DVB-S2 explained
DVB-S is the original Digital Video Broadcasting forward error coding and modulation standard for satellite television and dates from 1995. It is used via satellites serving every continent of the world. DVB-S is used in both MCPC and SCPC modes for broadcast network feeds, as well as for direct broadcast satellite services like Sky TV (UK) via Astra in Europe, Dish Network and Globecast in the U.S., Star Choice and Bell ExpressVu in Canada. The transport stream delivered by DVB-S is mandated as MPEG-2.
DVB-S2
Digital Video Broadcasting - Satellite - Second Generation (DVB-S2) is an enhanced specification to replace the DVB-S standard, developed in 2003 and ratified by ETSI (EN 302307) in March 2005. DVB-S2 will probably be used for all future new European digital satellite multiplexes, and satellite receivers will be equipped to decode both DVB-S and DVB-S2.Today the main use for this new standard is the distribution of HDTV, while the original standard was used mainly for SDTV services. The development of DVB-S2 coincided with the introduction of HDTV and H.264 (MPEG-4 AVC) video codecs.
The system allows transmission of one or more MPEG-2 audio/video streams , using QPSK or 8PSK or MAPSK (M-ary amplitude and phase shift keying) modulation with concatenated encoding.
DVB-S2 is based on the DVB-S standard which is used for satellite broadcasting, and the DVB-DSNG standard, which is used by mobile units for sending external footage back to television stations.
Two new key features which were added to DVB-S are :
- Changing encoding parameters in real time (VCM, Variable Coding and Modulation)
- ACM (Adaptive Coding and Modulation) which optimizes the transmission parameters for various users.
The authors claim that the DVB-S2 performance gain over DVB-S is around 30%. When the contribution of improvements in video compression is added, an (MPEG-4-AVC) HDTV service can now be delivered in the same capacity that supported an early DVB-S-MPEG-2 SDTV service, only a decade before.
Main features
- Source may be one or more MPEG-2 TS (MPEG-2 Transport Stream). Packet streams other than MPEG-2 TS are also valid (MPEG4-AVC/H.264).
- MPEG-2 TS are supported using a compatibility mode, whereas the native stream format for DVB-S2 is called Generic Stream (GS).
- Adaptative mode: this block is heavily dependent on the application that generates the data. This means
- synchronizing data using ACM and cancellation of null packets from the Transport Stream;
- CRC-8 encoding; used by a DVB-S2 for error correction;
- merging full stream and subdivisions in blocks for error correction encoding (DF, Data Fields).
- Backward compatibility to DVB-S, intended for end users, and DVB-DSNG (DVB-Digital Satellite News Gathering), used for backhauls and electronic news gathering
- Adaptive coding and modulation to optimize the use of satellite transponders
- Four modulation modes:
- QPSK and 8PSK are proposed for broadcast applications and they can be used in non-linear transponders driven near to saturation
- 16APSK and 32APSK are used mainly for professional, semi-linear applications, they can be also used for broadcasting but they require a higher level of available C/N and an adoption of advanced pre-distortion methods in the uplink station in order to minimize the effect of transponder linearity.
- Improved rolloff: α = 0.20 and α = 0.25 in addition to the roll-off of DVB-S α = 0.35
- Inexpensive to implement
- For forward error correction (FEC), DVB-S2 uses a system based on the concatenation of the BCH code with a low-density parity check. A single FEC datagram may be 64800 bits(normal) or 16200 bits (short). If VCM or ACM is used, any datagram may have a variable length, and the broadcast can be a combination of normal and short datagrams. There are 10 encoding values: 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 8/9 and 9/10 which depend on the modulation and the requirements that the system has.
- Encoding values 8/9 and 9/10 behave poorly under marginal link conditions (where the signal level is below the noise level). However, with targeted spot Ku or Ka band downlinks these coding schemes may be recommended to prevent out-of-region viewing for copyright or cultural reasons.
- Interleaving uses 8,16 or 32APSK modulation.
Performance can be configured to be within 0.7 dB of the Shannon limit.
Uses
DVB-S2 is used for:
- Broadcasting television programs (BS, Broadcast Services) in SDTV or HDTV. Optionally, this transmission may be backwards compatible with DVB-S (BC-BS,Backwards-Compatible BS), but does not benefit from the 30% extra bandwidth.
- Interactive Services with access to the Internet. Data generated by the user may be sent by cable(copper/Fiber), DSL or Satellite uplink(DVB-RCS).
- Professional applications, where data must be multiplexed in real time and then broadcast in the VHF/UHF band. These transmissions are not intended for the average viewer.
- Data content distribution and Internet trunking.
The DVB-S to DVB-S2 upgrade process
The conversion process from DVB-S to DVB-S2 is expected to take about 15 years -- probably in sync with the coming of HDTV. DVB-S has proven to be a well designed and flexible standard. This thus makes the upgrade process take longer for those delivering programming.
DirecTV in the U.S. is now switching over to offer their local and some national HDTV channels. They use the H.264/MPEG-4 AVC codec for this process.
Direct to Home broadcasters using DVB-S2
These broadcasters have used DVB-S2SNG for uplink and downlink to their centralized transmission facilities, but may not have instituted DVB-S2 transmissions.
The typical lifetime of a Direct to Home Set Top Box is about 5 years, with some lasting 7 years or more. Therefore it is to be expected that the conversion process to DVB-S2 should take about 10 years as a baseline estimate.
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