Connect with us

RGB to S-video/composite using AD-725

Discussion in 'Electronic Design' started by NoSp, Sep 20, 2007.

Scroll to continue with content
  1. NoSp

    NoSp Guest

    I need to convert an RGB signal to S-video and composite. Apparently
    the Analog Devices AD-725 (
    Data_Sheets/AD725.pdf) is ideal for this.

    There's a schematic in that datasheet which I've extracted as an image
    file here:

    but as a novice there are a few things I don't quite understand:

    1) there are pins for digital and analog power supplies. Do I really
    need two different power supplies for this, or can I simply connect
    pin 4 (analog +5V) and pin 14 (digital +5V) together to a +5V supply,
    then pin 2 (analog GND) and pin 13 (digital GND) together to GND of
    the power supply?

    2) I don't quite understand the oscillator on pin 4. I need a PAL
    signal, but I'm confused about the "OSC" as it looks like a simplified
    symbol rather than a component.
    Do I simply get hold of a 17.734475 MHz oscillator as in one of those
    metallic square thingies with 4 pins, or is the schematic referring to
    a more complex circuit you have to figure out yourself?

    3) Regarding the luma-trap which is supposedly going to improve the
    signal, at the end of the circuitry connected to pin 1 (encoding
    standard) and pin 12 (luminance trap filter) it says "NTSC/PAL" at the
    end. Where do I connect this?

    4) I can't see any reason for power down the circuitry in my
    application, thus pin 5 (chip enable) isn't needed. However, the
    datasheet says that a logic high (TTL) enables the encoding function
    while a logic low powers down the chip when not in use. Should I just
    connect this to the +5V line from the power supply?
    On second thought, I see that it says "..powers down the chip *when
    not in use*". Is there some sort of detection circuitry which senses
    when a TV etc. is connected to the S-video or composite outputs?

    5) my application will "tap" the RGB and sync signals several places.
    In other words, in addition to the S-video/composite circuitry the
    AD725 will provide I will also be able to connect an RGB monitor, TV
    and (S)VGA monitor to it. I suppose I just can't simply tap into the
    RGB output of the computer for all these devices, but need to buffer
    or recondition the signals first, right?
    I believe this is what the AD-8073 (
    UploadedFiles/Data_Sheets/AD8072_8073.pdf) is for as it shows an RGB
    monitor connected to its outputs:

    a) can I use several AD8073 circuits (copying the same connection as
    shown in the schematic) connected to the RGB source, then use those
    different outputs for all the various display devices mentioned above,
    or do I need a different circuit for this?

    b) next to the RGB monitor in the schematic it says "from VGA port"
    referring to the VSYNC and HSYNC signals. Is this simply the output of
    the "VGA connector" on the left in the schematic, or is it via some
    buffering circuitry etc?

    c) does it make any difference if I connect the AD725 to the outputs
    of the AD8073, and the RGB monitor directly to VGA connector, or do I
    need to follow the schematic and connect the AD8073 directly to the
    VGA connector and have the RGB monitor go to the outputs of the
    I guess this question relates a lot to questions 5 and 5a.

    Thanks :)
  2. Guest

    Depends. How noisy is your 5V supply? Sometimes all you need is a big
    LC filter to "clean up" your supply for the analog side.
    It is an entire circuit in itself. You need to have a clock that is
    within a known error of the clock that generated the RGB signal.
    A free-running oscillator will not work properly since it will have no
    known phase or frequency relationship with the RGB's clock.
    Unless you have access to the clock signal that is driving the RGB
    part of your project, you will need something like a 4FSC genlock
    I don't know.... Good luck!
  3. Jerry Lynds

    Jerry Lynds Guest

    Do you need to use the AD-725? Check out Sony CXA series of RGB encoders.
    Visit this link for some great info with lots of documentation. These chips
    were used extensively in older video game consoles like the Sega Genesis and
    Playstation. Find an old one of these units at a thrift store and you may
    even be able to cut out the entire circuit and use it...go here for a peek

  4. Not really. You should at least LC filter the analog +5 supply.
    You need an oscillator module (yes, one of those metallic square
    For PAL you connect this to GND.
    Just connect it to +5V (digital) if you don't need it.
    Yes, but you have to be careful of the signal levels. The RGB monitor
    is 75ohm terminated, so if you don't connect it you'll get double your
    volts into the AD725.

    So if you wanted to do it right you would have 75ohm terminators on
    your board and then another RGB buffer with x2 gain and 75ohm output
    series terminators to drive your external monitor.
    The AD8073 will do the job, just configure it for +2 gain as
    Tap the signal for your Ad725 from the input to the AD8073.
    You can probably just tap it.
    As I said, you can tap the output, but if you don't connect the
    monitor then you get double the volts, not a good thing.
    Take it from the (terminated) input and you'll always get the correct
    voltage level.

