# SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH

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```					4. SIGNAL TRANSMISSION
AND
CHANNEL BANDWIDTH

Prepared by
Sam Kollannore U.
Lecturer, Department of Electronics
M.E.S.College, Marampally, Aluva-7
 Picture – amplitude modulated
 Sound – frequency modulated
 Channel bandwidth – 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 - antenna size
- interference
- poor radiation at low frequency
Amplitude Modulation
 In amplitude modulation the intelligence to be conveyed is
used to vary the amplitude of the carrier wave.
    ec = Ec cos ωct is the carrier wave and
    em = Em cos ωmt is the modulating signal.
    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.
On substituting the value of A we get :
e = (Ec + kEm cos ωmt) cos ωct = Ec (1 + m cos ωmt) cos ωct
where m = kEm/Ec is the modulation index
At kEm = Ec , m = 1 and the corresponding depth of modulation is
then termed as 100%.
If the modulating signal consists of more
than a single frequency, as it would be for a
video signal, the equation can be extended
to include the sum and difference of the
carrier and all frequency components of the
modulating signal.

Therefore if the modulated wave is to be
transmitted without distortion by this
method, the transmission channel must be
atleast of width 2fm centred on fc.
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 carrier
frequency 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
Channel Bandwidth                             …contd

 Such a bandwidth is too large - limits the number of
channels 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 modulating
frequency fm from the USB or from LSB. Therefore it is
transmitted
 Thus results in simpler transmitting equipment
 Only needs an inexpensive diode detector at the
 In Television transmission we use VSB - Vestigial Side
Band transmission
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 the
picture
   Complete suppression of the lower sideband would result in phase distortion at
these 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
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
Demerits of VSB
Frequency modulation
FM Channel Bandwidth
For a 625 line system, bandwidth requirement = 2(50+15) = 130KHz - close to the
value calculated earlier
Television Signal standards

contd …
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

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Television Signal standards   …contd

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