DVB-S2

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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 in the U.S., and Bell ExpressVu in Canada. The transport stream delivered by DVB-S is mandated as MPEG-2.

[edit] DVB-S2

One of the first DVB-S2 tuner cards.

'Digital Video Broadcasting - Satellite - Second Generation (DVB-S2) is an improved and updated specification to replace the DVB-S standard, ratified by ETSI in March 2005. Today the main use for this new standard is the distribution of HDTV, while the original standard was mainly applied to SDTV services. The development of DVB-S2 coincided with the introduction of HDTV and H.264 (MPEG-4) video codecs.

The system allows transmission of one ore more MPEG2 audio/video streams , using QPSK or MAPSK modulation with concatenating encoding.

DVB-S2 is based on the DVS-S standard, used for satellite broadcasting, and the DVB-DSNG standard, used by mobile units for sending external footage back to the television station. Two new key features from DVS-S are :

- Changing encoding paramethers in real time (VCM, Variable Coding and Modulation), - ACM, Adaptive Coding and Modulation.

The authors claim that the DVB-S2 performance gain over DVB-S is around 30%. When the contribution of improvements in the video compression are added, a (MPEG-4) HDTV service can now be delivered in the same capacity that supported an early DVB-S-MPEG-2 SDTV service, only a decade before.

[edit] 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/H.264).
MPEG-2 TS (MPEG-2 Transport Stream) are supported using a compatibility mode, whereas the native stream format for DV-BS2 is the so-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).
Backwards compatibility to DVB-S (intended for end users) and DVB-DSNG (DVB-Satellite News Gathering)(used for backhauls and ENG)
adaptive coding and modulation to optimize the use of satellite transponders
four modulation modes:
- QPSK, as in DVB-S
- 8PSK, used in non-linear transponders near to saturation
- 16APSK
- 32APSK, for professional, semi-linear applications
improved rolloff: α = 0.20 and α = 0.25 in addition to the DVB-S α = 0.35
forward error correction
inexpensive to implement
For forward error correction, DVB-S2 uses an FEC system based on concatenation of the Bose-Chaudhuri-Hochquenghem code with a low-density parity check. A single FEC datagram may be 64800 bits(normal) or 16800 bits (short). If VCM or ACM is uded, 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 e 9/10.
Interleaving uses 8,16 or 32APSK modulation.

Performance can be configured to be within 0.7 dB of the Shannon limit.

[edit] Uses

DVS-S2 is used for:

Broadcasting television programs (BS,Broadcast Services) in SDTV or HDTV. Optionally, this transmission may be backwards compatible with DVS-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(coper/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.