  5. The AD725 does not need this, it will happily work with a free-running
    oscillator, as shown in the app circuit in the datasheet.

  6. Guest

    How is this possible? The display will not be very good then.
  7. Ask the guys who designed the chip maybe?
    It works just fine, I've used it myself.
    They wouldn't make it and advertise that it just needs a free-running
    oscillator if the quality was crap.

  8. NoSp

    NoSp Guest

    I don't know exactly what kind of supply I'll be using, but since the
    circuit is going to be used with a computer, installed inside the
    enclosure I assume I'll be getting some sort of switched mode power
    supply (the existing computer supply is too small for all the add-ons
    I'll be putting inside the rack-enclosure. It's an Atari ST computer
    by the way, in case you were wondering ;-)
    Will a normal computer power supply do for a joint +5V and GND?

    That's what I was hoping for :)
    I've finished my schematic now, which is taken from the example
    schematic in the AD-725 datasheet, but made complete and for PAL
    systems. Does it look OK?
    I've uploaded it here:

    Hmm... I'm a little uncertain about how to approach this as I have
    another schematic I'm working on which will turn the Atari-specific
    video output (demanding Atari colour or monochrome monitors) into
    selectable SCART and VGA in addition to the existing connector for
    Atari monitors, and now of course S-video and composite.
    Here's that schematic, but please disregard the AD-725 circuitry
    (which of course isn't complete) and one of the multiplexers (U1)
    which I'm still having trouble to figure out:

    I assume having the signals go through the AD8073 circuitry will
    distort it a little bit. If that's the case I would want the best
    possible signal to go directly to the (new) Atari ST video out (13-pin
    DIN) connector and the VGA output as those are the outputs I'll be
    using on a daily basis.
    I still want a good signal for those other ports, but I'll be using
    the computer mostly in "monochrome mode" as opposed to "colour mode"
    which is where the SCART, S-video and Composite outputs are used for
    TVs etc.
    Finally, there's already a composite output available directly from
    the Atari ST. I haven't yet tried it as I've heard discussions about
    it being just composite sync, or a complete composite video signal.
    If it's a complete video signal I'm not sure if I should use that for
    the composite output or use the AD-725 for composite and S-video.
    How do you suggest I approach all of this?
  9. Yep, it will do just fine. But as I and others have said, the +5V
    analog may need some filtering. A simple LC filter is fine.
    Looks ok apart from the outputs of the AD8073 shorted to ground via
    Better take out those grounds or you'll get a very black screen
    indeed :->

    Will have a look at the rest of your post when I get some more time...

  10. Guest

    They are not claiming it will work well. You are.
    Link to pictures or it didn't happen. How can you encode a color if
    your clock is free-running with regards to the source? There will be
    rainbow effects around edges and other defects.
    But lots of people are perfectly satisfied with crap, I know.
    Does it look like the picture on the right?
  11. NoSp

    NoSp Guest

    I must admit I don't quite know what an LC filter is or does, so I
    looked it up and found a schematic here:

    So basically it's a coil along the +5V line, and a couple of capacitors
    across the +5V and GND line?
    Which values should these components have?

    I also noticed that the circuit seems to need -5V in addition to +5V and
    GND. Do computer PSUs usually have all those voltages?

    Oops! ;-)
    I had a second look at the datasheet schematic, and I think I might have
    misunderstood the symbols for the *internal resistance* of the RGB
    monitor (which is indeed 75 Ohms and connected to GND).
    Does this look better?:
  12. NoSp

    NoSp Guest

    I definitely want the circuit to be as flexible as possible, so that
    everything will work equally well if or if I don't connect several
    display devices to it at once.
    I don't quite understand all the details about the doubling of the
    outputs and terminating the inputs, but I've been studying my current
    video output board circuit a bit more and have some thoughts about the
    whole thing...
    Here it is, with some comments/questions added with marked areas to
    illustrate what I'm talking about:

    If I'm not mistaken, the video signal is "tapped" 3 times:
    1) for the VGA monitor connector
    2) for the TV (SCART) connector
    3) for the AD-725 circuitry (providing S-video and Composite video)

    so I'm curious about if I need to add some additional circuitry at those
    stages to prevent the problems you've already mentioned.

    Next I'm stuck when it comes to the "U1" multiplexer as I haven't
    figured out which IC I need and how to connect it. The multiplexer is
    drawn by myself to illustrate what it's supposed to do and how it should
    work in my opinion.

    Finally, when it comes to the VGA connector (which will output a
    monochrome video signal) I thought it would be neat to add R-G-B level
    trimmers (which will be user-adjustable through the rear panel of the
    computer enclosure) so that it's easy to get a green, orange, red, blue
    or whatever colour you want for the display instead of just black/white.
    But I heard somewhere that such a setup for video is a poor solution. If
    that's the case, what should I use instead of trimmers directly at the
    R-G-B lines?
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day