Today, and even more so in the future, digital transmission technology will play the predominant role in satellite TV. The basic benefit to the programme providers is the fact that several channels can be transmitted on a single transponder, whereby the allocation can be freely selected. Instead of 12 TV channels, broadcasters can also choose to have just radio stations or a mixture of TV and radio. Internet data transmissions and any other types of data service are also possible. As well as additional flexibility, this also offers considerable economic benefits for broadcasters.
The combined total of all digital data transmitted on a transponder is referred to as a “transponder”. This includes the individual TV and radio channels. The transponder is what the receiver actually receives. The frequently published frequency, polarisation and symbol rate data always relate to a transponder stream.
Other than former analogue transponders, the bandwidth with digital transponders can be operated at almost any value. This makes it possible to operate older, weaker transponders with a reduced bandwidth and still obtain a wider range. Furthermore, a single transponder can carry several independent low-bandwidth signals (transponder streams), an option which is used most commonly by broadcasting vehicles. As a general rule, broadband signals (MCPC - Multiple Channel Per Carrier) tend to be used to supply signals to end customers, while signals with lower bandwidths (SCPC - Single Channel Per Carrier) are mostly used for relay transmissions, so-called „feeds“. ASTRA 1 almost exclusively transmits MCPC signals suitable for direct reception.
The symbol rate is an important figure to know, as it must be entered correctly into the receiver to ensure a successful channel scan if specific channels are to be searched. The higher the symbol rate, the higher the bandwidth, and the more channels can be transmitted on one transponder. However, digital technology allows many other ways to configure the transponder stream in addition to the symbol rate. Some providers use this to squeeze a higher number of channels onto one transponder, which results in poor image quality, low range and an increased error rate.
The range of each of the individual transponders varies slightly. This is sometimes used deliberately to focus the signal onto one area, sometimes this occurs more or less by chance. In addition to the transmitting power, the satellite also plays a role, as does the transponder configuration and the signal bandwidth. It is therefore normal for some channels to be received at the edges of the footprint, while other channels cannot be received there. Of course, modern automatic systems only scan digitally and therefore locate satellites from locations in which only few channels can still be received.
Normally, the TV image collapses almost instantly if the signal strength is insufficient. Interference usually manifests itself in the form of blocky/pixellated images or frozen images. This usually indicates that the signal strength is too weak. Other possible causes can include transmitter errors or an overload of the transponder stream. HOTBIRD often transmits channels which are affected by noticeable interference despite having a very powerful signal.
A clear view of the satellite is the key prerequisite for good reception. Buildings within the signal beam always block reception. Trees may, in rare cases, allow the signal to pass through unhindered, but this is not reliable. The geographic location also needs to be taken into account, as the signal is received at a lower elevation angle the further north the system is operated. In very northerly regions, the curvature of the earth can cause even topographic obstructions like mountains to prevent reception of satellite channels.
The larger the antenna is, the larger the reception range will be for a given transponder, and the more tolerant the system will be to poor weather. Damaged, i.e. dented or warped antenna dishes reduce the range significantly. High-quality LNBs show their increased capacity only at the limits of the footprint. Even in the centre of a footprint, e.g. in Germany, satellite operators recommend a minimum size for the satellite dish in order to ensure good reception even in poor weather.