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Broadcast For IT - Part 5 - PAL Line and Frame Relationships

In this series of articles, we will explain broadcasting for IT engineers. Television is an illusion, there are no moving pictures and todays broadcast formats are heavily dependent on decisions engineers made in the 1930’s and 1940’s. In the last article we looked at the incredibly complex relationship between lines and frames for NTSC systems used in the USA. In this article we look at the systems used in the UK and Europe.

Black-and-white television developed in Europe and the UK during the 1930’s used frame speeds of 25 frames per second, interlaced to give 50 fields per second. The field and frame rate originally chosen was based on the alternating current frequency of 50hz used in electrical power distribution. Due to the primitive and restricted electronics in use during the 1930’s and 1940’s, using a different frame rate would have interfered with the lighting to give a beat frequency causing flickering on the screen.

No Screen Flickering

As color developed during the 1950’s and 1960’s, UK and Europe was able to maintain the 50Hz field rate, unlike in the USA where the field frequency had to change to 59.94Hz (60/1.001). The screen flickering caused by interference between the color subcarrier and audio carrier, that dogged the USA NTSC system, was not evident on European 50Hz television.

As the European frame rate was lower than in the USA, European engineers had the opportunity to increase the number of video lines used and hence increase the vertical resolution giving 625 lines per frame.

Interlace was adopted to reduce flicker whilst maintaining fluidity of motion. But interlace technically halves vertical resolution, however, two fields combined to make one frame restores vertical resolution to 625 lines. As with NTSC systems, European television used some of the lines for scan-flyback to allow a cathode ray tubes electro-magnetic scan system to return the electron beam to the top of the screen in anticipation of displaying the next image.

576i is 625

In all, 50 lines are used for scan-flyback and transmission of ancillary data such as teletext and subtitles. Approximately 575 lines were displayed in analog television before LCD displays became popular, hence the reason standard definition digital formats are now referred to as 576i, the “i” means interlace, and “576” is the number of viewable lines in 625 televisions.

To maintain backwards compatibility with black-and-white television, color is modulated onto the luma signal allowing both black-and-white and color television sets to display the broadcast.

Analog broadcasts didn’t have the luxury of IP data streams to distribute subtitle data. Instead, some of the lines reserved for blanking and scan-flyback, just above the top of the image, were used. Data rates for subtitles are relatively low, and to maintain a high viewer experience and relevance, subtitles must be frame accurate with the images. Encoding subtitle data into invisible lines of video solved the problem completely.

Backwards Compatibility

Backwards compatibility with black-and-white televisions was a major requirement for European broadcasters migrating to color in the 1960’s. One method is to send black-and-white and color transmission separately, but this results in doubling frequency bandwidth. Broadcast engineers are in a constant battle - trying to maintain the highest picture and sound quality possible whilst keeping frequency bandwidth low. Generally, picture and sound quality are proportional to bandwidth – the better the picture and sound, the more bandwidth it uses, which in turn leads to higher costs.

As with NTSC systems, broadcasters decided to split the color television into two parts; chrominance and luminance. Chrominance (chroma) contains the color information and luminance (luma) the black-and-white. Chroma is modulated onto the luma. Existing black-and-white televisions had a natural low-pass filter in their vision due to the limited screen resolution of the CRT, and would remove enough of the modulated chroma for it not to be seen. Color television sets would use the chroma information to derive red, green and blue signals needed to display the color image.

PAL - Phase Alternating Line

European color television broadcasts started to emerge from 1967 and were referred to as PAL – Phase Alternating Line. One of the limitations of NTSC color transmissions is broadcast propagation effects caused by weather patterns influencing the subcarrier frequency to shift phase relative to the chroma modulated onto the luma, resulting in a change in hue on the viewers television. Most analog USA televisions had a “hue” control so that the viewer could correct the colors if required.

PAL overcame this by further modifying the color subcarrier so it changed phase on alternate lines. Averaged over two or more lines, the phase errors caused during transmission are negated giving consistently correct hue. The disadvantage of PAL is increased complexity, restrictions on editing and potentially desaturated color.

Line Frequencies

Frame, field and line frequencies are relatively easy compared to NTSC. With a frame rate of 25 Hz and 625 lines; one frame is 1/25 = 40milli-seconds, and the time taken for one line is 40mS/625 = 64 micro-seconds, or frequency of 15.625KHz (1/64 micro-seconds).

PAL’s 625/50i delivers an integer number of frames in one second making time counting much easier than in NTSC systems, where a non-integer number of frames is used – 29.97. 

Converting between USA NTSC 29.97 frames per second and Europes PAL 25 frames per second is incredibly difficult due to the lack of integer relationship between the frame rates of the two systems.

Converting between NTSC’s 29.97 frames per second and PAL’s 25 frames per second is a great challenge, and since the 1960’s much research and development has been committed to solving this problem – with varying degrees of success. Quite simply, there is no natural ratio between 25 and 30/1.001 (29.97) frames per second. Converting between the two requires computers to predict where objects will be between the discrete times of a frames position.

Difficulties of NTSC and PAL Conversion

Effects of poor conversion can be best seen at the end of a program originating from the USA and watched in Europe – the scrolling titles moving up the screen often judder and blur so much they’re unreadable.

Although NTSC and PAL may seem archaic by modern HD and UHD standards, their relevance cannot be over stated as they greatly influence the way broadcast television works in the 21st century. Frame rates of 29.97 and 25 are still the two main standards dominating the world today.

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