PAL-plus

"I wonder if you could tell me about PAL-Plus. The last time I was in Germany was in '84 so I've been out of touch with them."

(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

Oh boy, here goes another long story:

PAL-plus is an attempt to extend the life-cycle of terrestrial PAL transmissions by including compatible wide-screen (16:9) transmissions. It is an advanced variant of the letterbox format, this means that when you receive a PAL-plus widescreen program on an older 4:3 receiver you will see black bars top and bottom. It was originally developed in Germany (university of Dortmund in cooperation with German terrestrial broadcasters and some setmakers). Later a large consortium of European and Japanese setmakers took over and finished the job. Strangely, the German broadcasters seem to use PAL-plus only very rarely.

The PAL-plus standard comprises three extensions to the PAL-standard:

1. Vertical helper. In order to compensate for the fact that 1/4 of the video lines are not used, which would deteriorate vertical resolution for the widescreen viewer, the missing vertical information has been coded into the black lines in a manner as to be nearly invisible on a 4:3 receiver (you see some dark blue). The 16:9 PAL-plus receiver combines 432 visible lines plus 144 helper lines into 576 new visible lines.

2. Colour-plus. The PAL colour carrier is modulated in a slightly different way (making use of correlation between 2 fields) in order to give a cleaner Y/C separation in the PAL-plus receiver.

3. Signaling bits from which the receiver can conclude whether the transmission is 4:3/16:9/PAL-plus and adapt the display format accordingly. The bandwidth of these bits is low enough to survive recording on a VHS recorder.

In order to enable a poor-man's PAL-plus receiver, the standard permits using the mark "PAL-plus" if at least the vertical helper reconstruction is included. Colour-plus is optional, so you will find sets on the market with only half of the PAL-plus extension.

PAL-plus may also be combined with teletext, ghost cancellation reference, digital Nicam stereo, VPS, PDC and what-you-have more. Theoretically it can be broadcast over a satellite channel too, but it was not designed for that and some aspects of a satellite channel do indeed give interesting technical problems.

There are also sets marketed as "PAL-plus compatible". These are mostly widescreen sets without any PAL-plus processing at all, but they allow switching of the display format between 4:3 and 16:9. They may well do that automatically, based on the signaling bits.

There are 2 methods for displaying a 4:3 letterboxed signal on a 16:9 display, without using the PAL-plus helper lines:

1. Increase of the vertical deflection amplitude to display only the centre 432 lines.

2. Vertical interpolation without using the helper, to convert 432 lines into 576 lines and display on a 576 lines display.

Both modes may be called "movie expand". Only when you really convert to full-resolution widescreen will it be called "widescreen".

And there are 4 methods for displaying a regular 4:3 signal on a 16:9 display (regular PAL, has nothing to do with PAL-plus):

1. Decrease of the horizontal deflection amplitude, this gives black bars left and right.

2. Horizontal interpolation, to convert N pixels to 3/4*N pixels. Both modes may be called "4:3" or "normal".

3. Non-linear horizontal deflection waveform, called "Panorama mode" by JVC, works by increasing the S-capacitor value.

4. Non-linear horizontal interpolation, called "Superwide" by Philips, works with an advanced sample-rate converter.

With both modes, the left and right edges of the picture will be stretched to fill the left and right bars, but the aspect ratio of the centre part of the picture will hardly be affected.

Interesting, huh?