What is Satellite TV?
Satellite TV is the system by which viewers receive television broadcasts directly from communications satellites which have had this programming sent to them by service providers.
How does satellite TV work?
There are five basic steps associated with the transmission and reception of satellite TV. These are common to both satellite TV formats currently in use. The process starts with the programme source.
The programme source (e.g. HBO) creates the original satellite TV content. It then sends this content to the service provider via various means (e.g. satellite or fibre optics)
The service provider (e.g. DirecTV) collects together the content from all the programme sources that feed into it. It then compresses this aggregated content – typically using Mpeg-2 – into a signal. This compression reduces the bandwidth of this signal (i.e. how much room it takes up) by as much as 60%. This signal compression is important for two reasons; it removes repetitive data and allows more signals to be sent to its satellite than would otherwise be the case. The provider also encrypts the signal to prevent unpaid access to it. The provider then transmits this signal (known as uplinking) to its geostationary satellite.
The geostationary satellite is in an orbit 23,300 miles above the Earth’s Equator. To be in this orbit the satellite travels at the same speed and in the same direction as the Earth rotates. As a consequence of this it appears to stay in the same location over the Earth’s surface. That is why these satellites are in the southern sky. Geostationary orbits are favoured because it eliminates the expense of Earth stations having costly equipment installed to track the satellite as it travels through space. The orbiting satellite receives the signal transmitted by the broadcast (uplink) provider via a transponder.
A transponder consists of a transmitter and receiver. The transponder receives the uplinked frequencies then re-transmits them down towards the Earth (downlink) on different frequencies to that received. Different frequencies are used when re-transmitting on the downlink to avoid interference with the received signals on the uplink. This downlink signal is then received by the viewer’s dish antenna.
The dish antenna consists of two parts, a parabolic surface and a feed horn. The parabolic surface captures the provider’s satellite TV signal and focuses it onto the feed horn. The feed horn performs three functions.
First it amplifies the satellite TV signal focussed onto it by the parabolic surface.
It then converts this block – or band – of received satellite TV microwave frequencies to a block of lower microwave frequencies. It does this by using a Low Noise Block downconverter (LNB)
Finally this block of lower frequencies is then sent via co-axial cable to the set top box. There is a limit to the length of co-axial cable that can be used – without further amplification – as if it is too long it can attenuate (weaken) the satellite TV signal to a point where it is unusable.
The number of outputs on an LNB is important. A typical configuration for an LNB is to have four outputs, this can allow the viewers of up to four TVs in the household to select their own channels.
The final step is associated with the set top box. The set top box reverses what the service provider did to the original signal; it decompresses and decrypts the satellite TV programming. This produces audio and video signals, these are then sent to the television set where they are used to provide sound and vision.
The set top box can also offer the functions of displaying two programmes simultaneously, sending HDTV (High Definition TeleVision) broadcasts to the TV set, recording TV shows and pausing and replaying live TV.
In this post satellite TV has been defined and a basic explanation has been given of how these transmissions are generated and end up being displayed on the viewer’s television receiver. In the next article the features, benefits and drawbacks of satellite TV are explored and pointers are given on what leading U.S. DBS providers can offer.