DIGITAL VIDEO BROADCAST TERRESTRIAL. THE DIGITAL CLIFF DIGITAL VIDEO BROADCAST- SATELLITE. RAIN FADE

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Martin Blankenship
6 years ago
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1 DIGITAL VIDEO BROADCAST TERRESTRIAL. THE DIGITAL CLIFF DIGITAL VIDEO BROADCAST- SATELLITE. RAIN FADE DVBT DIGITAL CLIFF & DVBS RAIN FADE PAGE 1 OF 6

2 DIGITAL VIDEO BROADCAST TERRESTRIAL -_THE DIGITAL CLIFF QUESTION. Why is it that I can get near perfect television reception, but my next door neighbours get almost nothing? ANSWER. The Digital Cliff. MER, Modulation Error Ratio, though not the only digital signal criterion, is generally accepted as a good single indicator of quality factor representing composite parameters. It should be better than or equal to 25 decibels. DVBT DIGITAL CLIFF & DVBS RAIN FADE PAGE 2 OF 6

3 Digital, unlike analogue, will give you perfect reception or nothing at all, and unlike analogue, the window or margin between perfect or nothing is very small as the above graphs show. The fact that you are getting a near perfect picture is not the only measure of a good antenna system. The trick is to have your signal quality factor, (strength is important but more applicable to the olde analogue as a stand alone parameter) as far back from the digital cliff on the simplified graph as possible and well into the green on the technical graph, also with as much margin as possible. PinkTronix will strive to put your antenna system s performance well into the green on the technical graph and will demonstrate to the client on an easy to understand instrument, signal strength, quality result and a spectrum (real time graph), of the received signal at each TV outlet on completion of the job. Unless you encounter frequent picture break up you won t know how close to the digital cliff you are, but if you have problems concurrent with certain weather patterns or different times of the day, you can be pretty sure that your signal quality is in the yellow zone of the technical graph above. Like the vehicle in the simplified graph, you are not quite teetering on the edge but your margin for error (bits) is not too great. You are, by a small margin, to the left of the digital cliff. As for your next door neighbours, chances are that their signal quality is just to the right of the digital cliff flickering into the red zone on the technical graph or at the suicide point on the simplified graph. Perhaps their house, though very close to yours, is in a depression or surrounded by tall leafy trees leaving their television receiver with no performance margin to accommodate the poor signal propagation. Thus we hear the lament about the good old days of analogue when we always had some picture albeit a bit snowy. Like it or not, the analogue days are gone and the only way to be sure of a good picture is to bring your terrestrial installation up to scratch or install the free to air satellite service, V.A.S.T. DVBT DIGITAL CLIFF & DVBS RAIN FADE PAGE 3 OF 6

DIGITAL VIDEO BROADCAST SATELLITE - WHAT IS RAIN FADE? (This article is a précis of a technical paper modified for relevance to satellite television and ease of understanding by the general public.

4 DIGITAL VIDEO BROADCAST SATELLITE - WHAT IS RAIN FADE? (This article is a précis of a technical paper modified for relevance to satellite television and ease of understanding by the general public.) Several frequencies are used to carry satellite transmissions. The most popular frequency bands today are C-band and the higher frequency Ku-band, (VAST, Foxtel, on Optus C1/D3, ethnic and religious television on Optus D2) with Ka-band deployments increasing. At the higher operating frequencies of Ku and Ka-band, the satellite signal strength may be affected by heavy rain conditions. A problem arises when microwave and satellite transmissions have their signals attenuated, or weakened as a result of interference caused by raindrops. The raindrops weaken the transmission by absorbing and scattering the electromagnetic signals. How much the signal is weakened depends on many variables, but good network operators will include a practical rain fade margin when designing services. This is basically the amount of additional power added to the signal strength at the teleport and any additional receiving capability at the remote site terminal, (your home dish), to compensate for attenuation of the signal as it passes through the atmosphere. DVBT DIGITAL CLIFF & DVBS RAIN FADE PAGE 4 OF 6

5 A properly engineered circuit will generally have few ill effects as a result of rain. Note that rain can be a problem at either end of the link whether it is raining over the remote site or over the teleport or both. In either event the signal strength can be degraded resulting in higher error rates, and slower throughput due to retransmissions or complete loss of service in very heavy rain conditions. There is a direct relationship between signal quality and signal decode time. If you have a poor dish set up, in addition to increased pixilation you may also notice slower acquisition between channel changes as the error correcting circuitry works overtime to put corrected data into satellite decoder buffer memory before presentation to your screen. (See also THE DIGITAL CLIFF) The elevation of the satellite is also a factor, as the more atmosphere the signal has to traverse, the more rain it may potentially have to pass through. It is also worth noting that not all atmospheric moisture is visible and just because we have a sunny day does not mean that there is no atmospheric attenuation of the signal. Sometimes it is difficult to compare apples with apples. The amount of rainfall determines the affect of attenuation and the period of disruption or degraded service. In some tropical/equatorial regions you can expect short outages almost every day during the rainy season for Ku and Ka systems. DVBT DIGITAL CLIFF & DVBS RAIN FADE PAGE 5 OF 6

6 In more temperate climates, outages may be very rare and of very short duration, providing your dish is sized commensurate with service location and satellite signal level at that location. WHAT TO DO ABOUT RAIN FADE. As the industry paper below shows, one of the approaches is to use a bigger dish. Typically there is 3 decibels of difference between a commonly installed 60 centimetre dish and a 90 centimetre dish. Because of shoddy design, poor manufacturing technique and sloppy dish aim it is possible to have a smaller 60 centimetre dish out perform a larger 90 centimetre dish. PinkTronix has sourced a sturdy, durable and accurate 90 centimetre dish which shows good repeatability with received signal parameters. The gain increase of these dishes with respect to other dishes installed is noticeable, thus guaranteeing a robust signal and the best possible rain fade margin when compared to lesser installations. The performance of these dishes has proven to be thoroughly consistent from installation to installation when checked against the same satellite transponder under similar conditions. PinkTronix also reinforces the standard MIC dish mount before installation to ensure longevity of the complete installation and maximum stability of a meticulous aim. DVBT DIGITAL CLIFF & DVBS RAIN FADE PAGE 6 OF 6

Advanced Coding and Modulation Schemes for Broadband Satellite Services. Commercial Requirements

Advanced Coding and Modulation Schemes for Broadband Satellite Services Commercial Requirements DVB Document A082 July 2004 Advanced Coding and Modulation Schemes for Broadband Satellite Services Commercial