Amiga RGB to YPrPb convertor [Archive]

narmi

31 December 2009, 02:47

I started this project a long time ago, around the time I built my other adaptor (http://eab.abime.net/showthread.php?t=30752). 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.

http://www3.telus.net/narmi/circuit.jpg

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.

http://www3.telus.net/narmi/wb-composite.jpg

http://www3.telus.net/narmi/wb-component.jpg

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 :).

http://www3.telus.net/narmi/sysinfo-component.jpg

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.

dabone

31 December 2009, 04:04

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

narmi

31 December 2009, 04:28

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.

RizThomas

31 December 2009, 08:10

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

kriz

31 December 2009, 11:18

Cool project, keep us posted !

waltermixxx

02 January 2010, 03:59

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.

skan

05 January 2010, 16:33

OK, so: polish the board design a lil'bit more, put the whole stuff into an A520-like case and I'm getting one! ;)

narmi

12 January 2010, 06:45

skan

12 January 2010, 12:41

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! :p

http://bboah.com/display_photos/comm_vgaconv2.jpg

Stedy

13 January 2010, 23:04

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

EmuChicken

19 January 2010, 08:23

so, if the tv does not support 50hz (ie, PAL), then it wont display the image from this adapter?

narmi

20 January 2010, 05:45

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.

narmi

20 January 2010, 06:27

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.

http://www3.telus.net/narmi/yprpb-schem1.png

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.

pbareges

20 January 2010, 15:38

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.

narmi

21 January 2010, 04:30

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.

dw/style

22 January 2010, 01:30

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!

Stedy

22 January 2010, 22:06

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

narmi

24 January 2010, 00:13

Thank you for that information. I had not realized that the LMH6722 did not operate on +/-12V supplies, so it looks like the EL2045 (http://www.intersil.com/products/deviceinfo.asp?pn=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.

Stedy

24 January 2010, 21:42

@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

Calgor

25 January 2010, 12:30

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.

rkauer

26 January 2010, 00:53

Ian give you a good hint on where to source -5V. So here goes another couple:

1- use the 79L05 instead full-sized 7905. Less heat, less space taken and enough current (100mA at max);

2- check my old design where I want to fit the board inside the old A520. ;) There you'll find a chip who makes stable negative voltages from positive +5 or +12V.

screwtop

14 February 2010, 07:18

Nice work! I'd been looking for an RGB to YPbPr converter lately, and in the end found a ready-made commercial one, model CSY-2100 "SCART to Y-U-V Converter". I got mine from Jaycar electronics, but Google tells me there are other stockists out there too.

Input is a SCART connector, using the following pins:
Red: 0.7 Vp-p, 75 ohms
Green: 0.7 Vp-p, 75 ohms
Blue: 0.7 Vp-p, 75 ohms
Composite video (also works with Csync signal only): 1 Vp-p, 75 ohms

Output is 3 RCA jacks:
Y: 1 Vp-p, 75 ohms
B-Y: 0.7 Vp-p, 75 ohms
R-Y: 0.7 Vp-p, 75 ohms

The spec sheet says it's compatible with PAL, PAL M, PAL N, NTSC, NTSC 4.43, and SECAM. Power (0.5 A @ 7.5 V) is from an external adapter; internally it runs off +5 V.

I'll probably change the connectors on mine for better integration with the other hardware I have, but it works fine for now.

Silicon Chip magazine listed a schematic for a similar circuit (I went to Jaycar originally to buy a kit for one of these, rather than the CSY-2100, but they didn't stock the kit and it would have been more expensive anyway). Here's the teaser for the article:

http://www.siliconchip.com.au/cms/A_102648/article.html

Note that it assumes sync-on-green but there is a user-submitted mod which adds a separate composite sync input:

http://www.siliconchip.com.au/cms/A_104857/article.html

Matt020

14 February 2010, 14:41

Is it possible to explain the difference between YPrPb and s-video. Not in terms of how it works technically, but the difference in output and how it appears on a LCD or Plasma TV. Is there a display difference?

MagerValp

14 February 2010, 16:17

Basically, component video is sharper - theoretically as sharp as RGB, but on the Amiga there's a (very slight!) loss when converting from RGB to YUV color space. In S-Video the chroma component (U+V) is bandwidth limited, and is only suitable for SD content- it's barely able to keep up with 320x256. Component video has no such limit, and can easily support HD resolutions all the way up to 1920x1080, just like RGB can (typically in the form of VGA).

rkauer

14 June 2012, 08:11

As MagerValp wrote, YUV will not be better than the original RGB because the circuit will make a conversion.

Conversions from one output type to another will always make a loss in the image quality. So, beside the fact the YUV is superior to S-video (but not to original RGB), and the alarming fact that almost all TV and monitors from nowadays simply doesn't have Svideo any more, it's time to have other ways to connect our favourite computer to those.

Oh, and our good old CRT monitors will not last forever, either.

orange

01 July 2012, 01:22

upconversion can make video look better. however its probably not worth the effort with analog signal and would add some lag.

