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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
contd
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Television Signal standards contd
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Television Signal standards contd
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Television Signal standards contd
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Television Signal standards contd