DVB-S (short for English " D igital V ideo B road casting - S atellite" ; German "Digital Satellite TV ") and the successor standard DVB-S2 are names for the broadcasting of DVB -Signalen via satellite broadcast .
The broadcast of DVB via satellite (e.g. Astra , Eutelsat ) is the most widely used DVB variant. Thanks to the high data transmission rate, most television and radio programs as well as additional services are transmitted here (e.g. ARD and ZDF since August 1997). At the end of 2008, more than 1,500 radio and television programs were transmitted via the Astra satellites alone, of which almost 300 were television programs and telemedia services and around 170 unencrypted radio stations. In contrast to DVB-C (“C” for cable , cable ') and DVB-T (“T” for terrestrial ), DVB-S does not require any additional infrastructure ( cable networks , terrestrial transmitter chains ) and thus offers radio reception even in remote areas. There are parabolic antennas which, by automatically tracking the antenna, enable reception in airplanes, on ships or even in buses while driving. Therefore, the term “TV everywhere” applies more to DVB-S than to DVB-T. DVB-S sometimes even serves as a data supplier for the cable networks or DVB-T.
Equipment and costs
In contrast to DVB-C, there are no running costs with DVB-S ( apart from pay-TV ), since the satellite operation is paid for by the broadcasters . However, since older televisions cannot receive the DVB-S signal directly, they have to use a digital receiver .
An important advantage of DVB-S is that, in contrast to analog broadcasting, several programs can be broadcast on one transponder (MCPC). This represents a cost advantage for the program provider, since renting a satellite transponder is very expensive. The number of programs broadcast simultaneously via a transponder depends on the data transfer rate assigned to the respective programs. On average, the number of programs in SDTV is around eight full television programs with average resolution, and in HDTV around four programs.
Transmission technology and modulation methods
DVB-S contains optimizations for the satellite-specific properties (missing reflections , low carrier-to-noise ratio (CNR), transmission signal with a low crest factor ) in the transmission of digital data . Is used QPSK modulation. With MCPC signals ("Multiple Channel per Carrier", several channels per carrier frequency) very high symbol rates of more than 10,000 kSym / s are used, with SCPC signals ("Single Channel per Carrier", one channel per carrier frequency) low symbol rates of less than 10,000 kSym / s. Since external error protection ( FEC ) is necessary due to the transmission type via satellite, in contrast to digital cable signals (i.e. DVB-C) , there are high error correction components in the data stream of typically 1/6 to 1/3 of the gross data rate. With DVB-S2 (see below), despite the better correction method, the required error correction component is usually the same or even greater, since 8 PSK instead of 4 PSK (QPSK) are usually used.
To receive satellite broadcasting, a parabolic antenna with a digitally compatible signal converter (LNB) is required, which forwards the signals to the consumer using various cabling methods ( e.g. satellite block distribution or Unicable ).
There are numerous LNB designs that differ in terms of various criteria (according to frequency range, design, number of subscribers that can be connected, multi-switch functionality).
Program diversity of the frequency bands in comparison with other DVB transmission types
Two polarization planes are used per satellite (mostly horizontal and vertical, more rarely left and right turning). Therefore this frequency range can be used almost twice. Satellite groups consisting of several satellites can be stationed at each orbital position - at Astra at 19 ° East, for example, there are currently four satellites. All satellites in an orbital position share the 4 GHz bandwidth, provided they are all aligned to the same reception area.
Many frequencies and several satellite positions allow many programs. In theory, DVB-S is superior to DVB-C in terms of the number of programs only when receiving several satellite positions. Although the usable HF bandwidth per satellite position in the Ku-band is significantly larger at 4 GHz than in cable networks with DVB-C (0.8 GHz), the value is put into perspective when you consider the lower signal-to-noise ratio of DVB -S (works with QPSK) compared to DVB-C (mostly uses QAM ). Taking into account the Shannon-Hartley law , a similar channel capacity is calculated.
The direct comparison looks like this:
- per satellite : 4 GHz / 40 MHz = 100 digital QPSK transponders (4 GHz = satellite capacity , 40 MHz = bandwidth per transponder including space)
- Cable : approx. 800 MHz / 8 MHz = 100 digital QAM channels (800 MHz = cable capacity, 8 MHz = cable channel bandwidth)
However, to further increase the number of programs in DVB-S, several satellite positions can be used for reception and thus the number of programs can be increased by limiting the bandwidth of a satellite position in the Ku-band. With cable reception you would have to switch between different cable networks to achieve the same effect. In practice, the range of programs available via satellite (e.g. ASTRA) is therefore a multiple of the range offered by a cable network operator.
This calculation does not take into account that the bandwidth of the DVB-S is only currently used frequencies in the Ku band. An extension by other frequency bands is technically feasible at any time and means, for example with extension by the Ka band (17.7–21.2 GHz), more than doubling the usable bandwidth of a satellite position, which then corresponds to 250 digital QPSK transponders. In the future, the Ka-Band could also provide additional multimedia or program offers. Theoretically, it would also be possible to use the C-band to further increase the supply . However, this is unlikely because of the large antenna diameter required.
