DVB-S
DVB -S ( Abbreviation for Digital Video Broadcasting - Satellite, Digital Video Broadcasting via satellite ') is the name for the broadcasting of DVB signals via satellite.
The broadcasting of DVB via satellite (eg Astra, Eutelsat ) is the most widely used DVB- variant. Here most TV and radio channels and additional services will be delivered (eg, ARD and ZDF since August 1997) due to the large data transfer rate. Alone on the Astra satellites more than 1,500 radio and television programs are transmitted, which are nearly 300 television channels and 170 radio stations unencrypted. In contrast to DVB -C ( "C" for engl. Cable, cable ' ) and DVB -T ( "T" for terrestrial) requires DVB -S no additional infrastructure ( cable networks, terrestrial broadcasting networks ) and is therefore also in remote areas radio reception. There are satellite dishes that allow automatic tracking by the antenna reception in aircraft, ships or even in buses while driving. Hence the name "Everywhere TV " is more appropriate to DVB -S to DVB -T. DVB- S is used in some cases as a data provider for the cable networks (Analog and Digital) or DVB -T.
Equipment and costs
In DVB- S only pay the monthly license fee for the receipt of public television and radio stations. Other ongoing costs incurred in DVB -S DVB -C as opposed to not ( apart from pay-TV ), as the satellite operation is paid by the broadcasters. Since today but not all TVs can receive the DVB- S signal directly, the use of a digital receiver is usually necessary, as well as a receiving system that supports the high- band.
An important advantage of DVB -S is that on a transponder in contrast to analogue broadcasting several programs can be radiated ( MCPC ). This will not affect the program provider is a cost advantage because the rental of a satellite transponder is quite costly. The number of simultaneously emitted by a transponder programs is dependent on the data transmission rate which is assigned to the respective programs.
Transmission technology and modulation method
DVB -S contains optimizations for the satellite-specific characteristics (eg, missing reflections, rather poor signal -to-noise ratio) in the transmission of digital data. Is used QPSK modulation. In MCPC signals ( "Multiple Channel Per Carrier ," multiple channels per carrier frequency) can be very high symbol rates greater use 10,000 ksym / s, with SCPC signals ( " Single Channel Per Carrier ," a channel per carrier frequency ) low symbol rates less than 10,000 ksym / s Since (ie DVB -C), an outer error protection (FEC ) is required by the transmission via satellite as opposed to digital cable signals, resulting in the data stream high error correction shares of typically 1 /8 to 1 /2 of the total data rate. In DVB- S2 ( see below ) is smaller due to the better correction method of error correction required proportion.
Reception
To receive satellite broadcasting a satellite dish with digitaltauglichem signal converter (LNB ) is required, which passes through different wiring methods (eg satellite block distribution or unicable ) the signals to the consumer.
There are numerous LNB types that differ by various criteria ( by frequency range, design, number of participants 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 (usually horizontally and vertically, rare left - and right-handed ) per satellite. Therefore, this frequency range can be used twice. At any orbital position of satellite groups may be based, consisting of a plurality of satellites. All satellites orbital position shared by the 4 GHz bandwidth, provided that they are all aligned to the same coverage area.
Many frequencies and several satellite positions allow many programs. Theoretically, DVB- S DVB -C superior in the area of program number only when receiving multiple satellite positions. Although the usable RF bandwidth per satellite position in the Ku- band with 4 GHz is significantly larger than in cable networks for DVB-C ( 0.8 GHz ), but the value into perspective a bit if you lower the signal - to-noise ratio of DVB -S ( uses QPSK ) compared to DVB -C ( mostly used QAM ) is considered. Taking into account the Shannon -Hartley Act, calculated a similar channel capacity.
The direct comparison is as follows:
- Each satellite: 4 GHz/40 MHz = 100 digital QPSK transponder ( 4 GHz = satellite capacity, 40 MHz = bandwidth per transponder including space )
- Cable: 800 MHz / 8 MHz = 100 QAM digital channels ( 800 MHz = cable capacitance = 8 MHz cable channel bandwidth)
However, one can use to further increase the number of programs in DVB -S several satellite positions to receive and so increase the program number on the limitation of the bandwidth of a satellite position in the Ku- band. When selecting Cable you would have to achieve the same effect, switch between different cable networks. In practice, therefore, the programs offered by satellite (eg ASTRA ) is many times higher than that of a cable operator.
This calculation does not take into account that it is only currently used frequencies in the Ku-band with the bandwidth of the DVB -S. An extension by other frequency bands is always technically feasible and means, such as extension by the Ka-band ( 17.7 to 21.2 GHz), more than doubling the usable bandwidth of a satellite position, which then corresponds to 250 digital QPSK transponders. In the future, could the Ka-band also provide additional multimedia or program offerings. The use of C-band would theoretically to further increase in supply still possible. However, this is unlikely because of the required large antenna diameter.
