Written by V R Sunday, 31 October 2010 18:57
Pulse time modulation includes pulse position modulation and pulse width modulation.Both of these produce a form of pulse phase modulation and are sometimes called by that name.Pulse frequency modulation is also included,although it is not strictly a time modulation.
Written by V R Sunday, 31 October 2010 18:51
At UHF it is common practice to use a plane reflecting surface,either a flat surface or a corner of two surfaces,in plane of the single reflector element of Yagi.The reflecting surface must be at least one wavelength across in each direction and can be much larger.It may be a solid metal surface,or it may be wire mesh or a network of interconnected metal rods.The plane reflector is arranged so that the driven dipole is mounted a quarter-wavelength ahead of the reflector surface.A mirror image of the driven antenna in phase with the driving antenna radiations,but delayed one period.
Written by V R Sunday, 31 October 2010 18:45
The discone antenna is designed to radiate an omnidirectional pattern in the horizontal plane,with vertical polarization.It is broadband antenna with usable characteristics over a frequency range of nearly 10:1.It is usually designed to be fed directly from a 50 ohm coaxial line and is mounted directly on the end of that line.This type of antenna is ideal for base station operation for urban mobile communications systems,since it gives a good omnidirectional pattern,is physically very compact and rugged and is quite inexpensive to construct.Its directional gain along the horizontal plane is comparable to that of the dipole antenna.
Written by V R Sunday, 31 October 2010 18:37
There has lately been much interest in the possibility of upgrading the existing color TV systems to provide high-definition television.Much work has been done on the technical problems of providing HDTV,and international action is presently underway to establish a set of universal standards for HDTV,which will allow development of a universally accepted system.
The Japanese have led the way in this research,and in 1989 they demonstrated a prototype version of their system and planned to launch a direct broadcast satellite in 1991 to provide broadcast facilities for HDTV.European interests in 1989 began a joint research project called Eureka 95,with the aim of having an HDTV system on air in 1995.
Written by V R Sunday, 31 October 2010 17:51
The bandwidth required for transmitting a video signal is estimated by assuming that adjacent pixels on a scan line are alternating black and white.Under these conditions,two pixels will comprise one cycle of the maximum rate video signal.Scan lines are repeating at the horizontal synchronizing frequency fh.and each scan line can have a maximum of NL pixels on it.Based on this then the highest video frequency that must be passed is given by f=(fh*NL)/2.
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