Homage to pal

As I get older I find my piano repertoire fossilising - I play or think the same pieces over and over again. Those who listen to me have doubtless noticed but are too polite to say. One such tune that goes around in my head is Telstar named after a communications satellite launched in 1962 which relayed the first live analogue TV broadcast from the US to the UK.

Broadcast TV was started in earnest in the UK in 1936 using the 405 line system which ended in 1985. The higher resolution 625 line system was started in 1964 initially monochrome, colour being introduced in 1967. Broadcast analogue TV was finally switched off in the UK in October 2012 giving way to digital TV. The analogue video system still persists in older equipment but will soon be a relic of the past. But it is a relic worth paying homage to because of the way it makes good use of the technology available which to do is, of course, the remit of every good engineer.

NTSC-PAL-SECAM distribution

The main colour TV systems were the American NTSC, the French SECAM, and PAL. PAL is technically more elegant and gives a higher resolution and better colour rendition than NTSC. I have no experience of SECAM.

Young electronic engineers today, who work with digital video and flat screens in which a pixel is a well defined physical element, and who have access to software libraries like MPEG, may find it hard to grasp the difficulties in designing an analogue system for a severely limited physical medium. But even today the best engineers deal with cutting edge technology that pushes the very bounds of quantum physics.

Here are some of the clever aspects of analogue colour TV. Check out here for more detail.

Vestigial sideband modulation enables 625 line video (which needs about 6MHz of bandwidth) and sound to be transmitted within an 8MHz channel and yet is low cost to demodulate in the receiver.

The frame rate is locked to mains frequency (60Hz in USA, 50Hz in Europe) to reduce the effect of mains interference which otherwise would appear as a slowly moving band across the picture

25 frames of 625 lines are transmitted every second, but the lines are interlaced in two "fields" giving a field rate of 50Hz to reduce flicker yet giving full 625 line resolution for pictures with less movement.

Initially the 625 line system was designed for monochrome. Colour was added several years later and the extra information had to be squeezed into the same 6MHz bandwidth without compromising monochrome reception. The colour information is divided into YUV components. Y is the luminance and is transmitted at full resolution in place of the monochrome signal. Successive frames of video generally differ little and this translates, in the frequency domain, to most of the spectral energy being near harmonics of the field rate.

U and V are the second and third signals necessary to define tristimulus colour. These two signals are modulated in quadrature onto a sub-carrier at a carefully chosen frequency of 283.75 × line rate + 25 Hz. The 25Hz offset places the harmonics of the colour signal in the gaps between the harmonics of the luminance signal. Only the two can still get mixed up sometimes e.g. cross-colour.

The trick here is that the eye does not demand as high resolution for colour as for luminance - which perhaps is why tinted water colours work so well.

Colour tinted monochrome photograph

Water colour tinted line drawing

In the PAL system the colour phase is reversed with each line, which automatically corrects phase errors in the transmission of the signal by cancelling them out, at the expense of vertical frame colour resolution. The colour signals are also severely band-limited thus reducing horizontal resolution.

Transmitted PAL spectrum

After all that... the point of this post was actually not so much to pay homage to PAL or to recall fond memories of the BBC Research Department but as a kind of precursor maybe to my next post on music.