composite video signal amplifier

Discussion in 'The Projects Forum' started by jay.diem, Jan 12, 2009.

  1. jay.diem

    Thread Starter Member

    Jan 12, 2009
    10
    0
    Hello,

    I've been browsing here for last week or so, but decided today to join up in the community. :)

    I"m pretty new to this forum and just recently started to tinker with some electronics for my car.

    I'm currently struggling with an issue with a very weak composite video signal from an SD Card TV output player (bought it cheap on ebay). Pretty much my project is to output the signal from this card to a mobile video monitor unit in my car.

    The signal travels from this TV-out card, to a modulator box, and then to the video head unit inside my car. I was told by the vendor of this modulator box that if the tv-out signal is too weak, it will result in pure black and white picture, as I was able to find out shortly after my installation :(

    When I bring this tv-out unit and hook it up to my TV set inside my home, I am able to see full color picture just fine.

    To also test the modulator unit, I also hooked up my home dvd player to it, and it was able to produce color on my car's navigation display just fine.

    My question is : how can I produce a cheap, yet homemade in-line amplifier circuit so that the levels of this tv-out card would be high enough to produce a color picture? This amplifier would preferably be powered via 5VDC (voltage of tv-out card) , or alternatively 12VDC of car.

    Hope I explained everything right, please let me know if you need more info.

    Thank you, all!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Here is an apllication note from National about a video opamp.

    Greetings,
    Bertus
     
  3. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    As the display of a car is not big, why try not to get an analog line out video signal.. It would be much easier to handle.. Does it have one??
     
  4. jay.diem

    Thread Starter Member

    Jan 12, 2009
    10
    0
    Bertus, thanks for your input, I will definitely check this out.


    jj, there's only one video output of this SD card TV-out device and it is a 3.5mm jack connector. It is a composite signal (I assume this is already analog), and has an RCA adapter cable that you can simply connect to any RCA input of standard TV.

    The modulator unit I before mentioned converts this analog composite signal to digital so that my OEM nav head unit can display it.

    How can I handle this easily assuming the signal coming out of my TV-out device is already analog?
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Your problem may be nothing to do with signal strength.

    It may be to do with signal polarity.

    There are two standards, one using a positive going signal, the other a negative going one.

    Cross connection will result in a monochrome picture or no picture.

    If this is the case you need an invertor, not an amplifier.

    Read your manuals
     
  6. jay.diem

    Thread Starter Member

    Jan 12, 2009
    10
    0
    studiot,

    yes I suspected this could have been the case... hence as mentioned in original post i tried plugging the tv-out card to my tv, and was able to get color...

    in addition, i hooked up a regular dvd player (and also another different model SD tv-out card) to the
    car mobile unit and was also able to get color picture.
     
  7. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
  8. jay.diem

    Thread Starter Member

    Jan 12, 2009
    10
    0
    I appreciate everyone for their help.

    I did some research and was able to find this amplifier that should work well as a line driver. I just have trouble figuring out what I need to buy in addition to this amplifer in order to apply the circuit shown in figure 1 of this schematic?
    http://www.analog.com/static/imported-files/data_sheets/AD828.pdf

    What does the Rt and the Rbt figures mean? Can I assume that my TV-out card is already terminated at the source? How many 75ohm resistors do I need to buy?

    Also, should I be able to just hook up pin4 directly to ground without the decoupling caps since I'm just applying the single +5V supply?
     
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    In the U.S., there's only one standard, and it's NTSC, maximum black is maximum voltage. Most receivers blank the color if the color burst is less than 50% of nomnal.

    Most video amplifiers also have DC clamping circuits, as well, which generally cease to function properly if the video level is less than half of nominal. What you really need is a "proc amp" which regenerates all the components separately.

    eric
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Perhaps I should have been clearer, but I was away from my pigsty so I had to rush things.

    By standards I do not mean video encoding standards.

    By the way KL7AJ you also have more modern HD standards in the US.

    I mean plug and socket wiring standards and also signal levels. I have seen equipment that will accept or produce negative CVBS and/or Sync and other equipment positive CVBS and/or Sync. On The Hitachi studio monitor at Studio T there is a switch for just this purpose.

    Again, whilst most kit is content with 1v pk - pk, some accept produce more or less
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Composite video usually has sync tip as the most negative voltage, with white being the most positive luminance voltage.

     
  12. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    By the way did you find your amp??
     
  13. jay.diem

    Thread Starter Member

    Jan 12, 2009
    10
    0
    Eric,

    Both my HDTV and 10yr old magnavox was able to produce a color image, so I hope that the signal is sufficient enough levels (although a little low). I'll head to my university's lab later in the week to confirm.

    JJ,

    I decided to go with the following amplifer (see picture):
    http://www.analog.com/static/imported-files/data_sheets/AD828.pdf

    Can you explain to me the Rbt and Rt resistors (what are they)?

    Where is the amplified video out signal? At the 75ohm load?
     
  14. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    Rt is the Pull down resistor used to keep the output at sufficent levels and Rbt is for output impedance matching at 75ohms output. Normally the signals are taken at 75ohms..
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Actually, both are impedance-matching resistors. Rt is the (load) termination resistor, and Rbt is the back termination resistor.
    The little tube labeled 75Ω is an arbitrary length of coaxial cable. The coax gets your signal from the source to the destination with a minimum of crosstalk, because it is shielded. Coax has a property called Characteristic Impedance (Z0). If the coax is not terminated in Z0 on at least one end, reflections will occur with long lengths of coax. It is safer to terminate both ends in Z0, because you do not always have control of the other end. Also, if the load is highly reactive (usually capacitive), Rt alone may not provide adequate termination.
    For these reasons, both ends are generally terminated, unless the coax is only a few feet long (reflections get worse with length).
     
  16. jay.diem

    Thread Starter Member

    Jan 12, 2009
    10
    0
    Ron,

    Excellent, this really clears it up for me.

    The cable to be used is actually a composite video (RCA-yellow) connector. I assume this also has Zo of 75ohms?
     
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Probably not. How long is your cable?
     
  18. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287

    Should have specified that to mean one ANALOG standard....which is going away in a number of weeks!

    Also should mention that in NTSC, the maximum TRANSMITTER power (sync tip (extra black!) ) is actually zero volts at baseband. This is sometimes a point of confusion...and different from the Pal system, where maximum transmitter power is maximum white level.

    Again, this will all be a moot point when the FCC finally gives analog TV the lethal injection. Well it was fun while it lasted. *sigh


    Eric
     
  19. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
  20. jay.diem

    Thread Starter Member

    Jan 12, 2009
    10
    0
    About 3 inches...(after some splicing)...

    Originally it was 3 feet, but I found the noise was drastically reduced after I made it shorter.
     
Loading...