www.uncletony.co.uk

TV Stuff

uncletony created this page to give a basic introduction on how Television works and
some information specifically about the UK analogue and digital systems.

Selected TV-related links can be found at the bottom of the page.


How does TV work? An introduction to Analogue

Analogue raster scan
The raster scan

 


Composite Video Signal
The composite video signal

 

In the analogue television system the TV camera scans a scene in a series of horizontal lines, starting at the top left and ending at the bottom right. The output from the camera is an electrical signal that contains brightness (luminance) and colour (chrominance) information relating to the scene. Usually these two signals are combined together to form the composite video signal.

In the TV set an electronic scan accurately follows the scanning done by the camera, and at the same time it varyies the brightness and colour on the screen. The picture image is built up line-by-line by this scanning which is kept in step (synchronized) with the camera by the sync pulses. At the end of each line the sync pulse makes the scan fly back ready to start the next line of the picture. When the picture is complete a special group of sync pulses are transmitted to send the scan to the top of the screen ready to start the next picture.

The colour burst is an indicator that the video signal is in colour.

In the European TV system there are 625 lines per picture, and 25 full pictures per second.

 

How does TV work? An introduction to Digital


digital TV - pixel scanning
Digital screen 720 x 576 pixels


1st TV image 2nd TV image
Only the difference gets transmitted

In the digital television system the start- (the camera) and end-points (the TV screen) are the same as the analogue system, but the method of transmitting the information between the two is radically different.

The MPEG-2 system converts the analogue camera signal to a stream of digital 1's and 0's.

Each scan line is divided into 720 picture elements (pixels) on 576 lines (486 in USA). Each pixel is electronically sampled and the luminance and chrominance levels together with the screen co-ordinates are held for a short time in digital memory and converted to a digital number. The numbers are read out in a set sequence and form a data stream.
At the receiving end the data relating to screen position and picture luminance/chrominance are separated. The positional data is used to place the luminance/chrominance data into a particular location in memory. The picture data is then read from memory, converted back to analogue form and displayed on the screen.

Digital television is very hungry for spectrum space. In the analogue TV system 25 complete pictures are sent every second (30 in USA). Much of the visual content between one picture and the one preceeding it is the same. Some pictures may be identical, some almost identical. Digital television saves spectrum by identifying redundant information between pictures, and only transmitting the difference data.

A digital televison receiver has a huge amount of built-in memory, capable of storing a picture. The stored picture gets displayed on the TV screen. The next data stream transmitted from the studio only contains the difference between the new picture and the one currently in memory. The difference between successive pictures updates the TV's memory, and the new picture is then displayed.

This is one of three methods of redundancy, called temporal redundancy. The others are spatial (or intraframe compression) and statistical (which are beyond the scope of this introductory article).

Digital data is much easier to electronically process and distribute - many visual effects seen on TV could not have been created if the signal was in analogue form.

 

Analogue broadcasting:
There are five main broadcast networks in UK:
BBC 1, BBC 2, ITV , Channel 4, and 'five tv'. They transmit from a network of main stations operating in the UHF Bands IV and V. Each main transmitter has many low power relay stations associated with it to provide 'fill-in' coverage in areas of poor reception.

 

Brief Technical History
UK high-definition television broadcasts started by the BBC in 1936 and continued until the beginning of WWII in 1939. Transmissions resumed in 1947 on channels in VHF Band I. The system used was 405 lines, positive amplitude modulation with AM sound. In the mid-1950's a second (commercial) network (called ITV) started transmitting using channels allocated in VHF Band III.

To accomodate the planned expansion of TV broadcasting a decision was made in 1962 (the Pilkington Report) to move all TV broadcasting to UHF Bands IV and V and for the eventual closure of VHF Bands I and III. The new UHF network was engineered to support four nation-wide broadcasters using 8MHz wide channels numbered from 21 to 69 (470 - 862MHz). Transmitters were co-located at single sites and the channels were grouped together in geographical areas. Provision was made for the UHF output of the new-on-the-market video recorder (assigned to channel 36).

