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First PAGE |
Satellite TV: Introduction |
All satellite TV is transmitted from Geo-syncronous (or Geo-stationary) Satellite Transmitters. To appear stationary, as Arthur C. Clarke proposed in 1948, the satellite must be orbiting on the equator plane at 22,500 miles out in space. Usually large solar panels power the craft with backup rechargable batteries for when the sun is eclipsed by the earth or moon (night-time).
Because of the distance a few satellites even a hundred miles apart "appear" to be at the same place in the sky to a satellite receiver dish. This means Sky Astra Analog and Sky Digital Services are really supplied by a group of satellites on differing frequencies.
The dish is really a big mirror for radio waves rather than light. [picture of dish] Just like a large mirror telescope, it is parabolic in shape rather than spherical. Most domestic dishes are offset parabolic, [diagram of cutout] that is the actual dish surface is offset to the side of the axis of rotation of the true parabloid shape. This means the LNB (Low Noise block) at the end of the arm is at the focus, but not casting a shadow on the dish, giving a bigger signal for the same amount of dish metal.
Though the satellite microwave signals depending on band, region and up or downlink are anything from 2.2GHz to 20GHz, typical Sky Digital signals on the Downlink to your disk are 10.7GHz to 12.6Ghz. On the coax cable to your receiver they are 950MHz to 2000MHz (0.95 to 2GHz). Regular TV coax on UHF is 470MHz to 890MHz. The higher the frequency the more critical the design, quality and installation is.
The Dish "collects" enough signal for the LNB to work, much the same way a large mirror collects light in a big telescope to see faint stars.
The LNB converts the very high microwave band to a high UHF band. For Digital and some analog satellites the LNB may switch between two bands. A 22Khz tone on the cable activates the higher band.
Some points on Satellite installation:
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Different operators typically use different positions in the sky. Zero or 0 degrees is on the equator in line with Greenwitch, London, England. So 19.2E is pointing to the East and 5W is pointing to the west. If you lived on the Equator, the mount on the wall or the pole would be horizontal and you would just rotate the dish on the horizontal mount. As you moved further away from the Greenwich meridian (say America or Asia instead of West Africa) you would have to add or subtract a small amount of correction. If you lived at the North or South pole a vertical mount with the dish angled slightly down to the horizon would be fine. To pick a different satellite you would only need to rotate the dish on the vertical mount.
Here in Limerick we are at about 53 degrees. This means that with a vertical wall mount or pole, not only does the dish rotate to the left or right, but as you "scan" the sky you need to progressively tilt the dish up or down. The up/down tilt is called azimuth. If you have a pole or wall mount angled toward north according to your distance north/south (actually pointing at Polaris, the Pole Star), you have a Polar Mount. If you line up the azimuth setting on such an angled mount on any satellite, all satellites will have the correct tilt. As you rotate the dish it points highest at due south and appears to decline as you rotate east or west. The distance from Greenwich to Limerick is so much smaller than the 22,500 miles away of the satellite, that for all practical purposes the published angle is the angle to point the dish.
So if the dish only ever points at one satellite position, you don't need a polar mount. If you have a motor drive, the axis of rotation must be along an imaginary line pointing at the pole start or else your dish won't track along the plane the satellites are orbiting on.
Satellites today use more transmitter power than the early TV satellites. Additionally the system that digital transmission uses does not need as strong a signal. This opens up an alternative solution to multiple satellite reception.
With a polar mount and a single LNB, with a positioner and motor drive a 90cm Dish in the British Isles can pickup many satellites between 30Deg East and 30 Deg West. Some signals intended for North Africa, Eastern Europe or Scadinavia may need a 1.2m dish.
For most people (with 80 or 90cm Dish) there are only 3 interesting positions: 28deg, 19deg and 13 deg (Sky Digital, SkyAnalog/Astra Digital/Analog and Eutelsat/Hotbirds Analog Digital). This is possibly FIVE services on only three LNBs.
