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Video overlay generator

Discussion in 'Electronic Design' started by [email protected], Aug 13, 2007.

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  1. Guest


    Im starting a design process for a utility that needs to implement a
    video overlay generator. Previously we've been using the "classic"
    STV5730-chip for text-overlay but since this IC is quite outdated and
    hard to get hands on we've decided to implement our own overlay
    The reason for this is that we want to be able to display graphical
    items(such as a customer logo) and also because we need delecate
    control of synchronization signals.
    The video signal is CVBS in/out and could be either PAL or NTSC.
    We've thought of using these three major parts to implement the
    design: a video decoder (f.e. Texas Instruments TVP5150) that feeds a
    digital video stream into a FPGA (f.e. Xilinx's Spartan 3), that
    performs blending between the digital video stream and a "overlay
    buffer" that gets generated from a microcontroller, and finally into a
    video encoder (f.e. Analog Devices ADV7171).

    I would really appreciate some feedback or input to this solution if
    someone out there has done something similar.

    Should I count with a noticeable video quality loss due to the AD ->
    DA conversion?

    Our application has some specific requirements, f.e. there is a frame
    counter that has to be updated each frame which probably will require
    us to implement a character generator inside the FPGA.

    Do you think this is a viable solution - is there any existing ICs
    that will do some or most of this work for me?

    Best regards,
    Andreas Eriksson
  2. Andreas

    Andreas Guest

    No suggestions?

    Best regards,
    Andreas Eriksson
  3. Most of the loss will be in the chroma-luma separation. If you can
    somehow overlay a simple color graphic by replacing the video with the
    overlay (like an OSD), then you can pass-through the CVBS and only
    change the signal where the overlay is.

    The reason video / audio decoders are going out of style is because
    now they're all custom ASICs. Even for analog signals.

  4. MooseFET

    MooseFET Guest

    This sounds to me like a very bad way to go. Don't meddle with the
    signal for the normal picture. Only produce a "difference value" in
    your circuits. This way, your DAC and ADC aren't going to mess up the
    video and you have a much simpler processing job to do.

    A multiplying DAC can control the amount of normal video you pass and
    a second DAC can set the amount you add or subtract. The one I was
    involved in many years ago just used a couple of transistors as
    switches. One switch shifted the incoming video towards the white by
    cutting down the gain and the other added signal.

    A PLL is needed to lock your timing to the video. It doesn't have to
    be super good but without it, the edge of the inserted text flickers.
  5. Guest

    Yes, this is very possible, I have done it with some simple analog
    switches (HC4053).
    In case of color, you also need to insert the right chroma phase, that
    means in PAL
    that you need to phase lock a 4.43 MHz xtal and get the H/2 info too,
    from the H PLL and V sync.
    And, depending on how you create the chroma phase, also 2 90 degrees
    shifted apart
    It is all low power analog stuff, could easily fit on one board, if
    the graphics is fixed
    (for example a logo) you could clock from an EPROM (via a PLL locked
    clock to H), or else use SRAM and provide some processor on with an
    to update the SRAM.
    The digital crude way would be to digitize, add you graphics, and make
    again, using a high end PC for enough speed to do it in real time.
    Both methods likely require some programming knowledge too.
    What method depends on quality required and power available, knowledge
    of video
    basics.... time, money.... what not.
  6. MooseFET

    MooseFET Guest

    [....Video overlay...]
    You don't want to use a HC4053. They switch very quickly. This makes
    frequencies up into the chroma carrier frequency. You want a softer
    switching so that you stop short of that.

    You want to go up to 4 times the normal frequency. It is still low
    enough that HC counters will work and you can get you phase shifted
    components by using a Johnson counter.

    I've seen boards up to 12 x 18 inches. You are right it is small but
    perhaps you should have used a different way to express it.
  7. Well, this is not the case in _my_ case.
    I used it to add text (and colored text).
    When you use a slow chip, like the old CD4053, there is a _MUCH_ to long
    switching delay, causing visible artefacs eben in BW video.

    I _had_ to use the HC4053.
    Used the circuit to add subtitles to VHS recordings before the digital
    video times.

    Some company in Amsterdam asked me if I could add colored subs that way,
    so I made some experimental setup, it worked.
    But they did not order it...

    As to frequencies in the chroma range, sure, but there is a bit more to adding
    for example text, like lead delay and tail delay (character outline),
    I created those delays with some counters (say shift registers) and fast clock.
    Would have to look it up for the details.
    But if chroma bothered you, then for example you could perhaps decide to swith on zero
    crossing of the inserted chroma.
    You _MUST_ switch within a fraction of chroma period, say well below 10 nS.

