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