#### General:
Videoplus is the UK version of a system which started in the 'States a couple
of years ago under the name VCR+. There is a European version called ShowView
as I understand it. All these names are registered trade names of GemStar Inc.
It is not known if ShowView and Videoplus differ technically.

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 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.

### TV Schedule differences:
Because the US, UK and Euro ideas of television scheduling differ, different
variants of VCR+ exist. I don't know if UK Videoplus differs in more than
name from the mainland Europe ShowView, but certainly VCR+ is technically
different from Videoplus.

The US 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 length. 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 are bagged
by the 4 terrestial broadcasters, BBC1, BBC2, ITV and C4/S4C.

(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.


#### Trivia:
Because the 1 and 2 digit codes are the same on VCR+ and Videoplus, the
following trivia hold for both 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.

#### Dates:
Encoded along with the channel number and timeslot index is a programme date
(just a number from 1-31). A decoder will assume that if the decoded day
number is less than yesterday's day number, then it really refers to next
month. So, codes can be published up to 27-29 days in advance of the
programme's broadcast. There are "illegal" codes in the system, referring to
the non-existant dates at the end of certain months. Commercial decoders
produce an error report if these codes are tried, though the internet codec
will still encode and decode them. There also are a few "illegal" codes which
could in theory be validly decoded as occuring on the 32nd of the month. These
are not handled even on the internet codecs though.

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.


#### Credits:
VCR+ encoding was originally partly broken by a couple of guys in the States,
Ken Shirriff <shirriff@sprite.berkeley.edu>, and Curt Welch
<curt@oasys.dt.navy.mil> and their published sources on Internet were picked
up by me, Steve Hosgood <iisteve@iifeak.swan.ac.uk> and Douggie McClaggan
<mclaggan@movies.enet.dec.com> and were extended to cover Videoplus.

Neither of these published codecs can (yet) deal properly with codes of more
than 6 digits. This is under investigation. Currently *some* videoplus 7 and
8 digit codes can be coded and encoded only if certain similar codes are
'learnt' for the month in question (see "f1.exe").

These notes copyright (C) 1994,1995 Steve Hosgood.
Permission granted for non-commercial re-distribution.

VCR+ and Videoplus are registered trademarks of Gemstar Inc.
If the rumours about Showview are true, then Showview would be a registered
trademark of Gemstar Inc.