In addition to the Ku band common in Europe, the C band (3.4–4.2 GHz) is also used in America, Asia and Africa. This is characterized by a significantly lower susceptibility to failure when it rains. For the reception of most satellites, mirror diameters of 2 m or more are required. This volume offers only a few additional programs in German-speaking countries, but some very exotic programs.
The comparison between DVB-S and DVB-T is clearer: DVB-T has a lower usable bandwidth with a maximum of 0.5 GHz. The HF bandwidth is significantly lower, and the possible single-frequency operation does not solve the problem that only max. 15 percent of the frequencies can be used. The bandwidth also reduces the modulation that is usually only possible (COFDM in 16QAM or 64QAM) and the guard interval . All the effects taken together show that DVB-T allows about five percent of the data rate of DVB-C. Alternative topologies are only possible with great effort.
DVB-S2 is a further development of the DVB-S standard. The use of improved coding, modulation and error correction methods increases the data rate by up to 30%. In March 2005 , ETSI ratified the DVB-S2 standard under the number EN 302 307 . To switch the reception from DVB-S to DVB-S2 no new signal converter (LNB) is required, "only" new set-top boxes (receivers) or televisions with the corresponding DVB-S2 receiver.
Instead of 4PSK (QPSK) with DVB-S, DVB-S2 optionally uses the modulation types 8PSK, 16APSK or 32APSK. The adaptation ( ACM ) is optionally carried out by reporting the reception quality from the reference receiver. In this way, if the reception situation is poor, the modulation can be changed in order to avoid interruption of reception.
With the same bit error rate (BER), 8PSK requires a higher signal-to-noise ratio (SNR) of around 4 to 4.5 dB. At the same time, the more efficient error correction code LDPC is usually used (except for the Dish Network in the USA, for example ) , which requires about 1.5 dB less signal-to-noise ratio than DVB-S. This is one of the reasons why a higher net data rate is achieved compared to DVB-S. LDPC can also be used with QPSK to reduce the required SNR or for higher net data rates. Below the SNR required for error-free reception, however, LDPC leads to a total loss of signal more quickly, while DVB-S initially produces more and more image errors.
The use of better algorithms for image data reduction ( e.g. H.264 or MPEG-4 AVC instead of H.262 or MPEG-2 video) and better resolution ( HDTV ) is not necessarily linked to DVB-S2. WDR television sends its HD channels H.264-coded via DVB-S (on the same transponders as the SD programs). However, since new end devices with different demodulators and decoders are required for newer formats anyway, most providers also switch to a more data rate efficient and therefore more cost-effective modulation method if, for example, a new HDTV station is to be broadcast. For this reason too, DVB-S2 is often accompanied by a change in the audio codec in favor of Dolby Digital (AC-3), which the major broadcasters are already offering with DVB-S. Of the German-speaking HD channels, only the public broadcasters and Servus TV also broadcast with the MPEG-1 Audio Layer 2 (MP2) codec usually used for MPEG-2 audio .
DVB-S2X is a further development of the DVB-S2 standard. The improvements are less than those when switching from DVB-S to DVB-S2. The main changes are:
- Transmissions at low carrier to noise ratio (CNR) of −10 dBc to −3 dBc are possible.
- The amplitude phase modulation used can be operated with a high number of symbols at a high CNR (+15 dBc to +20 dBc) and thus allows a larger amount of information to be transmitted per symbol.
- A finer granular forward error correction (FEC)
- Use of hierarchical modulation methods possible
Compared to the limited divisible bandwidth of analog satellite television transmitters, digital data compression allows several digital transmitters to be broadcast in the same frequency range. For this reason, the analog satellite channels were switched off in April 2012 for bandwidth reasons and for economic reasons on the part of the broadcasters (and in some cases also the satellite operators).
Distribution using Sat-over-IP
Sat-over-IP technology is a standard for receiving DVB-S and DVB-S2 signals and distributing them via Ethernet networks. SAT-IP is an open, manufacturer-independent standard that has been certified by the European standardization organization CENELEC. Sat-IP translates received satellite signals into local networks using a Sat-IP converter, regardless of the Internet connection . This enables the mobile reception of the satellite signal via WLAN or LAN on devices that do not have an integrated satellite receiver. This means that it is sufficient to supply all devices connected to the respective network using a single LNB and satellite IP. This enables satellite reception on all network-compatible devices such as tablets, PCs, laptops, smartphones and all television sets via WLAN or LAN. The connection of each individual device to a twin LNB or quad LNB or a distribution station is therefore not necessary. The prerequisite is that the new devices are suitable for Sat-IP, otherwise a special Sat-to-IP router is required to convert the Sat signal into a network signal. These devices are also offered under the names of Sat-IP converters, Sat-IP servers or Sat-IP network transmitters and can transmit the converted signal to several end devices at the same time. SAT-IP-LNBs are available that have already implemented the SAT-IP converter software and convert the signal directly in the LNB.
- Digital television
- Digital radio all transmission methods for digital radio
- List of geostationary satellites
- Techview (PDF; 367 kB) EBU - Description of the design of DVB-S2 (English)
- Frequency and satellite tables