DVB Transmission

DVB builds upon MPEG-2 and uses MPEG-2 Transmission. It also defines additional private sections and providing a definition of the physical medium (modulation, coding, etc) needed to carry the MPEG-2 Transport Streams.

Typical Streams carried in a DVB Terrestrial Transport Multiplex

Each MPEG-2 MPTS multiplex carries a number of streams which in combination deliver the required services. Here is a sample break-down of the various MPEG-2 streams being used to provide a terrestrial 24 Mbps TV multiplex:

bit rate (kbps)
 Digital Teletext
 Total per Mux

Sample per-multiplex overheads

 bit rate (kbps)
 TV Video *
 Stereo Audio
Conditional Access
 Total Programme

Bit rate per programme

(* Video at 4-6 Mbps depending on content, Conditional Access may not be required)

This allows a standard 8 MHz channel to carry 5 TV channels or 4 higher quality channels without conditional access - or 3 high quality channels with conditional access. The remaining bandwidth may be used for other services such as Electronic Programme Guides (EPGs), Audio Descriptions for the visually impaired (~ 70 kbps), Signing for the deaf (~ 400 kbps using a separate window), house pages, digital data, software down-loads, etc.

DVB Bearer Networks

The DVB standards allows a DVB signal to be carried over a range of bearer networks. Various standards have evolved which define transmission over particular types of link:

Digital Satellite TV (DVB-S)

Satellite transmission has lead the way in delivering digital TV to viewers. A typical satellite channel has 36 MHz bandwidth, which may support transmission at up to 35-40 Mbps (assuming delivery to a 0.5m receiving antenna) using Quadrature Phase Shift Keying (QPSK) modulation.

The video, audio, control data and user data are all formed into fixed sized MPEG-2 transport packets. The MPEG TS packets are grouped into 8 packet frames (1503 B). The frames do not have any additional control information, but to enable the receiver to find the start of each frame, the TS-header byte is inverted (0xB8) in the first TS packet of each frame. The frames are then passed through a convolutional (organ-pipe) interleaver to ensure the data follows an approximately random pattern, assuring frequency dispersion of the modulated signal. At the start of each frame, the scrambler is re-initialised.

16 bytes of Reed Solomon (RS) coding are added to each 188 byte transport packet to provide Forward Error Correction (FEC) using a RS(204,188,8) code. For the satellite transmission, the resultant bit stream is then interleaved and convolutional coding is applied. The level of coding ranges from 1/2 to 7/8 depending on the intended application and available bandwidth. The digital bit stream is then modulated using QPSK modulation.

The complete coding process may be summarised by:

See also: Broadband Multimedia Satellite Systems

UK Digital Terrestrial TV Network (DVB-T)

Legislation passed in 1996, has opened the way to allow new companies to enter the terrestrial TV market, while protecting the interest of the main players. This lead to the granting of licences to various companies during 1997, and the planned roll-out of the first direct-to-home digital terrestrial services during November 1998.

In the UK, a standard TV channel is 8 MHz wide, accommodating the TV signal in Phase Alternate Line (PAL) format and the sound subcarrier. The same 8 MHz of bandwidth may be used to provide a 24 Mbps digital transmission path using Coded Orthogonal Frequency Division Multiplexing (COFDM) modulation. This may support up to 6 digital TV channels.

The information is transmitted in the following manner:

Some countries have chosen to use introduce a digital TV service which is based on more robust modulation, allowing mobile reception of the signal, or to pilot high definition TV - in both cases the bandwidth (number of channels) has been traded.

The UK will initially be providing 6 terrestrial TV multiplexes. These will be received in various parts of the UK. Some areas will receive all the multiplexes, while others will receive only one. Many parts of the UK will not be able initially to receive digital terrestrial TV, although these areas are generally sparsely populated. Satellite TV will also carry the terrestrial programmes.

 Multiplex  Coverage (% homes)  Programmes
 1. BBC  90%  BBC1, BBC2, Choice,News24
 2.  90%  ITV, CH4, ITV2
 3.  90%  S4C, C5, ...
 4. BDB  88% Subscription-based + Home shopping
 5. BDB 77% Subscription-based + Home shopping
 6. BDB 69% Subscription-based + Home shopping

See also :