Download presentation
Presentation is loading. Please wait.
Published byVictor Hudson Modified over 9 years ago
1
Prepared by Sam Kollannore U. Lecturer, Department of Electronics M.E.S.College, Marampally, Aluva-7
2
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
3
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%.
4
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.
5
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 = 10 + 1 + 0.25 = 11.25MHz
6
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
7
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
8
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
10
Demerits of VSB
11
Frequency modulation
15
FM Channel Bandwidth
16
For a 625 line system, bandwidth requirement = 2(50+15) = 130KHz - close to the value calculated earlier
17
Television Signal standards contd …
18
Television Signal standards …contd contd …
19
Television Signal standards …contd contd …
20
Television Signal standards …contd contd …
21
Television Signal standards …contd
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.