It has long been rumored that it is possible to build a Videocipher II video
decoder for a few dollars worth of Radio Shack parts. This may be true, but
most of the designs that I have seen have required a professional engineer
to get aligned and working. It is for that reason that I have invented the
"EASY VCII VIDEO" decoder. This small board gets all of the VCII encoded
channels as of today (October 16, 1988). Decoders such as the "Blank (Black)
box solution do work on the VCII signals. However, it takes a professional
grade oscilloscope and a good deal of time to get it set up and working. Then,
after you change channels, there are buttons and knobs to adjust on the front
of the box. This design, provided that all of the parts are good and hooked
up correctly, requires only that you adjust one pot until a stable picture
is obtained. That's it. When changing channels on G1 (which has most every
thing scrambled nowadays) the picture will lock in before you can get your
hand off of the channel changer!
Why??
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Why would you want to build one of these? After all, it does not get the
audio. Well some things do not have the audio scrambled (F4 TR 18, F1 TR 10)
some things don't need audio (S1 TR 18) and some things like Sunday night
football (G1 TR 9) are broadcast on the radio.
How it works:
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There are only 8 integrated circuits and 4 general purpose (2n2222)
transistors required for this project. It is so simple that you can not
believe how really well it works!
This decoder works by looking for the gap that occurs between the end of
the color burst signal and the start of the next line. This gap is there on
all lines, but the amplitude is different on the sync pulse that starts the
vertical interval. The way this decoder works, the gap at the beginning of
the vertical is skipped and a missing pulse detector generates a vertical
sync pulse.
This decoder gets by with only one adjustment by putting all of the critical
timing points into an eprom and clocking them out with an oscillator that
runs at about (but not exactly) 64 times the horizontal frequency.
The file timing.com displays the timing information inside the eprom.
The file sync3.dat is the data ready to put into a 2732 eprom. It's hard
for me to imagine a hobbyist that does not have access some type of eprom
programmer. (I personally own 3 different ones and have access to at least
4 more!)
After you get the circuit built, either on a perfboard or a PC board, connect
a scrambled source of unclamped video (how about Spacenet 1, TR 18 after
9:00 pm ? ) to the point marked "video in". Connect the "video out" to a
modulator, such as the video input on your VCR. Adjust the pot for a
clear picture. The adjustment is fairly critical, but easy to do if you use
a 10 or 20 turn pot for R13. I originally thought that the circuit might drift,
but since building the original perfboard of this circuit in July, 1988, I
have not had to readjust since. When you first turn on the power, it typically
takes less than 30 seconds for the picture to lock in. After that, locking is
immediate. If you leave the unit powered on all the time, then no warm up
period is required.
There is one minor difference between the schematics an the printed circuit
board layouts - cr4 through cr7 are not on the board. I used a calculator
type power supply for powering the board. This is the only VCII type decoder
that I have seen that works off of a single voltage supply. All of the others
have taken at least 3, sometimes as many as 4 or 5 different supply voltages
for operation.
Printing the drawings:
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You must have a IBM Graphics compatible printer to print the schematics
and PC board artwork layouts. The artwork is printed at 2:1 and is ready to
be reduced photographically for making your own pc board. Be patient! It
takes about 30 minutes to print each side of the board but the results are
well worth it.
The schematic is 6 pages long and is printed in sections. Just tape the pages
together after printing. I could have made this all fit on a single sheet of
paper, but it is not very easy to read.
There is no parts list included, but all parts are common, readily available
off the shelf from most any local or mail order place. Just make your
own list off of the schematic provided.
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That's it. You are now on your own. There are no boards available from anywhere
that I know of. There are intentionally no names anywhere in these files
because this is an experiment. I want to see how fast this particular program
circulates. (In other words, how long will it be before someone gives me a
copy!)
Anyone may use any of these files for any purpose that they see fit!
(and they will anyway, regardless of any notices!)
