanying sound. Television receivers are designed for color or black-and- white operation; both non portable and portable models are produced. Those manufactured in the USSR are capable of receiving signals from television stations transmitting in specifically assigned portions of thevery-high-frequency (VHF) band (48.5–100 megahertz and 174– 230 megahertz; 12 channels) and ultra high-frequency (UHF) band (470– 638 megahertz; several tens of channels).
Television receivers must simultaneously amplify and convert video and audio radio- frequency signals. They are usually designed with a super heterodyne circuit, and versions differ in the methods used to extract and amplify the audio signal. The principal components of a television receiver are shown in Figure1.
The tuner selects the signals of the desired channel and converts them to a lower frequency within the inter mediate-frequency pass band. The signal-processing circuits include an intermediate-frequency amplifier for the video signal, an amplitude detector, a video amplifier for the brightness signal, and, incolor receivers, a color- processing circuit for the chrominance signal. The processing circuit produces a brightness signal and a color- difference signal, which are fed to the control electrodes of a kinescope; an audio signal, which is fed to the audio channel; and horizontal and vertical synchronizing pulses (or a composite television signal), which are fed to a scanning generator. In the color television system used in the USSR , the color-processing circuit for the chrominance signal consists of a band- pass amplifier, in which the chrominance signal is extracted, channels for the direct and delayed signals, an electronic switching device, two frequency detectors for the color- difference signals, a matrix circuit, and amplifiers for the three color-difference signals. The color processing circuit has provisions for the extraction and decoding of the chrominance signal and for line selection, as well as chrominance disconnect circuits that operate when black-and-white transmissions are received.
The scanning generators include horizontal and vertical scanning circuits that produce sawtooth c urrentsin the horizontal and vertical scanning coils of the deflection system.
The high voltage for feeding the second anode of the kinescope is derived from a special high voltage winding of the line transformer or by rectifying pulses from the transformer; the volt age for the focusing electrode is similarly derived.
The kinescope’s interface includes static and dynamic white balance controls, switches for exting uishingthe electron guns, and regulators for focusing the beams. The demagnetizing circuit for a color kinescopecreates a damped alternating current in a demagnetizing loop that circles the kine scope screen. The current demagnetizes the shadow mask and tube rim, which are made of steel. The audio section consists of an amplifier for the difference frequency, which in the USSR is 6.5 megahertz, a frequency detector for the audio signal, and a low-frequency amplifier from which the audio signal is fed to a high- quality acoustical system, usually composed of several loudspeakers. The power- supply section converts mains voltage into the supply voltages for all components of the television set, including the kinescope and vacuum tube heaters.