PALplus
PALplus is an extension of the PAL analogue broadcasting system for transmitting 16:9 programs without sacrificing vertical resolution. A standard PAL receiver will display the image in letterbox format with 432 active lines, while a PALplus receiver can use extra information hidden in the black bars above and below the image to recreate 576 lines of vertical resolution.
A special signal tells the receiver when PALplus is in use, and also whether the original content was interlaced ("Camera mode" 576i50) or progressive scanned ("Film mode" 576p25). An additional signal can enable a "Ghost Cancellation" feature.
A separate feature related to PALplus is ColourPlus, which improves colour decoding performance.
Without PALplus, a 16:9 presentation has only 432 lines of vertical resolution. This reproduces noticeably less detail than the 576 lines used for 4:3 broadcasts. A fully decoded PALplus broadcast restores the 576 line vertical resolution. For compatibility reasons, the horizontal bandwidth remains at 5.0 MHz. This means a PALplus signal provides no extra horizontal resolution to compensate for the image being stretched across a wider screen. The result is a horizontal resolution that is 73% of the vertical resolution, or 51% when the Kell factor is ignored.
The PALplus standard comprises three extensions to standard PAL:
Vertical helper
A broadcaster creates a PALplus signal by scaling an anamorphic 16:9 picture with 576 lines down to 432 lines, so that the picture appears letterboxed on a regular PAL TV set. For luminance, the scaling is done using a pair of matching low-pass and high-pass filters, with the low-pass result appearing in the broadcast. One out of every 4 lines of the high-pass result is then hidden in the remaining 144 black lines at the top and bottom of the picture, using the U colour subcarrier. The filtering is such that this is enough to restore the complete 576 line resolution. The use of the colour subcarrier means the signals sometimes appear as blue and yellow patterns on a regular PAL TV set. The 16:9 PAL-plus receiver combines 432 visible lines plus 144 helper lines into 576 new visible lines.
In Film mode (progressive scan), the operation is performed on a per-frame basis, while in Camera mode (interlaced) the operation is performed per-field.
Colour-plus (or Clean PAL)
The PAL colour carrier is modulated making use of correlation between 2 fields, in order to give a cleaner Y/C separation in the PAL-plus receiver. Colour pictures on both standard and PAL Plus receivers are enhanced.
Signaling bits
A special WSS signal tells the receiver whether 4:3/16:9/PALplus is in use, and also whether the original content was interlaced ("Camera mode") or progressive scanned ("Film mode"). An additional signal can enable a "Ghost Cancellation" feature. The bandwidth of these bits is low enough to be recorded on VHS and allow the receiver to switch to the proper format.
PAL-plus compatible sets
The standard permits using the mark "PAL-plus" if just the vertical helper reconstruction implemented, with Colour-plus being optional.
Most widescreen sets without any PAL-plus processing will switch the display format automatically between 4:3 and 16:9, based on the signaling bits. These sets will display only the centre 432 lines of the 4:3 image, to fill all of the 16:9 frame.
In the late 1980s a new broadcasting standard was created, HD-MAC, that was HDTV and 16:9 capable (up to 2048×1152 picture resolution), twice the number of available lines in PAL. As a transitional standard, D2-MAC was established. Like HD-MAC, it is 16:9 ready, but with the same number of lines that PAL uses. When D2-MAC failed, the PALplus norm was created as another compromise, and failed as well.