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.  e c = E c cos ω c t is the carrier wave and  e m = E m cos ω m t is the modulating signal.  The equation of the modulated wave is :e = A cos ω c t  where A = (E c + kE m cos ω m t) when k is a constant of the modulator. On substituting the value of A we get : e = (E c + kE m cos ω m t) cos ω c t = Ec (1 + m cos ω m t) cos ω c t where m = kE m /E c is the modulation index At kE m = E c, 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 2f m centred on f c. 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 MHz0 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 = = 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 f m 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 contd …

Television Signal standards …contd contd …

Television Signal standards …contd contd …

Television Signal standards …contd contd …

Television Signal standards …contd