VCRPlus+® EXAMPLE

Gemstar designed a code which deliberately obscures patterns. The rules change within the system, Curt Welch says, but basically the cipher uses the following devices: scramble digits, assume constants, lookup values in tables, and generate final results with equations.

Let's look at a simple case to illustrate how the code works. Suppose you looked up your show and found the VCRPlus+® code 5 in the TV listing. What does that tell the VCR? A number, but the number isn't 5. First the VCR decoder adds the month, January, (1) to the code (5) to get 6; that's the number. But there's more. Now the decoder converts the number 6 to a 5-bit binary number (00110)--the kind that computers understand, just zeros and ones.

This number encodes the start time (T2 T1 T0) of your show and the channel (C1 C0) that it's on. The digits of the time and channel, however, are scrambled in the binary number 6 as the table shows:

0	0	1	1	0
T2	C1	T1	C0	T0

The decoder identifies the start time from the table: T2 T1 T0 = 010, which is binary for the number 2. It looks up "2" in a table similar to the one below to get the real start time: 7:30 p.m.

0	6:30 p
1	4:00 p
2	7:30 p
3	4:30 p
4	3:30 p
5	5:30 p
6	6:00 p
7	2:30 p

The channel (C1 C0 = 01) is the number 1, but first the decoder knows to apply a rule (add 1) to arrive at the real channel which is 2.

The VCRPlus code number, 5, is a single digit. This tells the decoder to assume the day of the month is the first and the show length is 30 minutes.

The VCR decoder gets the current month (December) and the day today (the 27^th) from its internal clock and assumes the month you want to record is January because the first of December is long past. So it cranks in next month's number in its calculations. The year doesn't matter. The VCR can only record shows up to a month in advance using VCRPlus+ codes.

The VCR now has the information it needs so it starts recording your show on Channel 2 at 7:30 p.m. on 1 January 2000 and stops recording 30 minutes later.