    Personally I am glad all that analog is gone, and all is digital now.
    The quality is much better, and procesing is much easier, just programing.

    If the guy just wanted _simple_ BW graphics and text he should use something like
    a SAA5246A, its does 25 lines of 40 characters in genlock on a composite signal
    PAL or NTSC.
    If you wanted to add outline and color you would need more stuff,
    the SAA has an RGB output you could use though.

    In most cases SAA5246A will be sufficient, if you can still get one.
  8. MooseFET

    MooseFET Guest

    The circuit I was involved in used a descrete transistor to do the
    switching. Actually there were two transistors. One shifted
    everything towards the white and the other adding the text.

    IIRC, the delay was one pixel of the inserted text.
    This would not work IMO. If the picture happened to have its chroma
    at its peak at the time you switched, you would still create artifacts
    at the edges.
  9. .....
    Yes there are as many ways to do that as there are semiconductors.

    Any color change in NTSC and PAL has some artefacts.
    The color bandwith is less then the BW bandwidth.
    Look at a color bar, it does just that.
    If you cut half a chroma period to zero at the top, then there would only be higher
    harmonics, double or more then the Fc, so 8.86 MHz in PAL, and way outside the video-band,
    An output lowpas (and likely any following equipment that accepts the FBAS) would remove that.

    But yes, digital is better, and if you must you can do digital RGB processing (as opposed to YUV).
    But then we are miles away from the simple solution the OP probably wants.
  10. On a sunny day (Thu, 16 Aug 2007 15:30:43 GMT) it happened Jan Panteltje

    Actually here is how I inserted the characters (horizontal timing):
    | |
    | |
    ------------- ------- character dot

    | |
    | |
    ---------- ------------ leading outline pulse

    | |
    | |
    ----------------- -------- trailing outline pulse

    ------------|||||----------- new chroma for character

    ||||||||||---------|||||||||| old chroma form picture

    The leading and trailing outline pulses are black or grey level (trimpot),
    those cause better visibility of the characters in a live background.
    Else for example white characters disappear in white background.

    The 'leading' pulse is in fact generated clocking one or more
    steps from the main pulse, the main pulse is delayed that much in a shift
    register, and after the main pulse ends, the trailing pulse is generated.
    You can get fancy by adding one line delay lines to make a vertical
    outline too.

    As you have the outline pulse remove chroma and video from the original,
    you can, by selecting the best pulse width for these, avoid color problems.
  11. MooseFET

    MooseFET Guest

    I quite agree. In principle, you could build the whole thing from
    2N2222s and 1K resistors if you used enough of them.

    Yes. The things I was refering to was the funny colors at the edge.
    When we had the switching wrong, the color would change instead of
    fading towards a lower saturation and back. It usually wasn't too bad
    but on strong colors it somethings looked funny.

    More on the self follow up.
  12. MooseFET

    MooseFET Guest

    The outline on the one I worked on was just an extra row of pixels.
    It really wasn't an outline so much as a shift to a lower saturation
    and brighter. The character's dots were darkened from that level.
    The font table, IIRC, was and even power of two tall but the actual
    characters within it were less than that.
  13. Yes, but then you have already a font with outline, and add in an analog way.
    On chips like the SAA5246A the font has no outline (but you can make a black
    background) so you have to add it yourself.
    When we go back to the fifties (last century) in my country the subtitles
    for the TV programs were printed on IBM type punch cards, a little camera was
    reading these, edges sharpend, clipped, and delay lines (analog delay lines!)
    were used to add the character outlines.
    A man pushed a button in real time, anytime a new title (=card) was needed.

    We have come a long way since that.

    I have my own software now to add subs the digital way:
    It is GPL'd so anyone can use it.
  14. MooseFET

    MooseFET Guest

    If the FPGA route was taken by the OP, I'd suggest building the
    outlining into the font. It has the advantage that you can code the
    the IBM PC.
    Have we really. When I turn off the sound on my TV, the closed
    captioning turns on. For many programs and ads the closed captioning
    reads like this:

    @[email protected]@[email protected]@[email protected]@[email protected]######@[email protected]@VA##[email protected]@[email protected]@A

    -next scene-


    -next scene-

    &&&&*******@[email protected]@AHHHHHHHHHHHHH

    .... and so on. It also leaves the aption up through several ads. It
    is sometimes leaves ones up that can be read from one ad into the
    next. This means that the car leaves your hair clean and shiny.

