The goal of the system is to encode the day-number, channel number and the *published* start time and duration of a TV programme as a number of no more than 8 digits (or 9 with some Showview codes) such that the encoding can be called proprietary, and that Gem-Star can raise revenue by selling the codes to newspapers. They also licence use of the system to be built into VCRs and stand-alone decoder handsets.

Codes are cunningly arranged to be small numbers in the case of prime-time material and progressively longer numbers for the more obscure time slots. It is done by arranging popular timeslots to be earliest in a giant lookup table.

The US broadcasters arrange 99% of their programming to start and end on 30-minute boundaries, frequently have programmes with lengths up to and including five hours (American Football or Baseball) and have a *lot* of channel numbers to choose from. There are thus 480 timeslots on 48 channels which get 6 digit-or- fewer codes in VCR+. There are 8 timeslots on the first 4 channels which get 3 digit-or-fewer codes in VCR+.

There are 2000 timeslots on 16 channels which get the 6-digit treatment in Videoplus because in the UK we use fewer channels on the whole but have a lot more variety in program lengths and start-times. The longest Videoplus programme with a 6-digit code is 3 hours long. As with VCR+ , there are 8 timeslots on the first 4 channels which can get 3 digit-or-fewer codes. In Videoplus these first four channels are allocated to the 4 original terrestial broadcasters, BBC1, BBC2, ITV and C4.

(Stupidly, UK Gold uses a rather US-like scheduling strategy, but is allocated a channel number greater than 16. This makes every UK Gold programme get a 7 or 8 digit code, yet if it had been allocated a channel number of 16 or less, almost every programme would get a short code, and the prime time slots would often be only 4 digits long. Same goes for Discovery)

Programmes whose starts and durations give them indexes 0-127 on the lookup table, and which are on channels 1-16 will actually have the same codes in VCR+ and Videoplus. This means all the 1,2,3 and 4-digit codes, plus some of the 5-digit codes are the same in the two systems.

1) One-digit codes can only occur on the first of the month.

2) Two-digit codes can only occur on the 1st,2nd,3rd and 4th of the month, but there will only be 4 possible slots on the 4th of the month that can have a two-digit code.

3) A timeslot/channel combination which gets a one-digit code on the 1st of the month *will* get a 2 digit code on the 2nd and 3rd day, but not necessarily on the 4th.

4) A timeslot/channel combination which does get a two-digit code on the 4th of the month will have had a two-digit code on the 2nd and 3rd too, and may have had a one-digit or two-digit code on the 1st.

VCR+ (or its variants) don't cope with unexpected changes to programme transmission times (that's PDC's domain) nor with the daylight-saving time switchover. Pity about that latter point, it could have been done easily enough.

Later again, our Videoplus sources were picked up by Daniel Minder in Germany and modified to handle Showview. See here for an online demo.

None of these published codecs can (yet) deal properly with codes of more than 6 digits. This is under investigation. Currently I (Steve Hosgood) have shown that Videoplus 7 and 8 digit codes can be coded and decoded but only if certain similar codes are 'learnt' for the month in question.

I make my current version of the Videoplus codec available under the GPL (GNU Public Licence) in a choice of four downloadable formats:

Videoplus Encoder/Decoder Program | |||

Sources (pkzipped) | Sources (TAR-ed and gzipped) | Source RPM | Binary RPM (Intel i386/Linux) |

These notes copyright (C) 1994-2000 Steve Hosgood. Permission granted for non-commercial re-distribution.

VCR+, Videoplus and Showview are registered trademarks of Gemstar Inc.