Interferance

Interferance:
 When the interfer¬ring signal has a frequency that lies within the channel to which a TV receiver is tuned, the extent of interference depends only on the relative field strengths of the desired signal and the interferring signal.
 If the interferring signal frequency spectrum lies outside the desired channel selectivity of the receiver aids in rejecting the interference.
(a) Co-Channel Interference
 Two stations operating at the same carrier frequency if located close by will interfere with each other. This phenomenon which is common in fringe areas is called co-channel interference.
 As the two signal strengths in any area almost equidistant from the two co-channel stations become equal a phenomenon known as 'Venetianblind' interference occurs. This takes the form of horizontal black and white bars, superimposed on the picture produced by the tuned channel.
 These bars tend to move up or down on the screen. As the strength of interferring signal increases, the bars become more prominent, until at a signal-to-interference ratio of 45 db or so the inter¬ference becomes intolerable.
 Co-channel interference was a serious problem in early days of TV transmission, when channel allocation was confined to VHF band only. This necessitated repetition of channels at distances not too far from each other.
 When a large number of channels in the UHF band are available such a problem does not exist.
 The sharing of channel numbers is carefully planned so that within the 'service area' of any station signals from the distant stations under normal conditions of reception are so weak as to be imperceptible.
 During a period of abnormal reception conditions (often during spring) when the signals from distant VHF stations are received much more strongly, co-channel interference can occur in fringe areas. The use of highly directional antennas is very helpful in eliminating co¬channel interference.
(b) Adjacent Channel Interference:
 Adjacent channel interference may occur as a result of beats between any two of these frequencies or between a carrier and any sidebands.
 A coarse dot structure is produced on the screen if picture carrier of the desired channel beats with sound carrier of the lower adjacent channel.
 The beat pattern is more pronounced if the lower adjacent sound carrier is relatively strong and is not sufficiently attenuated in the receiver.
 To prevent adjacent channel interference, several sharply tuned band eliminator filters (trap circuits) are provided in the IF section of the receiver.
 In addition to this, the guard band between two adjacent channels also minimizes the intensity of any adjacent channel interference. A space of about 150km between adjacent channel stations is enough to eliminate such interference and is normally allowed.
(c) Ghost Interference:
 Ghost interference arises as a result of discrete reflections of the signal from the surface of buildings, bridges, hills, towers etc
 Since reflected path is longer than the direct path, the reflected signal takes a longer time to arrive at the receiver
 The direct signal is usually stronger and assumes control of the synchronizing circuitry and so the picture due to the reflected signal that arrives late, appears displaced to the right. Such displaced pictures are known as 'trailing ghost' pictures.
 On rare occasions, direct signal may be the weaker of the two and the receiver synchronization is now controlled by the reflected signal.
 Then the ghost picture, now caused by direct signal, appears displaced to the left and is known as 'leading ghost' picture.
Preference of AM For Picture Signal Transmission:
 The distortion which arises due to interference between multiple signals is more objectionable in FM than AM because the frequency of FM signal continuously changes.
 If FM were used for picture transmission, the changing beat frequency between the multiple paths, delayed with respect to each other, would produce a bar interference pattern in the image with a shimmering effect, since the bars continuously change as the beat frequency changes. Hence hardly any steady picture is produced.
 Alternatively if AM were used the multiple signal paths can almost produce a ghost image which is steady. In addition to this circuit complexity and bandwidth requirements are much less in AM than FM. Hence. AM is preferred to FM for broadcasting the picture signal.