The...Musiczy...Of...thailand

Effects on ionosphere

2007-09-15T08:47:00.001-07:00

A solar eclipse causes a decrease in the critical frequency of E and F1 layer. The variation of critical frequency is approximately in phase with the optical eclipse. The ionization in E and F1 layer results from solar radiation having speed closely agreeing with that of light. The influence of the solar eclipse on the F2 region is relatively less. Meteors may produce enough ionization in the upper atmosphere so as to appreciably effect radio wave propagation. The meteors may account for sporadic E layer effects. Sporadic E layer effects are when radio waves are returned are returned to earth by the E layer at frequencies considerably greater than the critical frequency of the layer. These are believed to be caused by partial reflections from sharp boundaries to stratified ionization. Further the ionizing effects of the minute meteors continuously received by the earth may at least partially account for the ionization that remains in the ionosphere at night time. Irregularities in electron distribution in the ionosphere causes scattered reflections of radio waves. These reflected signals are generally weak, irregular and diffused and may be caused in any of the layers. Further these reflected signals some times become quite complex because of the interaction between layers. The vertical heights of scattered reflections depend upon the type of scattering and may lie between ninety to about fifteen hundred kilometers. Scatter reflections are also caused when an ionosphere wave on refraction from the ionosphere strikes the earth. The scattered energy then propagates away from the point of scatter refraction in the same general way as that from a transmitting antenna. How ever the signals so produced are some forty decibels weaker than the main signals. Scattering of radio waves may be radiated usefully utilized in the following waves that is scatter signal may be radiated within the skip region and with highly directional transmitting antenna, a receiving antenna lying appreciably to one side of the path of the main beam may also adequately receive the signal as caused by scattered wave traveling at right angles to the main beam.The effect produced by the ionosphere on a radio wave is not linear that is not proportional to the magnitude of the wave, hence two strong radio waves passing through the ionosphere concurrently may interact to produce cross modulation products. The magnitude of such a cross modulation may in some cases be large enough to produce appreciable effect. First such interference reported involved radio Luxemburg. Hence the effect is termed as Luxemburg effect. But the Luxemburg effect has not been generally observed with even most powerful transmitters. Hence there is a possibility that the effect might have been produced by external cross modulation instead of cross modulation in the ionosphere.

Fading

2007-09-15T08:46:00.000-07:00

By fading of a radio signal we mean that incident wave change in the field strength of signal received from distant transmitter. The fading of signal may be put into the following two categories namely general fading or synchronous fading and selective fading. The fading is caused due to interference between waves that have traveled by slightly different paths. The fading in general is slow taking some minutes to go from maximum to minimum signal strength. Generally the entire signal fades synchronously. Such a fading is termed as general fading or synchronous fading. Amplitude modulated signals traveling through ionosphere experience general fading. The fading is sometimes sensitive to frequency. Thus in amplitude modulated carrier wave the carrier and the two side bands may not fade in and out synchronously. Such a fading is termed as selective fading. Selective fading commonly results in signals involving large bandwidth such as frequency modulated signal or television signals. On the other hand the amplitude modulated signals involving only ten kilo hertz bandwidth suffer from selective fading only occasionally. Selective fading introduces distortion in the receiving signal. This distortion is particularly great when the carrier fades to small amplitude leaving the sidebands not attenuated. The general fading can be reduced to a small magnitude by use of automatic gain control in the radio receiver by use of highly directional antennas, by diversity reception and by MUSA receiving system. Frequency modulated signals transmitted over long distances under conditions where there is more than one path, suffer much greater distortion than to the amplitude modulated signal. The maximum distortion level is at two modulation frequencies and high depths of modulation. The distortion results from the fact that the instantaneous frequency modulated signal varies continuously varies so that when two waves arrive at the receiving antenna after having traveled unequal distances they have different instantaneous frequencies. The resultant signal then involves a new modulation involving both frequency modulation and amplitude modulation at a frequency which depends upon the path difference and is not harmonically related to the modulation product at the transmitter. By the term static is meant the radio disturbance caused by natural causes such as lighting, electrical sparks etc. the disturbances so produced in a receiver are sometimes called noise but usually the word noise is used to denote the electrical disturbance of man made origin. Static disturbances received by a receiver can be classified into four different categories like impulses of high intensity occurring by intermittently such as caused by local thunder storms, steady rattling or a cracking caused by electrical disturbances produced by distant thunder storms, steady hiss type of static obtained at high frequencies having interstellar origin and perception static