If you look in a mirror and move, you will see a different view. A Parabolic dish only has one true focus, on the axis. However there are an infinite number of off axis focus positions, albeit with a reduced clarity/ brightness on an optical parabolic mirror. So if you put a second LNB about 9degress WEST of your 19degree dish with LNB it is picking up a signal at 28 degrees east! Similarly a third LNB mounted 6 degrees East of the 19 degrees on axis LNB will pick up Hotbirds at 13 deg East. Because of the afore mentioned azimuth issue, the Western LNB for 28deg East must be raised slightly (about 1.5" or 3cm) and the Eastern LNB for 13 deg East (assuming original dish/LNB adjusted for 19deg East) lowered by about 0.5' (1cm) .
These figures depend on the size of your dish. For a typical solid 80/90 cm ovoid offset paraboliod dish, the Hotbird LNB is up tight or close to the main LNB [picture]. The Sky Digital LNB is separated by about the space of an LNB, from the original LNB. On such a setup the Sky Digital signal strength and quality may be better than on the mini-dish.
If you have or buy a 1.2m dish, you can fit five or six LNBs. Ensure the arm can take the weight!
Basically on the 1.2m dish you can pick up satellites only 3 deg apart, but on the 80/90 size dish the geometery limits you to 6 degrees.
The outer LNBs (more than 8 degrees off axis) are best for digital and are unlikely to be good for analog.
To connect to the LNBs we do:
Make or buy LNB change over switches for our Sky Digibox
Analog: PAL
Analog Satellite Radio is transmitted on TV subcarriers. Terrestial TV uses a 5MHz, 5.5 6.0 or 6.5MHz subcarrier for mono sound. Satellite TV just uses more than one subcarrier, typically up to using 6.5, 7.02, 7.20, 7.38, 7.56, 7.74, 7.92, 8.10, 8.28, 8.46. The first carrier is always mono. The subsequent carriers are often in pairs for stereo (7.02/7.20). The first subcarrier can be any frequency from 6.0 to 6.7MHz depending on system and operator.
Terrestial Analog TV uses AM mono vision, DSBSC colour, FM mono sound and NICAM coded digital Stereo in the blank bits of the picture. Satellite Analog uses FM video, with the colour and mono sound essentially the same as on the Terrestial signal. (Subcarriers). In both cases the Teletext is a digital signal inserted in the first few lines of the picture (you see the moving dots if the picture height is too low on your screen. Stereo on analog satellite is an older system than NICAM, with poorer quality. It simply uses two subcarriers, one for left and one for right.
Digital
Analog uses three main schemes:
Sky videocrypt
D2Mac is a hybrid analog/digital system which when properly implemented gives better quality than PAL and possibly better than Sky Digital. It was doomed from the start for two reasons: It never had a decent encryption system. It uses more bandwidth than PAL. MPEG digital uses 1/4 to 1/10th the bandwidth of PAL. D2MAC reception and decoding is typically implemented by an addon box that connects to the Analog Receiver.
MPEG digital can use essentially any scheme. Common schemes are:
Conax
The two most useful ones in British Isles are Viaccess and SkyDigital. Essentially with the Free to Air boxes a PCMCIA card or similar (Typically a CAM Common Access Module, immplements the scheme and has a slot for the viewing card), can implement any scheme the provider desires. Even the Sky Digibox could in theory use a module plugged into the rear.
Currently no digital receiver that is not a Sky Digibox can use a Sky viewing card nor can a Sky Digibox use any viewing card other than a Sky Card. A FTA receiver can pickup any FTA Sky Digital Channel. A sky digibox can pickup only FTA signals on other satellites within its own parameters (see below)
While Sky may close its analog services by June 2001 and in the long term Analog Terrestial and Analog Satellite TV is doomed, plenty of services have actually commenced on analog satellite. Some of the older transponders are not suitable or ideal for digital, thus may carry analog till the end of life of the satellite. A satellite may have a design life. But many of the transponders will work for years after that date. As long as it works at all, it costs very little, so the owner will sell the air space!
Comment to Martin Pickering of Satcure last October 2000
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First PAGE |
Satellite TV: Introduction |