AmmoJammo

02 November 2012, 14:15

A520? That would be freaking huge :). This is the board layout I finished yesterday.

http://www3.telus.net/narmi/yprpb-brd1.png

It's about 2x2" in size. I'm still waiting for parts so that I can test it before posting the schematic.

old thread, but your opamp appears to be backwards, and you've used a 25 pin connector instead of 23 pin ;)

did you ever end up finishing this?

kipper2k

02 November 2012, 14:19

old thread, but your opamp appears to be backwards, and you've used a 25 pin connector instead of 23 pin ;)

did you ever end up finishing this?

I don't think Narmi was been online for a long time now. (2 Aug 2010)

AmmoJammo

02 November 2012, 14:25

I don't think Narmi was been online for a long time now. (2 Aug 2010)

yeah, I noticed that after posting :p

kipper2k

02 November 2012, 14:32

yeah, I noticed that after posting :p

I do have a working component video schematic that i created, i won't sell it at the moment as it seems to be very picky on the TV/Monitor it is feeding. When it does work, it works great, when it doesn't there is no picture.

It uses a pair of LT6550 chips and produces a very nice picture. The Component schematic is actually in the datasheet for them. I have the schematic built onto my Svideo boards, but just don't populate that part of the board when i sell it

AmmoJammo

02 November 2012, 14:37

this I assume?

http://circuits.linear.com/images/circuits/158_circuit_1.jpg

kipper2k

02 November 2012, 14:45

this I assume?

http://circuits.linear.com/images/circuits/158_circuit_1.jpg

Yup, and i also tied it in to the Svideo and composite video schematic too, so it does all in one.

AmmoJammo

02 November 2012, 14:52

what issues does it have on some TVs?

kipper2k

02 November 2012, 15:00

what issues does it have on some TVs?

sync on green, some TV's do not use it, so therefore this board won't work on them. I do have a Sharp multi region TV that it works on fine in Pal/NTSC but i also have a Sony Bravia that won't work. My big LG works great too. This is why i am hesitant to sell it as a component video adapter, too many possible issues

AmmoJammo

02 November 2012, 15:06

you mean "Y"?

why wouldn't the ypbpr input have the sync signal on the "Y" input? if it's not there, where is it?

AmmoJammo

03 November 2012, 06:31

I've soldered one of these up as per the schematic, but the opamp I had isn't suitable... makes a picture, but it's horrible...

So, I've got some LT1254 on order, which are the same pin configuration, suitable for video, and will accept the +-12volts from the amigas video port ;)

I might update this when they arrive...

I'd like to build one circuit into my Sega Megadrive/Genesis, probably the "prototype" board I've just finished... and try to stuff one into a D23 back shell for the amiga ;)

AmmoJammo

03 November 2012, 08:22

http://imageshack.us/a/img844/245/20121103165955.jpg

Incase you're wondering what the silver box is....

http://imageshack.us/a/img696/3333/20121103170008.jpg

9 - 18volts goes in one end... regulated +-12volts comes out the other ;) ;)

As the Sega only has +10volts DC, it's perfect!

Before I cut, drill, or modify anything, I'm going to wait for the correct opamps to arrive... I connected the circuit to the sega, and once again, got a picture, but with rather odd colours...

lesta_smsc

04 November 2012, 01:50

Think the colours can be improved by changing the resistors... Not sure if it is worth putting 'pot' resistors that you can 'tweak' to give best colour/contrast?

AmmoJammo

04 November 2012, 04:02

Think the colours can be improved by changing the resistors... Not sure if it is worth putting 'pot' resistors that you can 'tweak' to give best colour/contrast?

the opamp I was testing with doesn't have the bandwidth needed to display the image properly, once the suitable opamps arrive, I'll see what the colours are like then ;)

Stedy

04 November 2012, 13:04

sync on green, some TV's do not use it, so therefore this board won't work on them. I do have a Sharp multi region TV that it works on fine in Pal/NTSC but i also have a Sony Bravia that won't work. My big LG works great too. This is why i am hesitant to sell it as a component video adapter, too many possible issues

The issue some TVs have is with the sync signal being the wrong type. YPbPr video should have a Tri-level sync signal, the Amiga outputs a normal bi-level sync. Some TVs will cope fine, others will not, it all depends on the analog front end of the TV.

With some creative circuitry you could create a tri-level sync from the Amiga, that should help.

Ian

AmmoJammo

04 November 2012, 13:18

well, that clears that up then ;)

so, basically, if you can feed the sync+green into the green (Y) connector on your tv, and get a monochrome picture, then a circuit like this will work ;)

AmmoJammo

22 November 2012, 02:48

This is the only image I've been able to get from this circuit...

I've replaced R16, R20 and R21 with potentiometers, and that makes no difference either... were these the wrong parts to change in at attempt to adjust the gain?

