23-05-2012, 05:19 PM
TV SIGNAL TRANSMISSION
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In most television systems as also in the C.C.I.R 625 line, the picture signal is amplitude modulated and sound signal frequency modulated before transmission. The channel bandwidth is determined by the highest video frequency required for proper picture reception and the maximum sound carrier frequency deviation permitted in a TV system.
Need for modulation.
The need for modulation stems from the fact that it is impossible to transmit a signal by itself. The greatest difficulty in the use of un modulated wave is the need for long antennas for efficient radiation and reception. For example, a quarter-wavelength antenna for the transmitting frequency of 15 kHz would be 5000 meters long. A vertical antenna of this size is unthinkable and in fact impracticable.Another important reason for not transmitting signal frequencies directly is that both picture and sound signals from different stations are concentrated within the same range of frequencies. Therefore, radiation from different stations would be hopelessly and inextricably mixed up and it would be impossible to separate one from the other at the receiving end. Thus in order to be able to separate the intelligence from different stations, it is necessary to translate them all to different portions of the electromagnetic spectrum depending on the carrier frequency assigned to each station. This also overcomes the difficulties of poor radiation at low frequencies. Once signals are translated before transmission, a tuned circuit provided in the RF section of the receiver can be used to select the desired station.
AMPLITUDE MODULATION
In amplitude modulation the intelligence to be conveyed is used to vary the amplitude of the carrier wave. As an illustration, an amplitude modulated signal
where
ec = Ec cos ωct is the carrier wave and
em = Em cos ωmt is the modulating signal.
Note that the camera signal is actually complex in nature but a single modulating frequency has been chosen for convenience of analysis.
The equation of the modulated wave is :
e = A cos ωct
where A = (Ec + kEm cos ωmt) when k is a constant of the modulator.
CHANNEL BANDWIDTH:
In the 625 line TV system where the frequency components present in the video signal extend from dc (zero Hz) to 5MHz, a double sideband AM transmission would occupy a total bandwidth of 10 MHz. The actual band space allocated to the television channel would have to be still greater, because with practical filter characteristics it is not possible to terminate the bandwidth of a signal abruptly at the edges of the sidebands. Therefore, an attenuation slope of 0.5 MHz is provided at each edge of the two sidebands. This adds 1 MHz to the required total band space. In addition to this, each television channel has its associated FM (frequency modulated) sound signal, the carrier frequency of which is situated just outside the upper limit of 5.5 MHz of the picture signal.