PAL
PAL, short for Phase Alternating Line, is a colour-encoding system used in broadcast television systems in large parts of the world. Other common analogue television systems are SECAM and NTSC. This page discusses the colour encoding system only. See Broadcast television systems and analog television for discussion of frame rates, image resolution and audio modulation. For discussion of the 625-line / 25 frame per second television standard, see 576i.
In the 1950s, when the Western European countries were planning to establish colour television, they were faced with the problem that the already existing American NTSC standard would not fit the 50 Hz AC frequency of the European power grids. In addition, NTSC 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 with a picture frequency of 50 fields per second (50 hertz), and sporting a better colour picture than 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 Générale de Télévision 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, principally European, African and Asian) television system, and to differentiate from a 525-line/60 Hz (480i, principally American and Japanese) NTSC system. Accordingly, DVDs are labelled as either PAL or NTSC (referring informally to the line count and frame rate) even though technically the European discs do not have PAL composite colour. This usage may lead readers to believe that PAL defines image resolution, even though it does not. The PAL colour system can be used in conjunction with any resolution and frame rate, and various such combinations exist. NTSC, by contrast does define the video line and frame format.
The basics of PAL and the NTSC system are very similar; a quadrature amplitude modulated sub carrier carrying the chrominance information is added to the luminance video signal to form a composite video base band signal. The frequency of this sub carrier is approximately 4.43 MHz for PAL, compared to approximately 3.58 MHz for NTSC. The SECAM system, on the other hand, uses a frequency modulation scheme on its color subcarrier.
The name "Phase Alternating Line" describes the way that the phase of part of the color 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 color 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 canceling; however this resulted in a comb like effect on larger phase errors. Thus, most receivers now use a chrominance delay line, which stores the received color information on each line of display; an average of the color 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 color resolution is poorer than the NTSC system's, but since the human eye also has a color 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 color detail) reduced greatly compared to the luminance signal.
For a 1:1 pixel aspect (square pixels) on a 50 Hz interlaced PAL signal the pixel rate should be 14.75 MHz.
spectrum of a system G (bands IV and V) television channel with PAL or SECAM colour).
Oscillogram of composite PAL signal - one frame.
Oscillogram of composite PAL signal - several lines.
Oscillogram of composite PAL signal - two lines.
The 4.43361875 MHz frequency of the colour carrier is a result of 283.75 colour clock cycles per line plus a 25 Hz offset to avoid interferences. Since the line frequency is 15625 Hz, the colour carrier frequency calculates as follows: 4.43361875 MHz = 283.75 - 15625 Hz + 25 Hz.
- CVBS is an initialism, but it does not stand for "composite video baseband signal", CVBS actually stands for (C)hroma, (V)ideo, (B)urst, and (S)ync; which are the four basic components of a composite video signal. That's why it's called "composite".
PAL vs. NTSC
NTSC receivers have a tint control to perform colour correction manually. If this is not adjusted correctly, the colours may be faulty. The PAL standard automatically removes hue errors by utilising phase alternation of the colour signal (see technical details), so a tint control is unnecessary. Chrominance phase errors in the PAL system are cancelled out using a 1H delay line resulting in lower saturation, which is much less noticeable to the eye than NTSC hue errors.
However, the alternation of colour information — Hanover bars — can lead to picture grain on pictures with extreme phase errors even in PAL systems, if decoder circuits are misaligned or use the simplified decoders of early designs (typically to overcome royalty restrictions). In most cases such extreme phase shifts do not occur. This effect will usually be observed when the transmission path is poor, typically in built up areas or where the terrain is unfavourable. The effect is more noticeable on UHF than VHF signals as VHF signals tend to be more robust.
A PAL decoder can be seen as a pair of NTSC decoders:
- PAL can be decoded with two NTSC decoders.
- By switching between the two NTSC decoders every other line it is possible to decode PAL without a phase delay line or two phase-locked loop circuits.
- This works because one decoder receives a colour sub carrier with negated phase in relation to the other decoder. It then negates the phase of that sub carrier when decoding. This leads to smaller phase errors being cancelled out. However, a delay line PAL decoder gives superior performance. Some Japanese TVs originally used the dual NTSC method to avoid paying royalty to Telefunken.
- PAL and NTSC have slightly divergent colour spaces, but the colour decoder differences here are ignored.
- PAL supports SMPTE 498.3 while NTSC is compliant with EBU Recommendation 14.
- The issue of frame rates and colour sub carriers is ignored in this technical explanation. These technical details play no direct role (except as subsystems and physical parameters) to the decoding of the signal.
PAL vs. SECAM
SECAM is an earlier attempt at compatible colour television which also tries to resolve the NTSC hue problem. It does so by applying a different method to colour transmission, namely alternate transmission of the U and V vectors and frequency modulation, while PAL attempts to improve on the NTSC method.
SECAM transmissions are more robust over longer distances than NTSC or PAL. However, owing to their FM nature, the colour signal remains present, although at reduced amplitude, even in monochrome portions of the image, thus being subject to stronger cross colour. Like PAL, a SECAM receiver needs a delay line.