Prepared bySam 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 ωctwhere m = kEm/Ec is the modulation indexAt kEm = Ec , m = 1 and the corresponding depth of modulation is then termed as 100%.

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.

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.

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

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

receiver for demodulation In Television transmission we use VSB - Vestigial

Side Band transmission

Channel Bandwidth …contd

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

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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

Television Signal standards …contd