|Digital Television can offer big advantages over existing terrestrial and satellite services. This page offers an intruduction to digital TV, and explains the features common to most digital TV services.|
|DIGITAL TELEVISION INTRODUCTION|
|DIGITAL TELEVISION TECHNOLOGY|
|DIGITAL TELEVISION FEATURES|
|UK DIGITAL TELEVISION SERVICES|
|HOMEPAGE / SITE MAP|
IMPROVED PICTURE AND SOUND?
may be expecting an improvement in picture and sound when you hook up
your new digital TV receiver. Indeed, the adverts would lead you to
believe that digital television improves existing analogue services
in the same way that DVD video improves on VHS video or laserdisc. Sadly,
this is not always the case. What you will be getting is a clear picture,
with great colours and contrast, unaffected by interference, and CD
quality digital sound the same as a perfect analogue signal - at least,
most of the time. In areas where reception of analogue television is
a problem, due to location or barriers that reduce the signal quality,
digital television will be just what you've been waiting for. Digital
signals, if they can be received at all in a given area, will arrive
unaffected by the hazards that play with an analogue signal. But if
you have a good reception with existing terrestrial television, the
only real advantages of going digital are the improved choice of programmes,
interactive services and possibly for taking advantage of widescreen
programmes, if you have a widescreen telly, that is.
However, even the 'perfect' picture can be effected by the very technology that makes digital television so good. Read on...
Digital data is recorded as individual 'bits', with each bit representing either an 'on' or 'off' instruction. The more bits sent during any given time period means more information is received which results in a better quality picture. Every item of digital hardware transfers digital bits at a certain 'bitrate'. This is where digital television experiences its first problem. Under normal conditions, the bitrate of digital TV is enough to ensure that all the required information across every channel gets through clearly. But when the bitrate is too slow to get the needed information to the receiver, like when several channels are showing fast, action sequences, the amount of information available to each channel is reduced. The effect of this is the screen resolution decreases and the picture is momentarily represented by small blocks, easily seen, rather than the usual high resolution pixel dots. This effect is known as 'blocking' and means that the information being received is not enough to complete the whole picture. Sadly, there is nothing that we, the end user, can do to solve this occasional problem. Only by increasing the speed of the hardware and increasing the available bandwidth at the transmission source can blocking be eliminated for good. Generally, though, blocking has been almost completely eliminated by the continual improvement of the source hardware at the transmission station.
The MPEG-2 system compresses the analogue signal into chunks of digital data that is transmitted and received by a suitable decoder. Only the changes from one picture frame to the next are sent, thus reducing the amount of data that needs to be sent in order to reconstruct the original picture. The picture you see is constantly being updated, or 'refreshed', as more data is received. The picture is not completely replaced with a new one - only the changes from one frame to the next are seen. For this picture to appear to be a continuous moving image, the hardware you are using needs a sufficiently high 'refresh rate' to update the picture fast enough to avoid jerky movements. For digital television, the refresh rate relies on the bitrate of the signal being received - more data means a higher refresh rate and a smooth scrolling image.
DIGITAL TV SOUND
The standard sound format for European digital television comes in the form of MPEG-2 Stereo. This system can carry two channels of CD quality digital audio, and can even carry additional channels interlaced with the stereo audio enabling the broadcast of Dolby Surround soundtracks for playback on Dolby Pro-Logic hardware. This may sound impressive, and, if you believe everything you read in the press releases, you would be forgiven for thinking that sound quality will be much improved with the digital system. Terrestrial television broadcasts in the UK currently use the NICAM Stereo Audio system for digital stereo sound with analogue television programmes, and have done for the last few years. NICAM has been providing CD quality stereo and Dolby Surround encoded television without any fuss - practically all stereo televisions and video recorders have a built-in NICAM decoder and has become so common that we hardly ever notice it. The MPEG-2 Stereo format is, therefore, a different method of doing what NICAM has been doing for a long time, and you're unlikely to notice any difference.
