PALplus
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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 ColorPlus, which improves color decoding performance.
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[edit] Operation
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.
[edit] Versions
The PALplus standard comprises three extensions to standard PAL:
[edit] 1. 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 color subcarrier. The filtering is such that this is enough to restore the complete 576 line resolution. The use of the color 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), the operation is performed on a per-frame basis, while in Camera mode (interlaced) the operation is performed per-field.
[edit] 2. 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.
[edit] 3. Signaling bits
A special 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.
[edit] 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.
[edit] Availability
Where is PALplus in use?
In Belgium, the Flemish public broadcasting service VRT has a policy that all of its self-created TV programmes are broadcast in PALplus. For example the news is broadcast daily in PALPlus, as are almost all of the weekly scheduled shows. Only old re-runs and American television movies are not. Newer films and series are mostly broadcast in PALplus. The commercial TV station VTM also broadcasts a lot in PALplus. The VTM news for example is also broadcast in PALplus. So are even the commercials. Special shows and films are always broadcast in PALplus too. Even the third broadcasting organisation SBS Belgium with its stations VT4 and VijfTV broadcast in PALplus for all of their new productions and they also broadcast the news and the commercials in P+. The Walloon public broadcasting service RTBF broadcasts bought 16:9 programmes in PALplus, like films, but chooses to create their own programmes in 4:3. Walloon commercial TV station RTL-TVi now broadcasts almost all its shows in PALPlus and tends to convert its old shows to 16/9 too.
In England Channel 4 broadcast selected films as PALplus during the late 90's.
In Germany all public broadcasters (ARD, ZDF, etc) comply with this standard. However, private broadcasters (RTL, Pro Sieben, etc) have shown no interest in either this standard or in the 16:9 format. Pay-per-view channels such as Premiere often broadcast in 16:9, but use a different standard that requires another kind of decoder.
In the Netherlands broadcasting in PALplus is sporadic. Most programmes are still in plain old 4:3, but sometimes special shows and series are broadcast in PALplus. 16:9 programmes acquired from another station are rarely converted to 4:3 and are often broadcast in their original aspect ratio in PALplus.
In Portugal, the private broadcaster TVI began broadcasting movies in PALplus in 1994, but some years after it left the standard behind. Today only public broadcaster RTP uses the system to broadcast selected programs and most widescreen movies.
In Finland, the commercial broadcaster MTV3 started broadcasting the youth music program Jyrki in PALplus format on 1997-08-18, but the experiment ended when the program ended some four years later. There are currently no PALplus broadcasts in Finland, and the public broadcaster YLE has announced that they prefer digital TV technology over the analog PALplus. It is unlikely that PALplus will ever return to Finland.
In Greece, there are still sporadic PALplus broadcasts, on the national television (E.R.T. - Hellenic Radio Television). Through the '90s, there where few attempts from commercial broadcasters to adapt the system, but all failed to gain popularity.
[edit] History
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.
[edit] Criticism
The decoding of a 16:9 picture in PALplus is very costly and requires the use of the two fields of the full picture. Because of this, only expensive TVs such as 100 Hz TV sets can decode the 16:9 mode of PALplus.
For all this, the takeup of PALplus has been marginal, and the future of this standard does not look very bright, since broadcasters are switching to digital television broadcasting (DVB).
