Signal Transmission & Channel Bandwidth

8/8/2019 Signal Transmission & Channel Bandwidth

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4. SIGNAL TRANSMISSION

AND

CHANNEL BANDWIDTH

Prepared bySam Kollannore U.

Lecturer, Department of ElectronicsM.E.S.College, Marampally, Aluva-7


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Picture amplitude modulated

Sound frequency modulated

Channel bandwidth determined by the highestvideo frequency required for proper picture receptionand the maximum sound carrier frequency deviationpermitted in a TV system

Need for modulation - antenna size

- interference

- poor radiation at low frequency


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Amplitude Modulation

In amplitude modulation the intelligence to be conveyed is

used to vary the amplitude of the carrier wave.

ec= Eccos ctis the carrier wave and

em = Em cos mtis the modulating signal.

The equation of the modulated wave is :e = A cos ct

whereA = (Ec+ kEm cos mt) when k is a constant of themodulator.

On substituting the value ofA we get :e = ( E c+ kEm cos mt) cos ct = E c ( 1 + m c o s mt) cos ct

where m = k E m/Ecis the modulation indexAt kEm = Ec, m = 1 and the corresponding depth of modulation isthen termed as 100%.


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Therefore if the modulated wave is to betransmitted without distortion by thismethod, the transmission channel must beatleast of width 2fm centred on fc.

If the modulating signal consists of morethan a single frequency, as it would be for avideo signal, the equation can be extended

to include the sum and difference of thecarrier and all frequency components of themodulating signal.


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Channel Bandwidth Frequency components present in the video signal extends from 0 to

5 MHz

Therefore Bandwidth required = 2 5MHz = 10MHz Attenuation slope of 0.5MHz is provided at the edge of the two side

bands (i.e. 2 0.5MHz = 1MHz)

Each channel has its associated FM sound signal whose carrierfrequency located at 5.5MHz (in the upper limit)

A small guard band of 0.25MHz for each channel So total Channel Bandwidth = 10 + 1 + 0.25 = 11.25MHz


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Such a bandwidth is too large - limits the number ofchannels in a given high frequency spectrum Two side bands are identical - only one is necessary -

thus saving 5MHz per channel - SSB Carrier conveys no information - but its presence is

necessary at the receiver for recovering the modulatingfrequency fm from the USB or from LSB. Therefore it istransmitted

Thus results in simpler transmitting equipment

Only needs an inexpensive diode detector at thereceiver for demodulation In Television transmission we use VSB - Vestigial Side

Band transmission

Channel Bandwidth contd


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Vestigial Side Band

Transmission In the video signal, very low frequency modulating components exist along with

rest of the signal

These components give rise to sidebands very close to the carrier frequency difficult to remove by physically realizable filters

Again the low video frequencies contain the most important information of thepicture

Complete suppression of the lower sideband would result in phase distortion atthese frequencies

Therefore we cannot fully suppress one complete sideband

As a compromise only a part of the LSB is suppressed

Radiated signal consist of :Full USB + Carrier + Vestige of the partially suppressed LSB

This pattern of transmission is known as

Vestigial Side Band Transmission or A5C transmission


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Vestigial Side Band Transmission contd

Frequencies up to 0.75 MHz of the LSB are fully radiated

Attenuation slope of 0.5 MHz at either end FM sound signal occupies a frequency spectrum of about 75

KHz around the sound carrier

Guard band of 0.25 MHz allowed on the sound carrier side for interchannel separation


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Demerits of VSB


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Frequency modulation


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FM Channel Bandwidth


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For a 625 line system, bandwidth requirement = 2(50+15) = 130KHz - close to thevalue calculated earlier


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Television Signal standards

contd


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Television Signal standards contd

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Signal Transmission & Channel Bandwidth