DIGITAL SURROUND SOUND
As with NICAM, the MPEG-2 Stereo sound can be encoded with the 4-channel Dolby Surround audio suitable for any home Dolby Pro-Logic decoder. Unfortunately, that's as good as it gets in the UK as far as surround sound with digital TV is concerned. The technology for encoding and transmitting digital surround sound, such as Dolby Digital or even MPEG-2 Multichannel, has not been included in the first generation hardware and software, and is currently incompatible with the digital television systems in use today. The official explanation as to why Dolby Digital has been excluded is because the terms of the UK Digital TV standard prevents the use of a proprietary sound system above the MPEG-2 technology used for both picture and sound data compression. This is a big mistake as far as I'm concerned, and a missed opportunity on the part of the digital TV providers. There are several countries around the world, including the entire USA, that have adopted Dolby Digital as the sound format for digital television, using true digital 5.1 channel audio. The latest news is that Dolby Digital has been chosen for use in Australia, and even some satellite channels available in Europe (but not SKY's ASTRA satellites). Following this decision, it has been hinted that the terms of the UK agreement may be revised to allow the adoption of Dolby Digital. The BBC have stated that existing technology could be modified to include multi-channel surround sound, but there are no plans to introduce it at the moment. We live in hope...
are quite a few digital channels transmitting widescreen programmes
and movies for viewers with a widescreen television. The widescreen
picture is transmitted beside the normal 4:3 programme. The decoder
will have an option in the set-up menu to select the type of television
it is connected to - either 4:3 or 16:9 widescreen. When set to widescreen,
the decoder automatically detects the presence of a widescreen broadcast
and activates the television's expand feature to display the full screen
size (via SCART cable) without you having to press any buttons. When
normal 4:3 programmes are received, the decoder returns to normal viewing.
a normal 4:3 television, you will receive a cut-down 4:3 picture from
widescreen programmes, or a picture with the black borders at the top
and bottom of the screen when viewing a widescreen broadcast in "Letterbox"
format. True widescreen broadcasts are transmitted as an "anamorphic"
picture - where the picture is squeezed horizontally into a 4:3 frame
and only appears in its original, un-squashed form when the image is
expanded by the widescreen TV.
Although the availability of widescreen transmissions is currently fairly limited, the general trend is towards the introduction of widescreen as the standard picture size for digital TV. The channels broadcasting in widescreen today include new programmes on BBC 1 and BBC 2, BBC Choice, BBC News 24, Channel 4 and Channel 5 (both limited to new programmes), Film Four movie channel, and the new SKY Premier Widescreen channel showing nothing but movies in their original cinema screen size every night.
The digital technology that allows your receiver to make sense of a digital signal can also encode information that can be sent away to another receiver. All digital TV receivers with interactive service capabilities have a telephone modem built-in that allows information to be sent along a standard phone line from your home back to the service provider. Interactive services are now available that include e-mail, television banking, shopping and games, all on your TV screen.
As new software and services become available, the digital system enables the receiver's software to be upgraded by sending the new programs alongside the normal television transmissions. The receivers will install and use the new software without you even knowing it. This is how most set top boxed and receivers will be upgraded. However, it is yet to be seen whether internal digital decoders included with some integrated televisions will allow upgrades without a physical modification.
EPG - Electronic Programme Guide
television services can transmit a detailed programme guide along with
the normal services, much better than using existing Teletext services.
This is particularly useful with the SKY Digital satellite service with
over 200 channels on offer. The feature is called an Electronic Programme
Guide, or EPG, and makes finding programmes and channel-hopping much
easier. Take the SKY Digital EPG, for example, called SKYGuide. You
have four direction buttons and a 'select' key. Pressing the TV Guide
button brings up the listings menu with different categories of programmes
such as news, movies, children, etc, as shown in the diagram above-right.
Scroll up and down to the menu you want and press 'select' - all the
programmes under the selected category are displayed. Scroll up and
down to highlight different channels, and scroll left and right to change
the time of day to see what's on later. You can even advance the time
to show what is on up to 7 days ahead. If you like the look of a programme,
highlight it and hit the " i " (information) key and a description of
the programme appears. If you want to watch a highlighted channel, press
'select' and the receiver tunes in.
whole thing operates like a massive spreadsheet as shown in the screen
shot above, with channels listed down the side and the time of day listed
along the top. You can move the highlighted area anywhere on the listing
using the direction keys on the remote control.
can even channel-hop to see what is on other channels without leaving
the programme you are currently watching. Pressing the direction keys
on the remote brings up a 'scan' box (shown on the right) at the bottom
of the screen which details the current channel, time, date, current
programme name and the name of the programme that is on next, including
the start time. Pressing the direction keys scrolls through the channels
and times in the same way as using the SKYGuide main menu, except that
all the information you need is displayed in the box. If you find another
channel that you would rather watch, press 'select' and the receiver
tunes to that channel.
The terrestrial digital service ONdigital can also remind you when programmes you want to watch are about to start. Scan through the channels using the EPG and highlight the programmes you want to watch. Five minutes before the selected programme starts, a reminder will appear on the screen, whatever channel you are currently watching.