MPEG

MPEG stands for Motion Picture Experts Group, an organization established in 1988 to develop digital compression standards for video and audio material in the multimedia world.

Its work is not exclusively within the field of broadcasting – its remit is much wider than that – and it now works on standards that are used over the Internet.

The MPEG broadcast standard defines the way the audio and video are compressed.

MPEG-2

This is the standard used for a wide variety of applications, including DTH broadcasting by satellite and digital terrestrial TV delivered to the consumer.

It is a truly global standard used in a very wide variety of applications from computers to HDTV, with a range of bit-rates from 2 to 80 Mbps.

The MPEG-2 standard was published in 1995 primarily for digital TV broadcasting, and is the dominant compression standard for DNG.

Profiles and levels

MPEG-2 consists of many different types of service, classified under Profiles and Levels, which allow ‘scaling’ of the video.

The principal standard used for digital video broadcasting is described as main profile/main level. It is far beyond the scope of this book to describe exactly what that means, but broadly the Profile defines the data complexity and the chrominance resolution (sharpness), while the Level defines the overall image resolution and the maximum bit-rate per profile.

4:2:0 and 4:2:2

There are two ways of referring to digital video signals, based on the way they are sampled.

Digital video regarded as ‘full-quality’ is referred to as 4:2:2, while 4:2:0 refers to a digital video signal which has approximately half the number of chrominance samples of 4:2:2.

The 4:2:0 standard is the compression used in DTH services. In the early days of MPEG-2, 4:2:0 encoders were the norm for DSNG, as the encoders were simpler and therefore cheaper, but with the advances in MPEG-2 encoder technology, it is common nowadays to use 4:2:2 encoders as they are now almost as cheap as 4:2:0 encoders (4:2:2 encoders can also operate in 4:2:0 mode).

The coding is either defined as 4:2:0 MP@ML (Main Profile @ Main Level) or 4:2:2 MPEG-2. 4:2:0 and 4:2:2 describe the type of luminance and chrominance sampling and processing.

DVB and MPEG-2

We have the option of multiplexing (mixing) two programme paths (or ‘streams’) in the digital domain by cascading two digital compression encoders together to provide a single digital data stream to the uplink chain.

Each stream is not affected by the other, but they are cleverly mixed together to form a single stream of data. At the receiving end, each programme stream can be selected and separated.

This is defined by the DVB standard, which enabled the start of DTH services via satellite in many parts of the world.

In the DVB standard a key feature is programme security, so that for services delivered to the home, only those who have paid for a service may receive it. As a result the DVB standard allows for conditional access (CA) using a security key.

This method of applying a security key (scrambling) is used on some DSNG feeds where there is a fear that the material may be ‘hijacked’ by the unscrupulous, or where the material contained in the feed has a high degree of exclusivity.

MPEG-2 transmissions are either transmitted as single channel per carrier (SCPC), or multiple channel per carrier (MCPC) feeds – combining several programme channels together.

However, at an individual programme channel level, both techniques use the same method for building a data stream containing the video, audio and timing information. This combination of compressed video and audio is termed a transport stream (TS).

Since many of the MPEG-2 encoders used for DNG are derivatives of those used in programme distribution, the ability to multiplex streams together is either an integrated part of the encoder, or an external multiplexer can be used.

The streams are combined at the uplink, and the downlink de-multiplexes the TS back into separate streams by using the packet identifier (PID) value, which is the channel identifier containing all the navigation information required to identify and reconstruct a programme stream.

PID values are entered into the MPEG decoder to enable reception of the signal, and are the same values as those entered into the MPEG-2 encoder at the origination of the transmission.