    Even though it is very unlikely that I will use it, I will thank you
    for adding that to what is available. In the unlikely event that I do
    need to do inserted text again, I will look it up.
  15. Yes, that can be confusing ;-) hehe.
    I see you are in the US, in Europe we have Ceefax / teletext,
    it looks like this in overlay:
    This is actually what SAA5246A output in RGB looks like
    (circuits are build in almost all analog TVs here).
    It is a very useful service, and can be used for subtitles too,
    in that case you only have some text at the bottom, with a dark background
    overlaying the picture.
    Things that are carried are the program schedules, news, weather etc.
    About 800 pages of text refreshed every minute, the signal is send
    in the vertical blanking.
    Now digital has come, Ceefax or teletext or Videotext (German) is
    just a substream in the digital transport stream, the service remains active,
    although BBC and some other UK stations no longer have it it seems.
    Maybe because it is free it never took of in the US?
    They have all sorts of adds here too that people can place on the local station.
    There are several PC versions too, in case a TV card is in the PC,
    or an external decoder, mine displays 4 pages at the time:

    You are welcome.
  16. Andreas

    Andreas Guest

    First of all, thanks a lot for the interesting discussion - at first I
    thought there would be no response, but after two days I find more
    than 10!

    Second, I would like to point out my view on going the FPGA-route.

    Using a FPGA+encoder+decoder seem to make a very flexible solution - I
    have almost all possible overlay techniques available, I could f.e.
    implement a alpha blending routine, vary the overlay resolution very
    easily and I should be able to not worry as much about flickering
    edges or trimming color/white levels.

    The cost of the system is not primary, rather reliability and picture

    The main concern of this route is that I'm not sure how much
    degradation of the video quality is to be expected(although there
    certainly are other factors like routing to take into consideration).

    Also, I'm more of a digital/firmware-guy so I feel "safer" knowing
    that I can control a lot more by firmware and HDL.

    The analog switching route also is interesting, but I feel a whole lot
    more uncertain in how to implement such a circuit reliably.
    Are there any "modern" (not to be obsoleted in the next year or so)
    circuits that are more suitable for this task that I should look for,
    or have reference designs that share my application?

    Best regards,
    Andreas Eriksson
  17. I agree you will have better control with HDL, depending upon the
    source of the video. So you're going to convert the CVBS to digital
    including the chroma sub-carrier? I've never seen it done this way,
    but it can work if you sample fast enough (~40 MSPS for a PAL signal,
    at 12 bits) if you are concerned about fidelity. You can get away with
    sampling 4X the chroma, but you can't avoid some analog processing ...
    such as clamping the signal - e.g. maintaining the DC component.

    The most difficult part might be the digital PLL. You will need to
    generate the chroma signal, assuming you need a color, not B&W
    overlay. A 32-bit NCO is probably enough. If your video is from a
    skewed time-base like a VCR, then you probably need to demodulate the
    chroma and correct the time-base first.

    If the time-base is clean, like *most* broadcast video, then you can
    get away with sampling the CVBS, re-generating the color sub-carrier
    with a DPLL, and "softly" inserting the signal digitally. If you use a
    lower sampling rate like 4x chroma, then the sample clock needs to be
    synchronous with the horizontal scan rate.

  18. MooseFET

    MooseFET Guest

    The floor clean does leave your hair clean and shiny BTW.
    Does it always completely take of the video or can it leave a dimmed
    version of the picture behind the text? One of the things I don't
    like about the system is that it covers part of the screen and you
    loose objects that travel into that area.

    Such things would never be allowed in the US. It is the law here that
    at least 83% of what is refered to as news must either be advertizing
    of programs or reports about Paris Hilton.

    In the US, networks are strickly a for profit business. The US, PBS
    system is nowhere near the BBC. If you watch a 1.5 hour movey on US
    TV, it usually takes 2 hours to watch it. They cut them down to make
    time for even more ads.
  19. It can show the picture at the same time.
    It is standard some buttons on the TV remote, so making
    it appear and go is just a button press.
    The remote has also buttons to select a page number, and
    'reveal' - 'conceal', showing hidden text parts.
    mm thought she went to jail some time ago, but yes, I have noticed
    a lot of preference treatment for her :)

    I guess (entering politics) that Hilton is there to draw attention
    away from the 4000 funerals of volunteer soldiers.
  20. Guest

    Is it really necessary to go to the trouble of converting CVBS
    directly to digital with a high speed ADC? My approach would be to
    find a circuit to decode from CVBS to digital YCbCr or RGB, inject the
    overlay information, and then re-encode back to CVBS using a video
    display circuit. Would the degradation in video quality due to
    decoding and re-encoding really be noticeable?

    Pure analog overlay injection could better preserve the video quality,
    but is the complexity of preserving the color integrity worth it? The
    Obsolete Motorola MC1378 "Color Television Composite Video Overlay
    Synchronizer" would simplify this task if it were still produced
    ( shows some inventory of these devices).

    -- David
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