Amiga RGB to S-Video Adapter

It was months ago on one of the Amigos Podcast episodes where I heard that John and Aaron were looking for a way to connect the A500 John has to his video capture device. They had purchased an A520 TV video modulator off eBay but it arrived DOA. I heard this and thought that it couldn’t be too complicated to build something that might help out. I was taught the basics of circuit design and assembly when I was in school and other than tinkering here and there I haven’t had much opportunity to put them to use. I started doing a little exploring and found a couple of posts on the English Amiga Board (post 1, post 2) where others had been doing the same thing and posted their circuit designs. I noticed at the heart of these circuits is the Analog Devices AD724 RGB to NTSC/PAL Encoder.

I examined the datasheet for the chip and it appeared to be a relatively easy circuit to assemble. I first re-drew the circuit so I would be more familiar with it. I then put together my parts list and started ordering.

Part
Unit Cost (CAD)
Project Need
Project Cost (CAD)
SOIC16 Wide to DIP adapter
$0.54
1
$0.54
AD724 NTSC/PAL RGB encoder
$2.03
1
$2.03
4-pin s-video female socket
$0.84
1
$0.84
RCA video socket (for Composite)
$0.34
1
$0.34
0.1uF 35V tantalum capacitor
$0.24
4
$0.98
10uF 25V tantalum capacitor
$0.18
1
$0.18
220uF 35V electrolytic capacitor
$0.07
3
$0.21
75 ohm 1/4W resistor
$0.03
6
$0.18
PCB
$0.67
1
$0.67
DB-23M
$5.72
1
$5.72
3.579454MHz crystal
$0.20
1
$0.20
30pF trim capacitor
$0.07
1
$0.07
Case
$4.02
1
$4.02
Rubber grommet (5/16″)
$0.22
1
$0.22
Total
$16.20

eBay was the cheapest place to order but it took forever for shipping as everything was coming from China. Though I did manage to pick up a few things at my local electronics shop. The first thing I did was breadboard a prototype circuit. You’ll notice right away the AD724 IC is up on “stilts”. Well….that’s what happens when I don’t pay attention to the datasheet for the AD724. I ordered the SOIC to DIP16 adapter PCB (the green board) but not the “wide” version, so I had to make do with what I had. I re-ordered a new adapter board, in case I need to make any others, which is being shipped now. The parts list above shows the correct adapter board. DSC_0308_resized_thumb6I plugged in the prototype into my A1200 and my Samsung plasma TV, crossed my fingers and turned everything on. It passed the first test, no magic smoke. 🙂 And after a short moment, I had a Workbench screen on my TV! Not satisfied with a boring black-and white Workbench 3.0 screen I immediately fired up a game I have on WHDLoad. Let there be the Misadventures of Flink! The colours were nice, not a lot of bleeding and for a prototype circuit I was very happy. Next was to take the components from the breadboard and put them on a PCB. I wish I had a nice cheap way to make PCBs but I don’t, so I had to use a pre-made PCB that is usually used for prototyping. All the components fit on a 5cm (2″) x 7cm (2.75″) PCB fairly easily. I tried out the new circuit and everything was still working! Nice! I adjusted the trim capacitor (the green circular thing near the cable that is zip-tied to the PCB) a bit to clean up a bit of noise and it was great. I was surprised at how poor the composite output was compared to the S-video output. But in the time of HDMI and everything digital I guess I’m spoiled. Finally, I needed to put it in the case. I notched the PCB so that it would sit secure in the case and drilled out the holes needed to expose the S-Video and Composite video connectors. And the final product looks like, (drumroll please).

Here is some actual output I captured on my TV. Sorry for the size, but an animated GIF get’s big fast. Changes that I would like to make are:

  • Have a switchable NTSC/PAL output option.
  • Have an LED to show when the adapter is powered on.
  • Use a better composite video connector. The one I used moves around too much for my liking.
  • Buffer the video input and put a DB-23F connector on the case so that a monitor could also be connected simultaneously as a TV to the S-video.
  • Get an actual PCB made up.

But those are all tasks for another day. I sent this adapter off to John and hopefully it will allow him to produce some Amigos Plays episodes on actual Amiga hardware. Finger’s crossed that it works for him!

2017-04-03 UPDATE

On request form tankboy I’ve attached a high-res version of the circuit schematic.  This circuit is for connecting to an NTSC S-Video display.

