PAL Television
What Is PAL TV ?
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| DVD Video Symbol With PAL Format |
VHS PAL DVD PAL
Most of us are used to seeing VHS PAL and DVD PAL labels on VHS cassettes or DVDs, but what does PAL mean?
It stands for Phase Alternating Line and is a technical term indicating the colour system that is used which applies to analogue Television signals.
Back in the 1950s when black and white TV was king, RCA of America developed a colour Cathode Ray Tube which meant colour TV was on it's way! Colour TVs would be very expensive to begin with and so millions would still be watching black and white for a while until mass production brought prices down. So how do you send the colour signal to the privileged few who could afford colour without effecting those who had black and white?
Luminance and Chroma
The black and white TV signal is also called the Luminance signal and is used to drive the CRT in varying intensity depending on the picture. You would need to keep this signal so that those with black and white TVs would be able to watch as normal. The colour, or Chrominance signal was sent separate from the luminance signal, in fact it was superimposed over the luminance signal, but did not interfere as the luminance circuitry could not 'see' it.
NTSC
The Americans were ahead of Europe when it came to colour TV and they developed the NTSC system which stands for National Television Systems Commission.
The Colour signal was sent on a subcarrier and the phase and intensity at a given time determined the colour that three RGB phosphor 'dots' would display. A problem with this was noise and phase errors getting into the signal, especially when being transmitted over the air, which would result in wrong colours and blotches being displayed. In fact, if you were around to watch TV programmes from USA in the 1970s and 80s, you would have noticed a lot of orange faces and other strange hues to the picture.
Walter Bruch
Who? Walter Bruch was a German engineer who invented the PAL system. He was working for the West German electronics company Telefunken at the time.
For Beatles fans - In the 'Let It Be' film, John Lennon wittily sang " Isadora Duncan works for Telefunken" - Sorry, couldn't resist it!
Back to Walter Bruch - He travelled to the USA to get some ideas, but could see [literally] errors with the NTSC system. So he came up with the PAL system.
Phase Alternating Line
His idea was to construct one TV Line of colour signal in a positive direction and the next TV line would Alternate and be in the negative direction or 180 degrees opposite. As it was assumed that the colour information changed very little between lines, the true colour information could be determined electronically by comparing the positive and negative signals. Any noise or errors that shouldn't be there could be ignored by the circuitry. So it went, the phase of the colour information would alternate 180 degrees between every TV line, hence - Phase Alternating Line or PAL.
In 1967, the powers that be very nearly chose the NTSC system for the UK, but were persuaded to use Bruch's Pal system. We can be thankful they did as it has served us well for nearly 70 years and is still used today with DVD players, video recorders, video games, etc etc.
Remember the PAL system is an analogue system. Most of the video we get today is digital and doesn't use pal. Most TVs today can still decode pal, as well as other formats such as NTSC, which had improved over the years to eliminate errors. The French also had their own system called SECAM.
It is also worth mentioning that although PAL refers to the colour format, there is also a difference in the number of TV lines and frame rate between the PAL and NTSC systems so in this way also, they are not compatible.
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