 

 

Power distribution of an analogue TV signal

Representation of power in the 8MHz-wide analogue TV signal

 

Transmissions were to be 625 lines, negative amplitude modulation, FM sound and PAL colour system. This spec is referred to as CCIR System-I.

 

Digital terrestrial broadcasting (DTT or DVB-T):
Within Europe the Digital TV systems are standardized to the following straightforward codes:
DVB-S - Satellite
DVB-C - Cable
DVB-T - Terrestrial

In mid-1990's the UK government invited applications to create a land-based digital television network. The long-term plan is to move all UK television to a limited number of UHF channels carrying digital signals by 2010 (although this date does seem to vary). The intention is to release most of the UHF spectrum within Bands IV and V for other (probably commercial) services.

 

DTT is broadcast to viewers using a group of 6 television channels known as 'Multiplexes'. Each Multiplex provides MPEG Transport Streams of around 24.1 Mbit/s data capacity. These Transport Streams carry the data of a 'bouquet' of TV, audio and data services. It is worthy to note that a Multiplex occupies the same 8Mhz of radio frequency spectrum as a single analogue television channel. Go here For a more technical description.

'On-Digital', a consortium of some ITV companies, won the bid and created a digital network of free-to-view and subscription channels which started in late-1998. Also included in the channel line-up were the five main broadcasters. However the overwhelming competition of digital satellite, poor advertising revenue, poor picture quality, small customer base and easily 'hacked' subscription cards spelt financial doom for the consortium. A last-ditch attempt to re-brand as ITV-Digital couldn't prevent the closure of the service in May 2002.

DTT was re-started by Freeview in late-2002 (a joint venture between BBC, Sky Television and Crown-Castle Communications). Changes were made in the technical specification and the line-up of TV channels. Less compression is used (see here) - the result is fewer channels but with much improved picture quality.

The re-launch did not include subscription channels, but in early 2004 'Top-Up TV' announced a bouquet of subscription channels. Top-Up TV service began on March 31, 2004, with only those subscribers with old the old type On-Digital (ITV Digital) receivers being able to receive the programming. In late-2004 a few receivers with a built-in access module and viewing card slot began appearing in the stores.

Freeview coverage in UK is poor. It is important to check that your area is served by Freeview before buying a DVB-T receiver.
The Freeview site has a useful checker based on your Postcode. An alternative website by Wolfbane Cybernetic offers a more accurace analysis and a lot of other interesting technical stuff).

For up-to-date information visit the Freeview site.

Link to Freeview web-site

 

 

COFDM waveform

Orthogonal Frequency Division Multiplexing (COFDM)


Digital channel spectrum

Representation of power in the 8MHz-wide digital TV signal

 

Link to TopUP TV website

 

For up-to-date information on digital terrestrial subscription TV visit the TopUp TV website

 

Aspect Ratio - Regular vs. Widescreen:
The ''Aspect Ratio' of a tv picture is the relationship between the width and the height. The origins of this are steeped in history dating as far back as the invention by Edison of silent movies.

The 'normal' screen has an aspect ratio of 4:3 (for example 20 inches wide by 15 inches high). This is sometimes quoted as 1.33 (ie 4 divided by 3). New 'widescreen' (or letterbox) format picture screens have a ratio of 16:9 (or 1.78).
(NB The term letterbox came about because the aspect ratio is similar to that of an American size 10 envelope!)

 

Although wider than a 4:3 picture the 'widescreen' picture area is actually smaller overall.

  • In this example consider a picture screen 40cm wide:
      • In 4:3 format the picture will be 30cm high.
      • In 16:9 format the picture will only be 22.5cm high (this is in effect a 25% reduction in height).