Pulse Code Modulation

2007-09-15T08:45:00.004-07:00

Pulse amplitude modulation, pulse width modulation and pulse position modulation form cases of analog pulse system. Pulse code modulation and delta modulation form cases of digital pulse modulation. Pulse code modulation in similarity with other pulse modulations uses the sampling technique but it differs from the others in that it is a digital process. Thus Pulse code modulation generator produces a series of numbers or digits, each of which represents the approximate magnitude of the signal sample at that instant. This approximation may be made as close as desired but at best it remains an approximation. In Pulse code modulation the total amplitude range of the signal is divided into a number of standard levels at equal intervals. These levels are transmitted in a binary code. Hence the actual number of these standard levels is a power of two such as sixteen, thirty two or one twenty eight. Practically systems use as many as one twenty eight levels. By the so called quantizing process, the level actually sent at any sampling instant is the one nearest to the standard level. Thus if the signals is 10.2 volts at any time, it is sent as the digit ten since ten volts is the standard amplitude nearest to 10.2 volts. This digit ten is sent at that time instant as a series of pulses corresponding to number ten. The signal is continuously sampled and quantized, each sample magnitude being converted to the nearest standard amplitude. The quantized number is coded using binary code converted into corresponding back to front binary numbers and then sent. If adequate quantizing levels are used the result closely resembles the corresponding analog transmission. Generally a supervising bit is added to each binary code group representing a quantized sample each group of pulses denoting a sample referred to a word is thus expressed by n+ 1 bit. The quantized signal some what differs from the actual signal. This amounts to introduction of same distortion called quantizing noise. The word noise is used here since the error so introduced is random in nature. This random nature results because the difference between the quantized levels or digit and actual signal at that instant of time is completely unpredictable or random. It is obvious that the maximum quantization error equals half the sampling unit. But it is wrong to assume that signal to quantizing noise ratio of this system is thirty two is to one because neither the signal nor the instantaneous quantizing error has always its maximum value. The above consideration and several others influence the quantizing noise. Thus quantizing noise for a given number of quantizing levels can be calculated only using statistical methods.

Space wave propagation

2007-09-15T08:45:00.003-07:00

By space wave we mean the radio wave which travel from the transmitting antenna to the receiving antenna through the space that is the earth’s troposphere. Earth’s troposphere extends from the earth’s surface up to about fifteen kilometers. The space wave consists of two components that is direct wave and the ground reflected wave. Frequencies above sixty mega hertz are never refracted back to earth by the ionosphere and ii is only under special circumstances that frequencies in the range of thirty to sixty mega hertz are so returned. Further the surface wave is attenuated extremely rapidly at that these high frequencies. As a consequence communication at frequencies above about thirty mega hertz is possible only through space wave propagation. When the transmitting and receiving antennas are both at the earth’s surface the two components of the space wave are equal in magnitude and opposite in phase. Hence these two components cancel each other at the receiving antenna leaving the surface wave as the only component of ground wave. This is the case of ground wave propagation at broadcast frequencies is medium waves but as the heights of the antenna expressed in wave lengths are increased the amplitude of the space wave increases rapidly and when the heights equal a few wave lengths or more the space wave becomes the principal part of the ground wave. In general the transmission is possible only up to or slightly beyond the line of sight distance. Hence the elevation of the antennas is the factor which determines the range. The horizon effective for the radio waves commonly called the line of sight is slightly beyond the horizon determined on the basis of a straight line path because of the refraction of the radio wave in troposphere. The dielectric constant of the troposphere is slightly greater than unity and varies with the concentration of water molecules having high dielectric constant. With the increase of height above ground the atmospheric pressure reduces causing corresponding reduction in the dielectric constant and refractive index. This decrease in refractive index with height causes refraction of the radio wave resulting in bending of radio wave towards the region of high dielectric constant that is towards the earth. Accordingly space wave propagation becomes possible up to range slightly beyond the actual horizon. The atmospheric pressure, the dielectric constant and the consequent refraction varies with time. But on the average the effect of refraction in troposphere may be accounted for by assuming straight line propagation and increasing the effective radius of earth. The curvature of earth produces two effects that is it reduces the difference in the path lengths between the direct and ground reflected waves by raising the point of reflection above that of a plane earth and as a result tends to reduce the strength of the received field.

