Analogue and digital

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The world is going digital. Some physicists even believe that the Universe is a huge hologram that becomes more complex as it expands because there are more possible on-off states for matter. Who knows? What is certainly true is that television and radio are becoming all-digital. The benefits are more channels and higher quality. Analogue television is progressively being switched off, but we may well use analogue to connect our equipment inside our homes for some time. The choice is a little bewildering. It's just a shame that most programmes are such rubbish!

To watch digital television the decision is simplest. You have to choose between cable such as Virgin Media, terrestrial freeview where you need an aerial, or freesat where you need a dish. The last two are covered here.

Connecting together your sound and picture equipment is more complex. There are several analogue connections and one digital. To view your new high quality pictures you can use a flat LCD or plasma screen or install a projector. For sound you can stick with stereo or have up to seven loudspeakers in your room.

 

Analogue video

Until recently all television systems have been analogue. The screen is made up of about 550 lines which together form the picture. Each line on the screen is sent as a voltage which varies from 0V to about 1V to show the brightness. For example there might be three voltages, each in a different wire, one for each colour, red, green and blue. The voltage can take any value between 0 and 1 so it is a model, or analogue, of the brightness. The voltage is continuous as it does not switch off until the end of the line. As the signal gets poorer, for example at the end of a long cable, the quality of the picture gradually gets worse until it becomes unpleasant to watch. It rarely fails completely, so analogue is very robust. Where data is stored digitally, for example on a CD or DVD, the player usually converts the signals back into analogue before sending to the TV, projector or audio amplifier.

 

Digital video

In a digital system the image is split up into tiny dots or picture cells, called pixels. These are arranged in rows and columns. Each pixel is made of three dots, one each of red, green and blue. If you look at a computer screen through a very strong magnifying lens you can see the dots. Each pixel has three data, each representing the brightness of one of the three colours. The data is made of 1s and 0s, called binary digits or bits, each of which is sent as a square-shaped wave which is either at zero or maximum volts. Normally there are 24 bits per pixel. This gives eight bits per colour, or 256 different brightnesses. When the three colours mix this gives a total of about 17 million possible pixel colours. This sounds a lot but many colours are unnatural and are not used. The most advanced high definition systems use more bits than this to allow an even more realistic range of colours.

HDMI (and DVI) uses Transmission Minimised Differential Signalling (TMDS). This comprises a blue signal to which sync signals are added, and separate green and red channels. These are called TMDS Data0, Data1 and Data2.

The quality or fineness of an image is given by its resolution, for example 1024 by 768 on a computer. The second number is the number of lines and the first is the number of pixels in each line. There are currently two standards for high-definition television (HDTV), having 720 or 1080 lines in each picture or frame, though higher resolutions are available such as 2560×1600. Multiplying lines by pixels per line gives the number of pixels. Each pixel is 24 bits, so the rate at which data has to be sent is great.

To reduce this, interlaced systems send alternate frames. The first has the odd numbered lines - 1,3,5 etc - and the second the even ones. The two sets of lines are sent so rapidly after each other that they appear to be simultaneous. Only half the data rate is needed, but the perceived quality is slightly poorer. This is called 'interlaced', shown for example as 1080i. Sending all of the lines in each frame gives a slightly better picture and is called 'progressive', for example 1080p. However while at present 1080i has 50 frames per second (fps) in the UK, 1080p will start with only 25 fps, which gives a 4% acceleration in the action as the film frame rate in cinemas is 24 fps.

To sum up, 720p50 format is 1280 pixels  × 720 lines progressive with 50 fields per second and is called 720p. The 1080i50 format is 1920 pixels × 1080 lines interlaced with 50 fields per second and is called 1080i. This is the standard currently used for high definition television. 1080p is the progressive or non-interlaced version used for blu-ray disks.

Digital audio

To digitise sound, the voltage generated by the microphone, which is an analogue of the loudness, is measured, called 'sampling', and turned into a binary number. For compact disks this is done just over 44 thousand times a second, and sixteen bits are used to store the voltage level or loudness. This gives 65536 possible levels. This must be done for each audio channel, which is two for stereo and potentially many more for home cinema. However the amount of data per second is very much less than for video.

What is a digital signal like?

A digital signal is a series of square waves to represent the bits. In HDMI and other digital connections a positive ( 3.3 to 5V) voltage is a one and zero voltage is a zero. When travelling along a cable, these square waves degrade in three ways. The height or amplitude drops, noise voltages are added to the wave, and the square edges become rounded. Circuits can restore the waves by amplifying and squaring them. This restores the data bits, but once a bit is very distorted it cannot be repaired and the data is lost. This is called 'drop out'. Computers have error detection, and carry out error correction or ask for data to be sent again, so this does not cause failure. Digital broadcast systems like freeview and freesat do have error correction, but there is no way that data can be re-broadcast. If the data channel or line becomes poorer, there might be so many errors that no image can be created at all. This is known as 'falling off the digital cliff'.

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(C) Peter Scott 2009

Last edit 26 December 2015