V

Video coding

So far all HDTV television systems around the world, including the DVB system, are based on the MPEG-2 video compression scheme and use as the basis an MPEG-2 Systems defined Transport Layer. This leads to a very high commonality of all HDTV systems in use around the world.

DVB is now underway to include more advanced audio/video compression tools such as e.g. H.264 (MPEG-4 AVC) and later maybe SMPTE VC-1 into its standards. DVB will provide the necessary tools for efficiently broadcasting HDTV in the coming years.

Even if new coding tools are becoming available, Set-Top-Boxes for the new HD standards will be backwards compatible with MPEG-2 MP@HL transmissions and current standard definition broadcasts in MPEG-2.

 

Video compression

The data reduction and compression of analogue TV signals into a digital stream. It is used by ASTRA to broadcast several channels through a single transponder.

 

Video formats

All around the world two main formats are in use for High Definition Television broadcasting. These two formats are abbreviated as 720P (where P stands for progressive) and 1080i (where i stands for interlaced).

1080i is a format defined in ITU-R Recommendation BT.709-5. It consists of 1080 active lines per picture and 1920 samples per active line.
The Aspect Ratio is 16:9 and the format uses square pixels.

720P is a progressive format defined by the SMPTE television standard 296M-2001. This format provides 720 active lines per frame and 1280 samples per line. Again the Picture Aspect Ratio is 16:9 and the format uses square pixels.

SES ASTRA and its industry partners have agreed to allow both of these formats to be used for HDTV broadcasting in Europe.

The table below lists the scan formats that European HD displays need to support in order to qualify for the HDTV logo.

 

Interlaced versus Progressive scanning
Traditional television has used so-called ‘interlaced’ scanning in order to conserve bandwidth. In interlaced scanning each frame is displayed on the screen in two passes. During the first pass all the odd numbered lines are drawn in a 1/50th of a second and during the second pass the even numbered lines are drawn in another 1/50th of a second. A complete picture is drawn
25 times per second.

On the other hand in a progressively scanned system the entire frame of pixels is conveyed in every scan sequence (every 1/50th of a second).

There have been many discussions on the respective merits of one or the other of these formats. There seems to be agreement on the following points:

  • A progressive format is easier to compress and leads to lower bitrates
  • Motion portrayal is better with 720P50, interlaced scanning can introduce image artefacts during rapid motion when shown on native progressively scanned displays (LCD, Plasma,..)
  • 720P50 provides overall fewer artefacts than 1080i

Production in 1080i is currently easier due to more available equipment. Overall 1080i is in wider use worldwide than 720P.

 

VSAT

Very small aperture satellite terminal

A small two-way satellite ground station with dish antenna normally under 2.4 metres in size. They access satellites in geosynchronous orbit and relay signals from small remote ground stations to other terminals or master earth station or hubs. They are also used for mobile satellite communications systems.