I'm using an LT1254CN opamp.

http://img546.imageshack.us/img546/6826/20121122111022.jpg

http://imageshack.us/a/img708/1813/20121122111127.jpg

http://imageshack.us/a/img11/5435/20121122111142.jpg

There is absolutely no green whatsoever!? and a seemingly excessive amount of red...

it appears I had more green when using an opamp that didn't even suit video... although I couldn't actually read anything on the screen... xD

http://img842.imageshack.us/img842/6268/20121102175324.jpg

AmmoJammo

22 November 2012, 03:15

Should I be adjusting R18 and R19?

kipper2k

22 November 2012, 16:09

the opamp I was testing with doesn't have the bandwidth needed to display the image properly, once the suitable opamps arrive, I'll see what the colours are like then ;)

The LT1254s are capable of producing a good component signal output. They are the chips used in the neobitz component video board which does work although i found the colour output a little pale. I did reverse engineer the circuit and played a while but moved on to the LT6551 amps which only require a few external components

http://www.linear.com/product/LT6551

Stedy

22 November 2012, 23:03

@Amojammo

The Green video signal from the Amiga is not being fed into the adaptor properly. A pinkish screen is caused by a lack of green in the Y component of the video and CrCb being near their maximums.

Check your wiring.

Ian

AmmoJammo

23 November 2012, 00:10

The circuit appears to have been assembled as per the schematic... should I perhaps replace R6/R7 with a potentiometer?

kipper2k

23 November 2012, 00:54

The circuit appears to have been assembled as per the schematic... should I perhaps replace R6/R7 with a potentiometer?

My thoughts are the sync level voltage is either too high or too low from the amiga. Try adjusting R29 up annd down and see what happens.

AmmoJammo

24 November 2012, 04:47

I've tried replacing:

R16
R17
R18
R19
R20
R21
R29

with potentiometers, generally two at a time...not one single modification gave me ANY green, other than perhaps what my brain was telling me was meant to be green...

AmmoJammo

24 November 2012, 05:08

I found the "Colour Wheel" thing in workbench... which confused me, as I was fairly sure there was meant to be some green in there?

This is with the LT1254CN Opamp:
http://www.datasheetcatalog.org/datasheet/lineartechnology/lt1253.pdf
Pinout on page 2!

http://imageshack.us/a/img515/1585/20121124142342.jpg

So I plugged my LCD monitor back in, which has a GBS8200 built in to the base... ah yes, green... so my circuit is displaying green as red, blue, black, and purple! as well as combining multiple shades of every other colour into one colour!

http://imageshack.us/a/img543/152/20121124142543.jpg

So, I then plugged the TL074CN opamp in to the circuit, which lacks the bandwidth required to display a proper image, date sheet found here, I believe the pinouts are THE same, and I've checked it four times now!
http://ampslab.com/PDF/tl074cn.pdf

and was greeted by this:
http://imageshack.us/a/img853/1120/20121124142746.jpg

yes, it does display black as bright green... but at least it displays green?!?!
Faults LT1254? I've tried two different ICs, and they give identical results....

I think I'll order/find some different opamps, and see what happens... but can anyone explain whats going on?!?

AmmoJammo

24 November 2012, 05:41

I found that the -12volt from the Amiga video socket was dropping to -10.7volts with the load from the opamp...

Thinking this could be the issue, I wired up a little +12volt to +-12volt module, now I have identical +-12volt rails.. but no difference to the video...

AmmoJammo

24 November 2012, 07:14

dJOS

04 January 2013, 09:11

How's your adapter going AJ, any more progress?

demolition

04 January 2013, 09:46

The EL2045 opamps originally used in this circuit are VOLTAGE FEEDBACK... while the TL1253 (and even the LMH6722 suggested earlier on) are CURRENT FEEDBACK!

they're completely different! and it explains why the TL074 gives correct colours, as it's also voltage feedback...

You can modify the schematic to make it work with current feedback opamps. The most important thing when using current feedback is that you cannot have any capacitance directly on any input pins. You need to put a resistor in series with the input pins. Even the capacitance from a ground plane below the opamp can cause it to become unstable. Also, decoupling is very important for these high speed opamps.

AmmoJammo

11 January 2013, 23:36

Havent touched it.
One day I'll get around to redesigning it for these different opamps.

AmmoJammo

03 February 2013, 03:49

I dont suppose anyone could offer some input as to how to calculate the correct resistors? :P

AmmoJammo

03 February 2013, 05:45

Adjusting R6/R7 gave it green, I added a 10k pot in parallel, but its a very fine line between right and wrong...

adjust the pot too low, there's no green.... but adjust it too high, and there's still no green!

Adjusting R4/R5 and R8/R9 make no difference, and the white still has a pink hue...

I'll be honest, I'm very impatient, I spent some time trying to find out how to set the gain of current feedback op amps.. but my brain just can't do it :p

http://imageshack.us/a/img534/6539/20130203145633.jpg

http://imageshack.us/a/img835/6949/20130203145842.jpg