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Old 31 December 2009, 02:47   #1
narmi
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Amiga RGB to YPrPb convertor

I started this project a long time ago, around the time I built my other adaptor. But this one is better. Finally, I can connect my Miggy to my lovely HDTV and view a full quality 480i image!

Here's a pic of the prototype. The circuit uses four EL2045 op-amps and a whole bunch of resistors to implement the math to convert RGB to YPrPb, plus a few extra elements to add CSync to the Y output.



Power comes directly from the Amiga's video port which has +/-12V available. I took some pictures of the output with my camera, but they don't show the difference very well. The first two pics show the top left corner of the Workbench. The top image is the A600's composite output while the bottom shows the output from my adaptor. The colour is different, and the text is sharper in the bottom image. I may have to tweak the circuit to get the colours right - they're slightly off.





In both of the above pictures, the resolution is NTSC Hires-Interlaced. This resolution is mostly useless with the composite output, but looks just fine with the component video output. The picture of SysInfo below shows how readable the text is. Unfortunately I forgot to take a comparison shot, so you'll have to trust me on this one .



I did take more pictures, including me playing Megaball, but they didn't come out so great. It's hard to photograph a CRT (yes, my HDTV is a big honking CRT that weighs more than I do). I'll try to get my hands on a Plasma or LCD TV in the next few days.

I've ordered a different op-amp which has four op-amps in one chip. That will make the circuit much more compact. Once I get that chip I'll build a prototype around it. If that works, I'll draw up everything in EAGLE and upload my project files here. Right now everything is recorded on paper only.

This circuit is MUCH CHEAPER to build than the s-video adapter. The AD724 chip in that adaptor costs $14. The op-amp I plan to use is $3. The total circuit cost is much less than half of the s-video adaptor, and the picture quality is much higher. Even better, the circuit is standard-agnostic. It should work for NTSC and PAL without any modifications or switches.
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Old 31 December 2009, 04:04   #2
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Could we use this

To display pal on a NTSC set?

(Sucks having an amiga and living in the usa.. My 1084s is so tiny, and flickerfixers are SO expensive)


Later,
dabone
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Old 31 December 2009, 04:28   #3
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No, this circuit won't convert between PAL and NTSC, but if you had a TV which would sync to a PAL signal fed in via the component inputs, this would allow you to connect your Amiga io it.

That reminds me, I should connect my A500+ (PAL) to my TV and see if my TV will display the PAL video.
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Old 31 December 2009, 08:10   #4
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Glad to see you did it again Narmi...looking forward to this project. I still have your adapter that you sent me and it is still working great!!

Have a good New Year

Rizal
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Old 31 December 2009, 11:18   #5
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Cool project, keep us posted !
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Old 02 January 2010, 03:59   #6
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Very Cool...

I would be very interested in building this...
I just downloaded eagle cad software (free trial version, because
I'm designing a circuite board for my Cosmac Elf project)...

and would love to build one of these RGB to Component converters for my Amiga 500 in the mean time...

Keep up the good work... I was going to build the RGB to Svideo project when I saw this one... this one looks very impressive...

Cheers!!!!

Enjoy the Vancouver Games!!!!

(we have an office in Vancouver and I've been there several times..)

cheers.
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Old 05 January 2010, 16:33   #7
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OK, so: polish the board design a lil'bit more, put the whole stuff into an A520-like case and I'm getting one!
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Old 12 January 2010, 06:45   #8
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A520? That would be freaking huge . This is the board layout I finished yesterday.



It's about 2x2" in size. I'm still waiting for parts so that I can test it before posting the schematic.
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Old 12 January 2010, 12:41   #9
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Quote:
Originally Posted by narmi View Post
A520? That would be freaking huge . This is the board layout I finished yesterday.

It's about 2x2" in size. I'm still waiting for parts so that I can test it before posting the schematic.
Ooops! Sorry, I was thinking of the VGA dongle actually, not A520!

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Old 13 January 2010, 23:04   #10
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Hi Narmi,

Your board looks good, the results impressive.

The slightly off colours could be due to the breadboard construction of your prototype. Breadboards are not good for high speed analogue signals like video. Hopefully your PCB prototype will be as good, if not better.

A suggestion to make on the PCB, add some bulk decoupling (10-22uF) capacitors. When video amplifiers switch they draw a lot of power.

How do you create the Y signal, is it 0.6 Green + 0.3 Red + 0.1 blue + the sync pulses from the composite video signal?

When you have finished the design, I will be interested to see the schematics.

Keep up the good work,

Ian
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Old 19 January 2010, 08:23   #11
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so, if the tv does not support 50hz (ie, PAL), then it wont display the image from this adapter?
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Old 20 January 2010, 05:45   #12
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The output from this adapter has the same timing as the input from the Amiga. The only change is a color space conversion from RGB to YPrPb. It's purely mathematical. Here are the equations I have used:

Y = 0.30R + 0.58G + 0.11B
Pr = 0.71(R - Y)
Pb = 0.57(B - Y)

The circuit computes Y first, and then uses it to compute Pr and Pb. There is no sync information in the RGB inputs, so I add CSYNC to the Y output.

None of the signal information is lost, so the picture quality is on par with RGB. This is as good as it gets.

Last edited by Jope; 21 January 2010 at 12:24. Reason: removed mention to a deleted post
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Old 20 January 2010, 06:27   #13
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If you want to build it yourself, here is the schematic I copied from my notebook. The only difference between this and the prototype is the op-amp. The prototype uses four single op-amps while I plan to use one quad op-amp in the finished product. This change saves money and board space.



