Unknown filter

so, to make it short i can throw it away.

I think it was around 1988 when I was one of the first persons to watch satellite television in my neighbourhood.
I remember that one of the first encrypted channels was BBC world service.
They used the SAVE system. What they did was adding an extra frequency to the video signal, with the result that everything looked like you were sitting behind bars.
I developed a simple PLL decoder, that first filtered out the disturbing frequency and generated the same frequency, that was added in opposite phase to the video signal. The result was a good, undisturbed picture.
A collegue of mine, who also had a satellite receiver found this very interesting and asked if I could build a similar decoder for him.
Of course, that was no problem and he payed me the costs of the components.
Just as a precaution, I removed the type numbers from the ICs...
Some time later, I sat with him in his car and found a 'commercial looking' copy of my decoder design...
It became clear that more people were interested in my decoder, especially because another channel, that was broadcasting 'nature movies' during nighttime, used the same form of encryption...
Soon after that incident, the encrypting was changed  , making all commercially produced decoders worthless...  

Moral: never trust your collegues! (Especially when they ask why you removed the type numbers from the ICs.)

In the 1980's when SKY TV first came on the scene, all the channels were free to view.

After a few months they introduced the first layer of encryption so people would have to pay. The forst encryption method was simply a big carrier somewhere in the video band. I remember at the time several small adverts for "Negative Trap Filters" or similar.
Many were rather dubious. One guy brought one to work and it was really nothing more than a roll of micro-coax in a box with a bit of a PCB and a couple of connectors. Another used a PLL to notch out the carrier and that was a lot more effective.

After a few months they added another level of encryption, like inverting packets of data, or something. There were loads of de-crypters being sold, but gradually the encryption became too complex and thos who bought decoders eventually could not buy new key-codes for their cards.

The only sattelite TV station I ever watched in the 80's and 90's was MTV, before it got WeIrDe.

Just a little diverse information :-)

BR Harry

E.g. read this:
http://www.dewtronics.com/tutorials/descramblers/notchfilter/neghtw.pdf

Does anybody know what is this good for?