Static and Noise

2007-09-15T08:45:00.001-07:00

By the term static is meant the radio disturbance caused by natural causes such as lighting, electrical sparks etc. the disturbances so produced in a receiver are sometimes called noise but usually the word noise is used to denote the electrical disturbance of man made origin. When most of static arriving at a receiving point is of distant origin, then these sources tend to lie in certain specific directions. It is found that the field strength of static on the average is approximately proportional to the frequency. This can happen if the electrical discharge which generates the static is a pulse of relatively long duration. When the static is of local origin the intensity is accordingly less the higher the frequency and becomes negligibly small at ultra high and micro wave frequencies. In case of static of distant origin the magnitude of static received at a receiving point depends further on the propagation characteristics. Thus distant sources cannot produce static at ultra high frequency and micro waves since ionosphere propagation at these frequencies is not possible. Similarly at any frequency in the short wave range, the magnitude of the interference produced due to distant source of static will depend upon the time of day, distance etc. at medium waves during day time, static of local origin alone is present since static of distant origin propagated using surface wave propagation dies down on having traveled short distance. During night time however medium wave range static of both local origin are present. Static in long wave range has very high intensity. This results because of two main reasons. Firstly natural sources of static produce large intensity static at low frequencies and low frequency radio wave propagation very efficiently over long distance under almost all conditions. The following means can be adopted to reduce response of receiver to static. Firstly by reducing the pass band of receiver to minimum required since the energy of static received by a receiver is proportional to the pass band. Secondly by use of a limiter to limit the signal and static to suitable pre determined high level. This removes all large amplitude static of impulsive nature. Thirdly by use of frequency modulation for communication automatically suppresses static through capture effect, if the signal magnitude appreciably exceeds the magnitude of the static. However, frequency modulation because of its large band width requirements may be used only at ultra high frequencies in which range of the static or interference is least severe. And lastly use of directional receiving antenna may be made to receive the maximum signal from the desired direction and to receive minimum static. Of course this expedient also fails if the direction of the signal is the same as the direction of static.

Super Refraction

2007-09-15T08:44:00.001-07:00

When the M curve has a negative slope the curvature of the ray is concave downward on a plane earth diagram and the true curvature of the rays is greater than the curvature of the earth. Hence rays which enter the duct with sufficiently small angles are bent until they become horizontal and then are turned downwards. This particular form of refraction is called super refraction. These rays are trapped in the duct oscillating between the ground and an upper level in the case of ground based duct and between two levels in the atmosphere in the case of an elevated dust. The conditions depend upon the position of the transmitter with respect to the duct. These cases are possible when transmitter is inside the duct, transmitter is below the duct or the transmitter is above the duct. Conditions most suited for duct formation is when the transmitter is inside the duct. Even with this condition fulfilled not all waves are trapped in the duct. The ground based ducts are formed generally when air flows from land out over cooler water. The evaporation of moisture from the water into the lower layers of air cools the air, increases the moisture lapse rate and produces a temperature inversion, a combination which usually results in formation of a good surface duct. Ground ducts are also formed by the nocturnal cooling of the surface of the earth. During a clear night the ground surface radiated its heat readily into outer space. Thus a clear dry land such as desert heats up considerably by the sun during the day time and it cools rapidly at night. This temperature variation may be relatively less in case of other types of land surfaces. This nocturnal cooling may cause temperature inversion in the lower layers of air on a clear night due to its contact with a cooler earth surface. However, over deep water the diurnal changes in surface temperature are, in general too small to cause the formation of ducts. The elevated ducts are generally formed be a metrological process called subsidence that is a slow downward setting of a large mass of air typically a stagnant high pressure area, combined with horizontal spreading of the air above the lower layers in the atmosphere. This subsiding air becomes considerably drier than the unaffected air below it, resulting in temperature inversion. This causes production of a duct with base several thousand feet above ground. Elevated ducts are typically found in the coastal areas at elevation of one thousand to five thousand feet and have depths typically varying from a feet to more than one thousand feet. A duct can be more often compared to a metal duct.

