Video Scrambler

Discussion in 'The Projects Forum' started by shipley3007, Dec 14, 2013.

  1. shipley3007

    Thread Starter New Member

    Dec 8, 2013
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    Hi everyone,


    I've been looking around for sometime now for schematics on building a basic video scrambling device, maybe just a sync suppressor or an in-band sync shifter. I've found so many descrambling circuits but none so far on the actual scrambling ones. I'd appreciate it if someone could point me in the right direction. Thanks guys.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Do you want to scramble and then unscramble, or do you want to just destroy the signal?

    Do you want this to scramble the RF transmission, or at the video line level?

    I think for anyone to help you, they'll need to what you're trying to do.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    I assume this is for the old analog video signal (?).
     
  4. shipley3007

    Thread Starter New Member

    Dec 8, 2013
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    Yep! This is just for the old analog signal standards like NTSC/PAL. I would like to scramble the signal on the video line level as I mentioned before, maybe just destroy the H-sync and then reconstruct it at the receiving end. I've heard about methods that would randomly "cut-and-rotate" the video signal but from what I understood that would require a somewhat complex implementation involving a microcontroller/FPGA, unless I'm wrong?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    One very simple method is to just invert the complete video signal, such as with a fast inverting op amp (a high frequency current feedback type op amp would work well for that). The TV won't synchronize due to the inverted sync and the picture will be the negative, black for white. At the receiving end you just need the same circuit to invert the signal back to normal polarity and then feed it to the TV.

    A more complex scheme would invert the video every other field or frame. For that you would need some method to sync the inversion generator circuit with the inversion recovery circuit.

    Another scheme is to invert only the sync signals.
     
  6. shipley3007

    Thread Starter New Member

    Dec 8, 2013
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    Thanks for the input crutschow. Any chance you have a schematic for wiring up any of these circuits? I also heard that inverting the whole video signal will only partially obscure the video as modern TV tuners are capable of locking in on at least some of the video frames even with the distorted sync and colors, is that truly the case to your knowledge?
     
  7. crutschow

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    I would think that if you invert the signal, then the TV would have a significant problem in displaying any viewable picture but I have no experience with that. What is the purpose of this scrambler? Who are you trying to keep from viewing the video? How secure must it be?

    Sorry, I don't have any circuits handy for that purpose. A Google search may turn up some. An inverter could be simply made using a high speed inverting op amp with a gain of one.
     
  8. shipley3007

    Thread Starter New Member

    Dec 8, 2013
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    I don't really need it to be bit encrypted or anything, just scrambled enough to prevent anyone from picking up the signal from a distance, if that were to happen it would defeat the whole purpose of the cameras since they're mounted around the house for security purposes. I thought about using the least TX power possible by attaching attenuators to reduce the transmission distance, so someone has to be really close by to pick it up, but I'm still not very comfortable with that. There are some digital encryption systems on the market too but they're prohibitively expensive.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    My guess is that you are only going to defeat a marginal group of people. Most people won't bother trying to capture your video feed and many of the ones that do will likely be equipped to examine the signal and recognize and correct most simple scrambling methods.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Hard wiring would be a low tech but very effective solution.
     
  11. shipley3007

    Thread Starter New Member

    Dec 8, 2013
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    Unfortunately wiring is too much of a hassle right now to be an option, would be cheaper to just get some Wifi encrypted video cams and be done with it. But I was really looking forward to this being an excuse to pull out my soldering iron :D haha! I think I'm giving crutschow's idea a try. Any suggestions on an appropriate op-amp for the job?
     
  12. WBahn

    Moderator

    Mar 31, 2012
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    I agree, though it may or may not be very feasible or practical.

    Still, despite what many ads are touting in order to sell their wireless alarm systems, I am not at all convinced that wireless systems are a good choice. For homes and mom/pop shops they are probably okay since the primary threat is from technically illiterate and unsophisticated burglars. But any system that uses wireless is going to be susceptible to simple barrage jamming. Even if the system responds to jammed communications with its sensors by sending in an alarm over a hardwired circuit, all the bad guys have to do is jam the system until it sends in the alarm and then drive away. Come back a couple days later at a different time and do it again. Keep repeating that until the owner/alarm company become so frustrated they leave the alarm off until it can be "fixed".
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If I recall, isn't the bandwidth on an analog TV channel about 6 MHz wide? I built a TV back in the day, and I remember it was quite difficult to build a gain stage with flat gain over that wide of a bandwidth. It required tweaking the gain response using peaking coils.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Just about any high frequency amp such as shown here should work with well more than adequate bandwidth for the NTSC signal. If you want a DIP package for ease of assembly then that limits your selection. An example of a DIP device is the LT1252. You would want to set it for an inverting gain of 2 and use a 75Ω resistor in series with the output if you want to drive a 75Ω coax line. Otherwise set it for an inverting gain of 1.

    Note that the feedback resistor for a current-feedback-amplifier (CFA) is generally limited to a narrow range of values as shown in the data sheet in order to maintain the proper frequency response. The table on page 3 of the LT1252 shows recommended resistor values for various gains.

    To get a flat frequency response you should mount the device on a circuit board with a ground plane (can be a vector type board) with short connections for all decoupling capacitors (0.1μF ceramic in parallel with a 4.7μF electrolytic) from the supply pins to the plane. Keep all connections as short as possible and keep the output away from the input.
     
    Last edited: Dec 18, 2013
  15. shipley3007

    Thread Starter New Member

    Dec 8, 2013
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    Thanks crutschow, I really appreciate your input. I just went ahead and ordered a couple "LMH6714MA"s like the ones you mentioned, though some info on the spec sheet did not make any sense to me. I was wondering for example if a bandwidth of 400MHz and a slew rate of 1800 V/us was an overkill for this application? Keeping in mind that I would like absolutely no loss in video quality after inverting/re-inverting the signal, maybe you guys can shed some light on these specs. Also it seems to my untrained eyes that the TI ICs have the best performance among them all yet they're somehow the cheapest it seems "http://tiny.cc/3otf8w" how come?
     
  16. shipley3007

    Thread Starter New Member

    Dec 8, 2013
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    By the way I just stumbled on this module made by some German company called VTQ, the device is called "Video encryption for Micro PLL Transmitter" from what I understand it's supposed to invert the video signal supplied to it, wouldn't this module achieve the same thing I'm trying to do here? Invert the video and the sync signals?
     
  17. crutschow

    Expert

    Mar 14, 2008
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    The video bandwidth is only about 6MHz so just about any current-feedback op amp with a bandwidth of 20MHz or greater should work. The main disadvantages of bandwidth overkill is that the tendency for the amp to oscillate is somewhat higher and the noise level is also higher.

    Without knowing more abut the German device I can't tell you if it will work, but it would appear to, if it handles standard analog video signals.
     
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