PAL (an initialism acronym for Phase Alternating Line), is a colour encoding system for analogue television used in Broadcast Television Systems. Other common colour encoding systems are NTSC and SECAM, but all systems have a 4:3 aspect ratio. PAL is a term used informally and somewhat imprecisely to refer to a colour 625-line / 50-field (25 frame) per second display.
Commodore home computers which used CRT television screens to output a video display had to be synchronised in order to produce a valid picture. Therefore depending upon the national Broadcasting Television Standard, there are different PAL and NTSC-variant Commodore home computers. At the time, these systems were (and the associated software was) generally incompatible.
Broadcasting Television Standards
The Broadcasting Television Standard adopted by a country is directly related to the utility frequency at which the national electricity distribution system operates. This is in order to avoid intermodulation (also called beating) between the television screen deflection system and nearby mains generated magnetic fields; the consequence being a flicker of the television picture or rolling bars on the screen.
In countries using the PAL Broadcasting Television Standard, the frequency is 50hz (although there are exceptions).
Ignoring colour, all analogue television systems work in essentially the same manner:
- A monochrome image is divided into horizontal scan lines, a specific number of which make up a frame.
- In television broadcast signals the frames are interlaced; even-numbered scan-lines are displayed followed by odd-numbered scan-lines. Smoother motion is achieved with half the frame rate by using interlacing.
- In computer broadcast signals the frames are non-interlaced or progressive; where rows are displayed in numerical sequence.
- Each half of the frame is called a video field, and the rate at which fields are displayed is related to the Broadcasting Television Standard.
The PAL Broadcasting Television Standard generally uses 50 interlaced fields per second or 25 progressive frames per second.
For comparison, cinematic film has a transmission rate of 24 frames per second.
The number of scan lines differs between Broadcasting Television Standards due to a number of reasons including (and not limited to) cost, technical capability, and bandwidth. The scan-line figure had to be reliable, stable and generated by analogue means. For interlacing reasons, the figure also had to be an odd-number. At the time, the only practical method of frequency division was through the use of a chain of vacuum tube multivibrators. Each of the separate divisors had to replicate relatively small odd-numbers due to the problems of frequency drifts within vacuum tube devices (due to component age, temperature changes or voltage fluctuations). The overall divisor was therefore a product of several vacuum tube devices.
In 1962, when PAL Broadcasting Television Standard was developed, the maximum accepted resolution (for mass population take-up) was 600 scan lines. To achieve this, the closest practical sequence of vacuum tube device factors was 5 x 5 x 5 x 5 or 625 scan lines.
However as the CRT circuits could not be instantly reset from the bottom-right to the top-left of the screen, a relative number of scan-lines are needed to retrace the path back to the start. As the CRT beam is switched off during this period, it is known as the vertical blank (or VBLANK).
PAL Broadcasting Television Standard systems require 49 scan-lines to retrace the path. Consequently there are only 576 visible lines in a frame.