Exit Festival - Probably the Best Festival in the World!

2007-09-15T08:42:00.000-07:00

A record number of festival goers descended on the picturesque town of Novi Sad, with over 10,000 Brits enjoying delights of the Petrovaradin fortress for this year’s Exit Festival in Serbia.Eager revealers arrived early and the campsite was overflowing by nightfall and as the first day of the festival arrived, the atmosphere in Novi Sad was buzzing with anticipation. Says EXIT’s Rajko Bozik “The English audience was great – friendly and in a good mood. There was a great atmosphere in the festival’s campsite and on the fortress, and many friendships were started among visitors from UK and Serbia/Balkans.”As the sun began to set on the eve of the first night, the view from the terrace on the fortress was simply stunning. With the thousands of people walking in expectation across the bridge like an army of ants making the pilgrimage to the festival entrance. Tanya Stephens and Groove Armada began the proceedings on the main stage, but it was The Prodigy that shook the 18th Century fortress walls with their opening track Breathe. Revving up the crowd to insanity, Exit was left in a state of euphoria and the band’s car was totally mobbed when they tried to leave the site. “It was the best festival we played this year!” exclaimed Liam Howlett. While all this was going on the crowd were treated to an amazing fireworks display before rock legend Robert Plant & The Strange Sensations took to the stage giving the crowd all the Led Zeppelin classics they had been waiting for and then culminating with the Stanton Warriors & Rodney P until sunrise.Meanwhile on the Dance Arena the 25,000 strong crowd were treated to the delights of Aussie Deepchild, and then Trentemoller live, with one fan from Scotland saying “It’s so good it’s not right!” Next up was Green Velvet and Richie Hawtin who was took things tougher. “Exit was one of the most interesting gigs for me this year. The location was amazing and to feel the warmth of the people dancing and freaking out to my music was inspirational! Respect to the people of Exit for pushing the boundaries with their choice of lineup and bringing great new music one step further east!” enthused Richie Hawtin. Magda concluded the night with her selection of choice techno and her appreciation of the faithful crowd who showed her a true Serbian welcoming.As the sun rose so did the temperature and the party people made their way back to their beds across the river in Novi Sad. But no time for rest as the beach party started at midday with Jo Mills bringing a slice of DC10 to the banks of the Danube. Pete Gooding and Alex Gold were also playing over the next few days for those who did not wish to sleep (or couldn’t). Onto Day 2 and Friday was the turn of all girl group The Pipettes to kick things of on the Main Stage. “We were hit immediately by the enthusiasm of the crowd, we've never played a better party!” exclaimed Gwenno from the band. They were followed by the Brazilian band CSS with singer LoveFoxx modelling some very unique outfits including a sequined jumpsuit. Then it was the Beastie Boys turn to take command of the crowd with all their biggest hits and humorous stage antics. Followed by Zinc back to back with Friction and EXIT resident MC Dynamite bringing in the morning.Cut to the Dance Arena where Ibiza’s We Love… crew played host, bringing with them the legendary Danny Tenaglia, 2020 Soundsystem live, Nic Fanciulli, Paul Woolford and Serge Santiago. Outstanding performances by all with a perfect ‘Exit moment’ when Danny Tenaglia donned a pair of large gold comedy sunglasses and busted some dance moves on the stage. Priceless - A true festival moment!Exit festival has over 25 stages of all sizes and musical genres, a walk through the mystical fortress is like a magical wonderland…. Featuring the Café del Danube, the Elektrana Stage, Reggae Stage, Latino Stage, Urban Stage, Explosive Stage, Cinema, Happy Novi Sad Stage, Futureshock and trillions more. First up is the World Music Stage where the amazing group of percussionists from Novi Sad called Bateria SambaNsa were playing Brazilian rhythmic music. Next up, the Reggae Stage hosted by Brixton’s Terratone crew who have been performing at the festival for 6 years and are considered residents of Exit! “The Serbian