Glossary: A


ACS

Access control system

A system/device that ensures broadcast services are only accessible to those entitled to receive them. Usually achieved by coding (scrambling or encrypting) the service to make it unintelligible to unauthorised viewers.



ADSL

Asymmetic digital subscriber line

A system that allows high-speed transmission and reception of digital information over telephone lines.



Advanced modulation

 DVB-S2 is the successor to the very successful DVB-S standard. When combined with Advanced Audio and Video Coding it provides the optimal solution for HDTV broadcasting in the coming years.

DVB-S2 provides a capacity increase of +/- 30 % under the same transmission conditions.

Conversely it can provide a ruggedness increase for the same spectrum efficiency.

The new Variable Coding and Modulation (VCM) scheme can provide different levels of error protection to different service components.

Due to the increased number of HD services that DVB-S2 can convey it increases the gains possible in statistical multiplex environments.

 


All-or-nothing effect

The sudden breakdown or loss of digital signal reception when it contains more errors than the error correction system can cope with. Up to this point there is no degradation in reception quality; a significant advantage over analogue where there is a gradual deterioration.



Analogue

The traditional method of transmitting and recording sound and pictures where the signal varies smoothly and continuously over time.



Antenna

See "satellite dish".



Aspect ratio

The ratio of the width of a TV picture to its height, usually expressed as two numbers. Traditionally, UK television sets and broadcasts have been based on a 4:3 aspect ratio, but this is being superseded by a widescreen and high-definition standard based on a 16:9 format similar to cinema films. Widescreen will eventually become the standard aspect ratio for all TV broadcasting and television sets.



Asymmetric transmission

Transmission where the required bitrate for the forward path and the return path is different (e.g. high forward path and low return path data rate).



Audio coding

HDTV doesn’t just mean great picture, it also means great sound. Dolby Digital can deliver immersive 5.1 channel surround sound to perfectly complement the improved visual experience offered by HDTV.  A Dolby Digital 5.1 broadcast can captivate you in a way which stereo cannot, for films delivering a true cinematic experience, for sport placing you with your team right ensuring you don’t miss the action.  In fact for any content Dolby Digital delivers the experience the audience demand.

Dolby Digital is the format of choice for film with over 40,000 cinema screens equipped and standard for DVD worldwide entertaining millions of people every year.

Dolby Digital is the format to deliver surround sound for films, sport and drama but its flexibility also allows for mono and stereo broadcasts with seamless switching between mono, stereo and surround its ideal for all direct to home broadcasting.  Increasing the flexibility yet further is the ability of Dolby Digital Decoders to take 5.1 sound tracks and create stereo, a process called downmixing, for situations where a 5.1 system is not available.  It’s even possible to downmix right down to mono for a portable TV.

Dolby Digital is also easy to use both for the broadcaster and for the end user in the home.  In the home it’s as simple as making one digital connection from the set top box to a surround system.  Even surround systems available now are easy to set up usually having intuitive colour coding for connections and simple on screen menus to guide you.  Speaker positioning is quite straightforward and usually takes only a few minutes, often smaller satellite speakers are used which are even easier to place.

Set-top boxes often also feature a stereo analogue output feature for cases when a surround system is not in use.  These outputs can deliver a downmix of the Dolby Digital stream avoiding the need for the broadcaster to transmit an addition stereo track using valuable bandwidth.

Discrete means that the sound information contained in each of the six available channels is distinct and independent from the others. These six channels are described as a "5.1-channel" system, because there are five full-bandwidth channels (left, centre and right at the front together with left and right at the Back) with 3 Hz to 20 kHz frequency range plus one "low frequency effects" (LFE) subwoofer channel devoted to frequencies from 3 to 120 Hz.

 


Azimuth

Angle between true north and a particular satellite. Can be measured with a compass.