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Skriven 2009-05-04 15:43:00 av JOE NICHOLSON (1:123/140)
Ärende: TV systems 3/3
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History of the PAL standard

In the 1950s, when the Western European countries were planning to establish
colour television, they were faced with the problem that the NTSC standard
demonstrated several weaknesses, including colour tone shifting under poor
transmission conditions, earning it a comically maligned acronym "Never Twice
the Same Color". For these reasons the development of the SECAM and PAL
standards began. The goal was to provide a colour TV standard for the European
picture frequency of 50 fields per second (50 hertz), and finding a way to get
rid of the problems with NTSC.

PAL was developed by Walter Bruch at Telefunken in Germany. The format was
first unveiled in 1963, with the first broadcasts beginning in the United
Kingdom and Germany in 1967.

Telefunken was later bought by the French electronics manufacturer Thomson.
Thomson also bought the Compagnie Generale de Television where Henri de France
developed SECAM, historically the first European colour television standard.
Thomson also co-owns the RCA brand for consumer electronics products, which
created the NTSC colour TV standard before Thomson became involved.

The term PAL is often used informally to refer to a 625-line/50 Hz (576i),
television system, and to differentiate from a 525-line/60 Hz (480i) NTSC
system. Accordingly, DVDs are labelled as either PAL or NTSC (referring
informally to the line count and frame rate) even though technically the discs
do not have either PAL or NTSC composite colour. The line count and frame rate
are defined as EIA 525/60 or CCIR 625/50. PAL and NTSC are only the method of
the colour transmission used.



Technical details

The basics of PAL and the NTSC system are very similar; a quadrature amplitude
modulated subcarrier carrying the chrominance information is added to the
luminance video signal to form a composite video baseband signal. The frequency
of this subcarrier is 4.43361875 MHz for PAL, compared to 3.579545 MHz for
NTSC. The SECAM system, on the other hand, uses a frequency modulation scheme
on its two line alternate colour subcarriers 4.25000 and 4.40625 MHz.

The name "Phase Alternating Line" describes the way that the phase of part of
the colour information on the video signal is reversed with each line, which
automatically corrects phase errors in the transmission of the signal by
cancelling them out. Lines where the colour phase is reversed compared to NTSC
are often called PAL or phase-alternation lines, which justifies one of the
expansions of the acronym, while the other lines are called NTSC lines.

Early PAL receivers relied on the imperfections of the human eye to do that
cancelling; however this resulted in a comblike effect on larger phase errors.
Thus, most receivers now use a chrominance delay line, which stores the
received colour information on each line of display; an average of the colour
information from the previous line and the current line is then used to drive
the picture tube. The effect is that phase errors result in saturation changes,
which are less objectionable than the equivalent hue changes of NTSC.

A minor drawback is that the vertical colour resolution is poorer than the
NTSC system's, but since the human eye also has a colour resolution that is
much lower than its brightness resolution, this effect is not visible. In
any case, NTSC, PAL and SECAM all have chrominance bandwidth (horizontal
colour detail) reduced greatly compared to the luminance signal.
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