Amiga RGB -> YPrPb Adapter Parts List

IC1 - LMH6722 Quad Wideband Op-Amp

C1 - 1uF 35V Radial Capacitor Tantalum
C2 - 1uF 35V Radial Capacitor Tantalum
C3 - 0.0047uF 50V Ceramic Capacitor
C4 - 0.0047uF 50V Ceramic Capacitor

R1 - 6.8kohm 1% Resistor
R2 - 6.8kohm 1% Resistor
R3 - 1.3kohm 1% Resistor
R4 - 12kohm 1% Resistor
R5 - 8.2kohm 1% Resistor
R6 - 3.9kohm 1% Resistor
R7 - 1.8kohm 1% Resistor
R8 - 5.6kohm 1% Resistor
R9 - 5.6kohm 1% Resistor
R10 - 75ohm 1% Resistor
R11 - 75ohm 1% Resistor
R12 - 75ohm 1% Resistor
R13 - 1.0kohm 1% Resistor
R14 - 3.0kohm 1% Resistor
R15 - 3.0kohm 1% Resistor
R16 - 3.0kohm 1% Resistor
R17 - 75ohm 1% Resistor
R18 - 360ohm 1% Resistor
R19 - 360ohm 1% Resistor
R20 - 360ohm 1% Resistor
R21 - 360ohm 1% Resistor
R22 - 68ohm 1% Resistor
R23 - 39ohm 1% Resistor
R24 - 220ohm 1% Resistor
R25 - 39ohm 1% Resistor
R26 - 130ohm 1% Resistor
R27 - 75ohm 1% Resistor
R28 - 100ohm 1% Resistor
R29 - 680ohm 1% Resistor

You can substitute the LMH6722 with another op-amp suitable for video (i.e. wide bandwidth). Something like the LF347 will not do because it's bandwidth is too narrow.
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Old 20 January 2010, 15:38   #14
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hi my friend,

would you be willing to build and sell such units ? i'm really eager to put my hands on one of these!!! can you tell what you estimate would be a faire price (i don't ask you to commit, i just want to get an idea) ?

keep up the good job!

best of luck to you.
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Old 21 January 2010, 04:30   #15
narmi
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Can we keep this thread on topic please?

@pbareges: I haven't yet figured out how much these will cost to build. When I do I'll post that here. I'm willing to build and sell them provided that they're not too hard to build, and I can get all the parts easily.
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Old 22 January 2010, 01:30   #16
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A word of caution

Hello

A friend directed me to this post because I wanted to build an adapter similar to this some time ago.

One of the things that made me lose interest in the project was that apparently not all amigas have -12v on the rgb out connector ( at least according to http://pinouts.ru/Video/AmigaVideo_pinout.shtml ). Also, the ones that do seem to have a maximum current of 10mA on the -12V.

I was looking into using the MAX4451 quad op-amp, but it's not available in DIP format. The LMH6722 you're using looks very interesting, but according to the datasheet on National's site, it's a +-5V part, so you might want to double check that.

Hope this helps, and hope you finish this project. The image quality from this should be excellent!
 
Old 22 January 2010, 22:06   #17
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Hi

@dw/style

You are indeed correct, the A3000 has +/-5 and +12V on the video port, the A600/A1200 have +/-12V and +5V on the video port. Confirmed this by inspection of the schematics.

Could not find any current limiting components on the supply rails for the video port.

The LMH6722 does need +/-5V supplies and it must not be used with the resistors shown in the schematic, it will go unstable! The LMH6722 is only characterised with a reedback resistor, Rf of 300 ohms, I know from experience, that even 1K feedback caused the amplifier response to behave a little wildl at PAL video frequencies as the gain was not flat.

@thread
You have to be very careful with video amplifiers, they are fast and power hungry. A quad op-amp like the LMH6722 driving 3x75 ohm loads will draw 45mA to drive the output loads and another 15-20mA to supply the internals so if there is a limit on the power pins, beware and add more decoupling.

Some amplifiers are only stable with a gain of 2, others have weird feedback networks and will only work with the values shown in the datasheet.

Bye,

Ian
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Old 24 January 2010, 00:13   #18
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Thank you for that information. I had not realized that the LMH6722 did not operate on +/-12V supplies, so it looks like the EL2045 is what I should stick with. I would rather have a quad op-amp to keep the part count down, but it looks like that is not possible.

I'm glad I only ordered samples of the LMH6722 instead of buying parts . It's time to revise the schematic and create a new PCB layout.
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Old 24 January 2010, 21:42   #19
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@Narmi,

There is a simple solution to allow continued use of the LMH6722, provision for a 7905 -5v 1 Amp linear regulator. Add ajumper link, one option connects the -5V of the A3000/A4000 etc if present to the board, the other connects the -12V supply to the input of the 7905 and it's output supplies -5V to the board.

The existing +5V supply from the amiga is of course used.

If you change R14-R16 from 3K ohms to 330 ohms you will improve the stability of IC1D.
R1-R2 around IC1C need changing to approx 300 ohms but you then need to re-check the gain calculations.

Good luck,

Ian
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Old 25 January 2010, 12:30   #20
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The user manuals usually have the maximum rated amperes specified. Whether they are correct or reflect empirical reality, not sure.

I have seen most models specified 100mA max on positive voltages, and mostly 10mA on negative pin 21, be it -12v or -5v on different models. A3000 manual has an error in it on the negative voltage (incorrectly says both -12v and -5v on pin 21, so also not sure to believe the higher 50mA figure)

I think A1000 may have had different ampere ratings.
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