Rgb to svideo convertor - cheap:-)

Discussion in 'Desktop & Laptop Computers Forum' started by Werner, May 16, 2005.

  1. Werner

    Werner
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    Well, this isnt much a question, more of a call for collaboration.

    I've been looking for an rgb to svideo convertor, but didnt feel like paying the £39 maplin wanted. I found some circuits online, but the cost fo components +time involved seemed to tally to about £39 also...not much good.

    So, here comes the new way. :D

    About a year ago I picked up a Bush IBX100 "internet set top box" for £10 from a pound store. I got the impression they were all over the place as every pound store seemed to stock them. For £10, you got a risc pc, with 8mb ram and a remote control that flipped open keyboard style. Not bad.

    Inside, you find two large pcb's, first one (the bigger one) is the actual computer part, the second pcb is a rgb to composite convertor using the AR724 chip
    http://www.analog.com/en/prod/0,,761_798_AD724,00.html

    You can see the pcb the chip is mounted on in this picture
    http://www.camilla.org/stb/I~000008-640x480.shtml
    It is the lighter green rectangular board on the right

    The Bush stb only uses composite for compatability, the chip itself can convert the rgb into svideo easily.

    I first thought of removing the chip, making the circuit according to the ad724 datasheet, but on closer examination it seems the Bush stb uses the reference design for it's own circuit , and that they just havent connected the svideo pins!!!!!!! This means, for £10, you can rip open the guts, solder a few wires (honestly) and have a rgb to svideo convertor:) :thumbsup:

    If you check here
    http://pwp.netcabo.pt/pscoelho/vgatv/ludovico/LC-AD724.html you will find the chip being used to go from vga to tv, but for this to work the vga signal has to be at tv refresh rate first (i.e. the circuit actually takes a tv standard rgb and converts to svideo. This circuit is practically mirrored in the bush stb, on an easily removable seperate pcb.

    I hope this helps someone, it is just theory so far, but my soldering iron is warming up:)
     
  2. CTID

    CTID
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    you are a mad man (and a cheap skate ;))!!! Fantastic idea, I haven't got a poundshop reound here parts tho, but if this works I'll go on the hunt for one of the STB's
     
  3. Chris Muriel

    Chris Muriel
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    Interesting idea.
    On the face of it it ought to work ok.
    I wouldn't expect "broadcast quality" as the AD724 (and AD725) were designed for low cost consumer markets.
    You have correctly deduced that, for use with input from a PC, the PC must be able to output in an interlaced mode (some of customers miss this point and come to us to ask what is wrong).

    Some other applications hints :
    It expects to see (for PAL) a 50 Hz , non-interlaced signal.
    The NTSC and PAL specs require a tolerance of +/-10Hz and most monitors require that the sub carrier frequency be within a few hundred hertz of the nominal standard. The required tolerance is therefore dependant on the monitor you intend to use and how close you wish to be to video standards, it is not limited by the AD724.
    There is one point - equalization pulses : The AD724 does not insert serration or equalization pulses. You can insert these pulses, which are at twice the normal Hsync frequency, into the HSYNC signal and they will appear on the output.
    Also : The AD724 requires that all three video inputs, R,G and B do NOT have sync on them. Therefore, sync should be removed from the Green signal before it is input to the AD724 if your input is SOG (Sync On Green) with a sync stripper .

    Chris Muriel , Manchester (who happens to work for Analog Devices).
     
  4. Werner

    Werner
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    HI Chris

    Thanks for your valued input on this...it's starting to feel like a very small world:)

    What I intend to do is to "borrow" the circuit used in the bush stb (which mimics the example circuit in the ad724 datasheet) and re-use it to connect from the rgb scart socket of an ntl /pace Di4000N cable box, have the ad724 circuit convert the signal into svideo and have that input into a capture card.

    As the bush box looks like it already fed an equivalent rgb signal to the circuit, I'm hoping that substituting the input from the IBX100 video board with that of a pace cable box (rgb, tv standard) will work ok.

    If it does, I'll be the first to be doing backflips down the high street.:)

    the only reason for doing this is that consumer rgb-->svideo convertors tend to run anywhere from £40 to £100 depending on the phases of the moon, and more than likely use this same circuit, or something equivalent to it. Not only will I save a pew pennies, but recycling some old hardware is always a good thing to me.
     
  5. CTID

    CTID
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    Chris, does this not also apply to £40 maplin cheapies as well?
     
  6. Chris Muriel

    Chris Muriel
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    I am sure it does. It's just that we also make many professional broadcast standard ICs ((less than 0.1 degree differential gain and phase error etc.) so some people expect the same from ALL our chips !
    The high quality side is why I managed to get to NAB (Las Vegas) on the company time/expenses this year as one of our video anoraks :)
    We had a couple of demo suites there where I helped out.

    Chris Muriel, Manchester.
     
  7. Werner

    Werner
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    Well, back on topic...
    It didnt work. I tried it, but no joy.

    I need to add an Lm1881 IC (8 pin jobbie) and feed it the composite signal that is also present on the scart socket. The LM1881 will then produce a sync signal from this and I can then feed the sync signal into the AD724 chip. THEN it will work :thumbsup:

    I just dont have an Lm1881, so I guess maplins here I come...
     

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