A number of Television standards now coexist the worldwide. It also offers detail listings of standards prevalent in various countries.Television broadcast commenced approximately 50 years ago. The knowledge gained over the years has helped evolve better standards. As result of this, US which saw the birth of wide spread commercial television broadcasts,the first system which predictably, is also the most primitive. Subsequent television systems have learnt from earlier mistakes.
Vestigial Side Band Modulation:
As mentioned last lecture, the two methods for generating SSB modulated signals suffer
some problems. The selective–filtering method requires that the two side bands of
DSBSC modulated signal which would be filtered are separated by guard band that allows
bandpass filters that are used to have non 0 transition band (so it allows for real
filters). The ideal Hilbert transform for phase–shifting method is impossible to build,
so only an approximation of that may be used. Therefore, the SSB modulation method is
hard, if not impossible, build. A compromise between the DSBSC modulation and the
SSB modulation is known as Vestigial Side Band modulation. these types of
modulation is generated using similar system as that of selective–filtering system
for SSB modulation. The following block diagram shows VSB modulation and
consolidated CCIR system-B standard :
CCIR System B was the 625-line analog broadcast television system which at its peak was system used in the most countries. Currently (as of 2012) it is being replaced Asia and Africa by digital boardcasting.
The video bandwidth is 5.0 MHz. The video signal modulates the carrier by amplitude modulation. But portion of lower side band is suppressed. This technique is known as the vestigial side band modulation (AC3). The polarity of modulation is -ve, meaning that increase in instantaneous brightness of the video signal results in a decrease in the RF power and vice versa. Specifically, sync pulses result in maximum power from the transmitter.
System B was the first formal 625-line broadcasting standard in the world. The European 41-68 MHz band television allocation was agreed at the 1947 ITU (International Telecommunications Union) conference in 1947, and the first European channel plan (i.e. the use of channels E2 – E4) was agreed in 1952 at the ITU conference in Stockholm. The extension to VHFBand 111 (i.e Channels E5 – E12) was also agreed in the 1950s.
Q1: Give Example of VSB Signal ( TV Signal)?
Ans: Television picture signal has nominal bandwidth of 4.5MHz If DSB modulation is used, it requires at least 9MHz for each TV channel.
So, the VSB modulation is used so that whole TV signal is confined to about 6MHz.
Q2: What do you mean by Heterodyning ?
Ans: • Heterodyning means the translating or shifting in frequency.
• By heterodyning the incoming signal at ωRF with the local oscillator frequency ωLO, the message is translated to an intermediate frequency ωIF, which is equal to either the sum or the difference of ωRF and ωIF.
• If ωIF = 0, the bandpass filter becomes a low-pass filter and the original baseband signal is presented atoutput. This is called homodyning.
Q3: What is sound offset rate in System B ?
Ans: 5.5 MHz.
Q4: Write down the particulars of linear modulation?
Ans: linear modulation. Inparticular,
• DSB-SC, Double sideband suppressed carrier
• DSB-LC, Double sideband large carrier (AM)
• SSB, Single sideband
• VSB, Vestigial sideband.
Q5: What isVestigial Sideband Transmission?
Ans: Vestigial Sideband Transmission :
• Vestigial sideband (VSB) transmission is a compromise between DSB and SSB
• In the VSB modulation, 1 passband is passed almost completely whereas only a residual portion of the other sideband is retained in such a way that the demodulation process can still reproduce the original signal.
• VSB signals are easier to generate because some roll-off in filter edges are allowed. This results in system simplification. And their bandwidth is only slightly greater than that of SSB signals (-25 %).
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