Unfortunately it is only programme material originating in a TV studio that will be broadcast in the standardized aspect ratios of either 1.33 or 1.78.
DVD's, movies and music video's use many alternative ratios, for instance 1.85, 2.00, 2.35, 2.40, 2.50.

A CinemaScope™ movie is shown in 2.35. It is inevitable that black bands will appear above and below the picture, even when a 16:9 (1.78) tv is used (see the diagram opposite).

  • Other film/movie formats include:
      • VistaVision (Paramount) = 1.66
      • MGM / Disney = 1.75
      • Universal / Columbia = 1.85
      • CinemaScope (20th Century Fox) = 2.35
      • Cinerama = 3.0

 

16:9 and 4:3 aspect ratios
This picture shows the difference in picture height between 4:3 and 16:9 aspect ratios.


Black-banding on any TV screen
Illustrating the position and size of black bands when a wide-screen movie is viewed.

 

High Definition TV (HDTV)
The standard MPEG-2 digital TV system has 720 pixels on each line and 576 viewable lines per picture (486 in USA). This is often referred to as an SDTV (Standard Definition TV) picture.

One of the several HDTV standards specifies increasing the number of pixels per line to 1280 and number of lines to 720. The picture is made up by scanning the odd numbered lines first, then interleaving the odd lines with the even numbered lines. This is known as 'interlaced' scanning and this format is described as 720i. As a complete picture takes two scans sometimes flicker becomes a problem, particularly on very large screen sizes.
Interlaced scanning is currently used in all regular SDTV broadcasts.

A further increase in the number of pixels per line to 1920 and a corresponding increase in the number of lines to 1080 (giving a wide-screen aspect ratio of 1.78 = 16:9) is the most common current broadcast format. It is known as 1080i. An alternative broadcast standard, at the expense of an increase in bandwidth, is the 1080p system. With this system a complete picture is made up by scanning each line progressivly (odd, even, odd, etc). This creates a much more stable and flicker free image.
Computer screens use this method of picture display.

 

Comparision of screen displays
Comparision between standard Digital TV and HDTV

 

 

Milestones in British TV history

  • World's first high-definition (405 line) TV broadcasts start in UK in 1936.

  • TV transmisssons cease in 1939 for duration of War, then resume in 1947.

  • April 1964 a new BBC channel (BBC 2) opened, broadcasting on UHF only. When the UHF network opened the BBC and commercial stations eventually transmitted simultaneously on VHF and UHF.

  • Mid-1967 regular PAL colour transmissions started (BBC 2 only initially, then on all UHF stations).

  • In 1982 a fourth UHF TV network started (called Channel Four).

  • In 1985 the 405-line VHF transmissions ended.

  • 1989 Direct-to-home satellite services introduced BSB and Sky (later to merge as BSkyB)

  • 1989 - NICAM ("Near Instantaneous Companded Audio Multiplex"), a digital system for broadcasting stereo sound, was added to regular UHF transmissions.

  • March 1997 roll-out of 'Channel 5' started.
    Due to engineering constraints it proved impossible for 'five tv' (the re-branded name for Channel 5) to be transmitted in all areas of the UK.
    At best the service coverage is only some 80% of the UK population.

  • End of 1998 - Terrestrial Digital TV starts.

 


uncletony's TV Links

For a listing of main analogue and digital TV transmitters and their relay stations follow this link to the ITC website

For up-to-date status of DTT transmitters go here

Visit the Satcure satellite TV site for the largest source of information about Sky Digital.

Visit The-Cool-Book-Shop.com for e-Books about satellite TV and terrestrial digital TV.

For independent news on Freeview and the DTT scene go here

For additional information on worldwide television standards go here

For a detailed timeline on UK TV history go here

For a (slightly out-of-date) detailed explanation of DTT go here

For a very technical discussion of terrestrial and satellite DVB go here

For technical, entertainment news and forums on all digital TV and Radio go here

New Digital Satellite Catalogue is available here

For uncletony's Satellite TV page go here

 


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