Digital Television

Section Headings

• The Fundamental Basis
• SDTV
• HDTV

The Fundamental Basis

The majority of broadcast TV applications, known as DVB (Digital Video Broadcasting), use MPEG-2 at lower quality levels or "bit rates". These bit rates are expressed in megabits per second or Mb/sec - Note that the b is lower case - MB/sec would mean megaBYTES per second which would be a wholely different measurment. DVDs typically use bitrates of between 2 Mb/sec and 10 Mb/sec (the maximum allowed). Some digital TV applications fall below even 2 Mb/sec, unfortunately. Further the process of encoding video into MPEG-2 for broadcast/DVD is a technically complex proceedure. The majority of DVDs are encoded "non-real-time", in other words they are analysed by a computer frame by frame over the course of several hours before the final result is available. This results in a much better quality picture at a lower bitrate than could be achieved if the MPEG-2 encoding has been done "in real time". Unfortunately most DVB systems do not use any material already coded into MPEG-2 (from the DVD master for instance) - most are coded "on the fly" in real time. This again makes for inferior results.

DVB systems also use the slightly inferior 4:2:0 method of colour encoding (in common with DVD), although there is a variant of MPEG-2 for broadcast use that uses the industry standard 4:2:2 method of coding. This encoding method reduces the amount of detail in the colour information part of the picture. Many of the worlds DVB systems also include some form of scrambling technology to allow for subscription and pay-per-view broadcasting, and these again differ between various countries, territories and suppliers. Others also include facilities for Interactive Television which tries them to running a specific "environment" on which the computer code for these services runs.

SDTV

The term SDTV (Standard Definition Television) refers to the use of a digital broadcast to carry TV pictures of the same physical parameters as today's existing systems. These are interlaced signals (ie each frame is divided into two fields) and the only new feature they add over existing analogue TV distribution from a picture viewpoint is automatic support for both 4:3 and 16:9 aspect ratio operation.

HDTV

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