Television Receivers
This resource contains some information about television receivers. It contains brief information about analogue terrestrial television receiver, color decoding, color burst processing and switched mode power supply.
Television Receivers
Analogue Terrestrial Television Receiver :
1) The tuner selects the UHF carrier frequency for the TV channel as chosen by the user and converts it to an intermediate frequency of 38.5 MHz.
2) The modulated IF is then amplified through several stages of amplification and demodulated to obtain the original composite video blanking and sync (CVBS) signal.
3) The video in the form of CVBS from the video selector is fed to the color decoder to reproduce the original RGB signals which following the RGB matrix are fed to the CRT via individual RGB amplifiers.
4) The 6-MHz sound inter-carrier is taken off at the video detector stage using a surface acoustic wave (SAW) filter. The FM sound signal is detected, amplified and fed into the loudspeaker via the sound select chip.
5) The sync pulses are clipped from the video information at the video output stage, separated into line and field, and taken to the appropriate time base.
6) After amplification, line and field pulses are used to deflect the electron beam in the horizontal and vertical directions via a pair of scan coils.Color Coding :
1) To separate the chrominance from the composite video which is carried out by a comb filter.
2) To break up the composite chrominance signal into its two component parts U and V each of which must be demodulated separately to obtain the original color difference signals.
3) The weighted color difference signals U and V are separated from each other by a unit consisting of a delay line (usually known as PAL delay line) and an add or subtract network.
4) Two separate signals U and V are produced which are fed to their respective demodulators B-Y and R-Y. Each demodulator is fed with a 4.43-MHz signal at the correct phase from the reference oscillator and burst channel.
5) Two gamma-corrected color difference signals B-Y and R-Y are thus obtained.
6) The third color difference signal G-Y is obtained from the first two by the G-Y network. A luminance delay line is therefore inserted into the luminance signal path to ensure that both signals arrive at the matrix at the same time. Color Burst Processing :
1) Color burst processing separates the sub-carrier burst signal from the chrominance information so it may be used to recreate the sub-carrier which has been suppressed at the transmitter.
2) The sub-carrier has to be restored in both its frequency and phase to ensure correct color reproduction.
3) Two sub-carriers at 90° to each other have to be produced, one for each color difference, B-Y and R-Y.
4) In the PAL system, the sub-carrier for the R-Y demodulator has to be phase reversed on alternate lines.
5) The burst processing section is also used to provide automatic chrominance control (a.c.c.) as well as the color killer signal for monochrome-only transmissionsSwitch Mode Power Supply (SMPS) :
1) Apart from reducing alternate current ripple switched mode power supply are more efficient than the linear types dissipating less heat and are smaller in size.
2) The SMPS is basically a direct current to direct current converter, it converts direct current into a switched or pulsating direct current and back again into a direct current. The switching speed determines the alternate current content or ripple frequency at the output