Amiga NTSC RGB to S-Video Adapter

Modifying to work with a PAL display shouldn’t be difficult:

  • Connect pin 1 (STND) of the AD724 to ground instead of +5V.
  • Replace the 3.579545MHz crystal oscillator with a 4.433619MHz one.  Pin 15 on the video connector is also a clock signal from the Amiga, so if you don’t have a crystal oscillator you could use that in a pinch.  But I found that the clock generated from an external crystal oscillator produced a better quality image.

23 thoughts on “Amiga RGB to S-Video Adapter

  • February 8, 2017 at 7:01 am
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    You read my mind. It's on my list. I'm currently researching it. From what it looks like I'd need to not only just convert the raw RBG signal to HDMI, but also scan-double it. So it's a bit more complex of a circuit.

  • February 8, 2017 at 5:27 pm
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    Yeah except that one is way nicer. 🙂 It does look like we're using similar circuits. It's odd that the 75Ω resistors on the portion of the circuit near the DB-23 connector aren't soldered on both pads. It's like he's removed them from the circuit. I wonder why. I would think that this would throw the impedance off from what the buffer providing the RGB signals from Lisa would be expecting. Interesting…

  • April 3, 2017 at 11:27 am
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    This is an awsome project. Is it possible that you share a more high res version of your circuit schematics? It would be of big help for us DIY-ers 🙂 Cheers!

    • April 3, 2017 at 7:35 pm
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      I’ve uploaded a PDF copy of the circuit schematic. Hope you find it useful.

    • August 3, 2017 at 8:22 pm
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      Thanks! It was definitely a fun project. If you decide to give it a try and need a PAL version I could probably modify the circuit schematic.

  • September 2, 2017 at 4:17 pm
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    hey, please can u help me just to make the circuit with db15 instead of db23 thnx a lot!!!!

  • September 6, 2017 at 10:10 am
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    thanx a lot Jason, and yes a pc and i saw it while i was searching but i can not understand the part of osc where to be connected and what is that if you please make me understand this part i want it for PAL mixer to be distributed to 6 TVs thanx!!! 😀

    • September 10, 2017 at 10:59 am
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      Ah. The OSC is a FSC or 4FSC (FSC = Colour Subcarrier Frequency) clock, whereas the CRYSTAL is a 4.433620MHz crystal oscillator. The parts are used exclusively, meaning you’d either use a OSC or a CRYSTAL but not both. I’ve found that it’s way more common to use crystals and would definitely recommend it.

      I was unable to find a 4.433620MHz crystal, but 4.433619MHz crystals are all over the place on eBay for cheap (http://www.ebay.ca/itm/Lot-of-10PCS-4-433M-4-433MHz-4-433619M-4-433619MHZ-Crystal-Oscillator-HC-49S-/321424207218).

      The other part of the circuit in the datasheet that may have you curious is the use of the AD8013. From what I can see they are used to buffer the original VGA video signal and amplify it for use by a monitor. But if you’re not going to connect the PC’s VGA output to monitor and an S-Video input simultaneously you could probably not worry about that portion of the circuit.

      I’m just heading out of town for a bit, but while I’m on the road I can try and put together a circuit diagram for you. When I get back home I should be able to breadboard it and test it.

  • December 28, 2017 at 1:25 pm
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    Nice work, thanks Jason!

    It is a bit regrettable though to convert RGB to S-Video so that it can be captured by a card which will then convert it back to RGB. It is true that the loss of quality is not enormous but still this feels a bit wasteful.

    Your project reminded me of the great dbGrafxBooster made by dbElectronics (a Canadian retro electronics kitten) which takes the RGB output of a PC Engine TurboGrafx-16 and can convert to many different formats (RGB, CBVS aka composite, S-Video) and offers many different connectors to guarantee easy connection to most available retro or modern displays: https://db-electronics.ca/product/dbgrafx-booster-ttp/
    I bought one for my TurboGrafx-16 and I am very happy with it and it struck me as easily convertible to the Amiga RGB output. With only a few modifications, the PCB should be able to accept Amiga input and thus allow Amiga owners a very wide array of quality output formats and connectors.

    • December 28, 2017 at 4:05 pm
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      A TurboGrafx-16! Nice. That’s on my list if I ever run across a reasonably priced one.

      That dbGrafxBooster is a pretty cool product. Yeah I’d imagine modifying that to accept the Amiga RGB input would be pretty trivial. Just a matter of connecting the RGB and sync singals to the right connector inputs on the dbGrafxBooster. Funny thing is that I actually was considering the Sony CXA1645M (the IC the dbGrafxBooster is based on) too when I was looking at RGB encoder IC’s. I like how it buffered the RGB input and allowed for the RGB signal to be passed through the IC which would allow for simultaneous connection to an RGB monitor (like a 1084S) and an S-Video display. But I think I opted not due to cost.