In addition to the usual in Europe Ku-band and the older C-band ( 3.4 to 4.2 GHz) will be used in America, Asia and Africa. This is characterized by a much wider margin in the rain. Here mirror diameter are needed from 2 m for the reception of most satellites. This volume offers a few additional, but some very exotic programs.
Significant comparison of DVB -S with DVB- T goes from: DVB- T has a maximum of 0.5 GHz a lower usable bandwidth. The RF bandwidth is significantly lower, and the possible simulcast does not solve the problem that only max. 15 percent of the frequencies can be used. Further, the bandwidth reduces the usually only possible modulation ( 16QAM 64QAM also ) as well as the guard interval. All effects are considered together, that DVB- T allows about five percent of the data rate of DVB -C. Alternative topologies are possible only with great effort.
DVB-S2
DVB -S2 is a further development of the DVB- S standard. By use of improved encoding, modulation and error correction method, the data rate is increased by up to 30 %. In March 2005, ratified the ETSI DVB- S2 standard under number EN 302 307 To convert the DVB -S to DVB- S2 no new signal converter (LNB ) is required, only new set -top box (receiver) or TV cards.
Instead of 4 PSK (QPSK ), DVB -S DVB- S2 uses the optional 8PSK modulation types, 16APSK or 32APSK. The adaptation (ACM ) is optional by feedback of reception quality by reference receiver. Thus, with poor reception, the modulation can be changed to avoid a receive abort.
With the same bit error rate (BER ) requires 8PSK a higher signal -to-noise ratio ( SNR ) of about 4 to 4.5 dB. At the same time (except, for example, the Dish Network in the U.S.) the efficient LDPC error correction code used most in need of about 1.5 dB less than DVB -S. Among other things, therefore, a higher net data rate over DVB -S is achieved. LDPC can also be used with QPSK for the reduction of the required SNR or for higher net data rates, which makes, for example, Sky Germany with its HD programming. Below the need for missing free reception SNR LDPC leads but faster to totalem signal loss while in DVB -S initially, developing more and more artifacts.
The use of better algorithms for image data reduction ( for example, H.264 (MPEG -4 AVC) instead of H.262 (MPEG -2 video ) ) and higher resolution (HDTV ) is not necessarily linked to DVB -S2. Thus, the WDR television broadcasts its HD channels H.264 encoded via DVB -S (on the same transponder as the SD channels). But for newer formats anyway new devices are needed with other demodulators and decoders, change most of the providers on a data- rate- efficient and thus more cost-effective for them modulation methods, such as when a new HDTV channels to be broadcast. For this reason also goes with DVB -S2 often a change of audio codecs in favor of AC-3 associated ( send from the German HD channels, only the public and Servus TV additionally in MPEG -2 audio ), which is the major broadcasters offer in addition, however, already in DVB -S.
There are already several transponders on different satellites that broadcast in DVB -S2 mode.
Due to the selected phase for the DVB -S2 newly added types of modulation can mixing DVB -S and DVB -S2 signaling on a transponder is possible. Thus, a transmitter, for example for older DVB- S receivers offer a transponder, a number of channels in SDTV, a DVB-S2 receiver receiving on the same transponder, but may additionally decode one or two transmitters as superimposed 8PSK modulation in DVB -S2 lie on the 4 PSK signal of the DVB -S ( see also simulcast ).
Criticism of DVB -S
Because digital signals can be encrypted simply, critics fear at a later date at a later introduction of paid offers. Supplement: That's come so ( for example, "SKY " as German provider). The standard that is used today, "HD " and is called " HD Plus", even if the offers are not designed for continuous HD. Behind this lies, as expected, a pay-TV concept, which aims to offer HD content. In part, the special offers, but in many cases also deals in normal resolution (DVB -S1 or SD) are freely available, for a fee, this also in HD (DVB -S2) are offered.
They also see the possibilities of free access and use of the media by using digital methods and thereby possible digital rights management (DRM ) through smart cards, HDCP and the like. considerably restricted.
Analogue switch-off
Compared to the limited bandwidth of divisible analog satellite television allows the digital data compression to spread multiple digital channels in the same frequency range. Therefore, the analogue satellite channels bandwidth reasons and also also for economic reasons on the part of the sender ( in some cases also, the satellite operator ) in April 2012 were shut down.
Distribution via satellite over-IP
To distribute a new alternative, DVB- S and DVB -S2 signals terrestrially, the satellite -over- IP technology (also called satellite IP ) that enables the conversion of the satellite signals into local networks. With the basic configuration possible on IPTV -enabled devices on the network up to four free to air TV channels.