people are a joy to be amongst and treat everybody like royalty, the very little they have they openly share with you, it is truly touching” Wenti Coke enthuses. And this is a sentiment shared by many UK visitors to the festival. Said Mark, a punter from Manchester “The people of Serbia where so welcoming and friendly towards the festival goers to the point where we have made friends with many of them. The festival has a friendly atmosphere and is set in an excellent location.”And there’s the MTV Movement stage – an intimate and rocking charity driven stage supporting social issues at EXIT. This year they launched the campaign ‘Movement’, promoting Tolerance and Diversity and encouraging people to free their minds from prejudice. Acts included The Loose Cannons, Kissy Sell Out, Rob Garza (Theivery Corporation), Bez (Happy Mondays) and Tayo, with surprise sets from Tom Finlay’s Groove Armada and Green Velvet doing an 80’s set no less. Diverse to say the least!And onto the Fusion Stage where Serbian and regional bands perform. A group of Serbians explained that although the festival is a great party, it still means freedom to them. EXIT festival started as a student initiated project against the Milosevic regime in 1999 and although it has grown to into an internationally recognised event, EXIT still stays true to its roots and supports issues that effect the Serbian youth such as student mobility, youth activism and visa abolishment. Says EXIT’s Rajko Bozik “Because of visa regimes young people from Serbia and Balkan can’t travel easily, so having so many international visitors to EXIT was great opportunity for our local audience to find out more about festivals in UK and Europe. We hope very soon it would be easy for young Serbs to go to Glastonbury, just as it’s easy for young Brits to come to EXIT.” And comparisons to Glastonbury are rife. With the variety of unique stages and performers, this year’s EXIT also featured Ms Lauryn Hill playing the diva, Basement Jaxx belting out their hits and the ultra smooth Snoop Dogg bringing the ‘bling’ to Main Stage, finishing with the delectable sounds of LTJ Bukem & MC Conrad.Saturday also saw Roger Sanchez, the Godfather of house Frankie Knuckles, Buraka Som Sistema live, Justin Martin and Hannah Holland on the Dance Arena. "It’s great to see Serbia progressing in providing a place for people from all over the world to come together and enjoy electronic music. There were a lot more European attendees and for me it is very positive to see people like Frankie Knuckles on the line up. I really enjoyed playing this festival with a great crowd and wonderful organization" stated Roger Sanchez after rocking the 25,000 throng of revellers.Sunday, the last night of the four day event featured the Audio Bullys, Pendulum DJ set, another amazing fireworks display and the mighty Wu-Tang Clan on Main Stage. With Wu Tang’s Masta Killah exclaiming after their performance "Man, they are crazy out there, crazy". Meanwhile in the Dance Arena, Sebastien Leger preceding John Digweed, both playing at EXIT for the first time before handing over the decks to Eric Prydz, another first time performance. Said John Digweed “One of the best festival gigs that I have ever done. Amazing venue, thumping sound system and great crowd”. And as the sun came up on the last morning, it was left, as tradition has it, to local hero Marko Nastic to end EXIT’s four day musical and magical fest. EXIT is truly an amazing experience and although it gets bigger each year the sentiment is still the same. It is great to be part on an event that actually means something and makes a stand to make things better for the young people of Serbia.EXIT’s General Manager Bojan Boskovic sums things up “Every year we aim to out-do the previous year and this year is no exception. We, as the State of EXIT, wish to bring various people together, mainly inviting international and regional bands, artists and performers to Serbia in order to promote the interaction of different cultures, traditions and beliefs through music and social activities. EXIT has become a social force that is a symbol of togetherness and cultural diversity, that every year brings more and more people from various backgrounds and cultures, together.”Long may the State of EXIT reign!Copyright Nikki Wright 2007. All rights reserved.