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High Definition Television can be defined as having a picture with significantly more detail than the current standard (currently of 625 or 525 lines). The picture will look clearer and colours more natural with a larger range of reproduction. Corresponding sound accompanying the picture will also improve in quality.

The Basics:

Standard definition TV in Europe has 625 lines, of which, only 576 are used for the picture.

High definition TV has a larger number of lines or PIcture ELements (pixels).

720 lines with 1,280 points or 'pixels' per line or
1,080 lines with 1,920 pixels per line.

Standard definition signals are comprised of 576 lines with 720 pixels per line. The space in the signal for widescreen and the older squarer picture style are the same, so the widescreen picture is pre-squashed (Anamorphic) for sending. Then, in the tv set, the widescreen picture is unsquashed.


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Advanced: ...the compromises of sending...

Standard definition TV pictures are made and broadcast in a way which is a compromise in order to save space (to give more channels etc.) The picture is sent in two halves called fields. Two fields make a complete 625 line frame.

This two-field dodge is called interlace. Why is this useful to know? The higher HD standard of 1080 lines uses interlace. Field 1 containing the odd numbered lines is sent first, followed by Field 2 containing the even numbered lines.

See demonstration of how the two fields fit together to make the one complete frame of 625 lines. As only the lines with pictures are counted, you may see these talked about as 576i (i=interlaced).


HDTV using the 720 standard uses a single frame with no separate fields so this is known as 720p. (p=progressive). So for 1080i, the field of odd numbered lines are followed by the field of even numbered lines similar to standard definition tv. 540o +540e = 1080 lines

This 'workaround' saves valuable space as it is effectively 540 line tv but with each field between the other - effectively doubling the number of lines. BUT the penalty is a compromise in the picture. There may be interaction between the camera and any movement in the studio producing interference. The flickering between fields also means an effective reduction in definition to about 700 lines. See Interlace demonstration here.

Very Advanced

Current SDTV uses MPEG-2 coding and sent using DVB-T for terrestrial and DVB-S for satellite. Technology has now moved on and most HD transmissions will use MPEG-4 or H.264 with sound in the AAC format. Improvements in coding mean that newer standards of DVB-S2 and DVB-T2 will be used for HD and also for newer SD services.

Extremely Advanced

Many imported programmes such as Lost that are imported from the USA are made in what's called the Common Interchange Format (CIF). They are made in '1080, 24P'; Twenty four frames per second scanned progressively. This is comparable to the cinematic frame rate for films/movies.

To convert this progressive picture into an interlaced 1080i channel, the progressive picture is split or segmented into two halves and transmitted like an interlaced signal. The difference between interlace and this segmented frame (SF) format is as follows:

An interlaced camera will capture picture movement (such as an actor moving) during each individual 540 line field in sequence: Field 1 then Field 2.... Thus, there will be movement between each field.

A progressive camera captures a complete single frame of 1080 lines AND THEN studio equipment splits it into two segments for signal processing - There will be NO inter-segment movement. The two segments are then transmitted just like two fields. The technical difference between interlace and segment is (roughly) how the camera photosensitive elements are apparently switched.
This segmented signal is then sent to the receiving digibox along with a signal 'flag' commanding a suitable display to to be reassemble the two 'segments' into a 24P progressive format for display. Viewing 24 frames per second would be quite fatiguing. Some displays repeat the whole frame twice or three times to reduce display flicker - Though any original camera judder will still be seen. No SF flag? The signal is treated like a conventional interlaced picture.

Some displays attempt to overcome judder electronically by incorporating a function called Motion Flow. The circuitry averages out the difference between whole frames or fields and inserts a new frame or many new frames between the original two, giving the effect of reducing the judder and improving the movement of subjects within the picture.

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