      I have another project in the works for Amiga video output that I’m going to hopefully start soon. I don’t want to disclose too much (in case it doesn’t work), but hopefully I can get something cobbled together soon as a prototype. Stay tuned! 🙂

      • December 28, 2017 at 9:37 pm
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        Yes, the dbGrafxBooster is really neat. It can be connected to many C= monitors directly via either CVBS or Chroma-Luma (= S-Video) or in RGB via the “MegaDrive/Genesis” like DIN output.
        The latter is especially practical given that there are many quality MD/Genesis RGB-to-SCART cables maker out there, way more than Amiga specific cable makers.

  • April 19, 2018 at 4:28 pm
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    Nice work Jason – this looks to be just what I am looking for!

    Just one question though. Do you, or did you, have any concerns about pulling 5v from pin 23 of the monitor port? The reason I ask is that I was looking at another similar application where somebody raised the question. There being a few hundred mA capacity on that line – exceeding it could damage the Amiga. But I’m assuming your circuit must be using well within the current limits – as it appears you have been using it with no problem. Just wondering out of curiosity if you have checked the current draw.

    • April 19, 2018 at 8:56 pm
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      Do you have a source for the few hundred mA current limitation on that line? From what I can see on the A500 schematics, this line is pretty much directly connected to the +5V power rail. Which is weird indeed since it would make sense to not support high current output on such a line.

      This said, assuming that limitation stands, is is relatively easy to implement current limitation by adding a sense resistor and shutting down the circuit in case too much current ends up flowing there.

      • April 19, 2018 at 9:06 pm
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        So, I was initially looking at a way to connect my Amiga to an SVGA monitor, and came across this blog using the GBS8200, which seems to be a popular way to do this:

        http://amigamodblog.blogspot.com.au/2012/06/amiga-500-scan-doubler-vga-converter.html?m=1

        In it he is powering the circuit by pulling 5v off pin 23 as you have. A nice way to do it, rather than having a separate power supply, but then I noticed a comment by “Wally b” towards the bottom, warning about a 100mA limit, and potential damage etc. I don’t know HIS source, and I am certainly no expert – so this is why I am asking. I am just just getting into restoring and repairing some old Amigas and haven’t studied any schematics yet…

        • April 21, 2018 at 1:56 pm
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          I looked at the Hardware Reference Manual published by Commodore but they do not list any limitation for current on pin 23 of the video port. On some versions of the manuals they do list current limitations for other connectors but not the video one.

          For example, the DB9 joystick connectors are listed as 125mA max and the schematics show a 4.7 resistor between VCC and the +5V pin (pin 7) but for pin 23 of the video connector there is no such thing: it is listed as directly connected to VXX, which itself is separated from VCC only by an EMI filter.

          I suspect that whoever indicated a current limit on that pin had good reasons to do so as this seems to be good practice but so far I have not found any official document mentioning it.

          This said, it is indeed a good idea to self-limit the current, just in case.

          Sources:
          HRM: http://ada.evergreen.edu/~tc_nik/files/AmigaHardRefManual.pdf
          Schematics: http://www.amigawiki.de/dnl/schematics/A500_R6.pdf

          • April 21, 2018 at 2:49 pm
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            There are many pinouts on the net which do list these “A3000 specific” limitations but unfortunately they do not all list their sources and when they do obviously forget to add a corresponding link. 😉

            This said, the A3000 manual (cf link below) does indeed list a 100mA limit and the A3000 schematics do indeed mention a separate “+5V User” line which is connected to pin 23.

            It looks though the A3000 does indeed have different specs than the A500 as far as power output is concerned. As mentioned before, the A500 seems to use the regular power rail.

            But then again, it is probably a good idea to assume a 100mA limit for all machines, which Jason’s device seem to respect since it only draws 45mA.

            Sources:
            A3000 Schematics: http://www.amigawiki.de/dnl/schematics/A3000.pdf
            A3000 Manual: https://ia801707.us.archive.org/30/items/Introducing_the_Commodore_3000_1991_Commodore/Introducing_the_Commodore_3000_1991_Commodore.pdf

      • April 21, 2018 at 11:19 am
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        @Laurent, good point about adding current limiting with a sense resistor. I’ll have to keep that in mind for future projects.

    • April 21, 2018 at 11:17 am
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      Hi JayGee! I’m glad you liked the circuit. That is an interesting question about the current draw from the Amiga’s 5V pin. I haven’t outright measured the current draw of the circuit. I should do that sometime. But I honestly wasn’t too worried when building it because the datasheet of the AD724 indicates a current draw of about 45mA when encoding.

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