Amiga RGB to YPrPb convertor - Page 3

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.

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

AmmoJammo · 22 November 2012, 03:15

Should I be adjusting R18 and R19?

kipper2k · 22 November 2012, 16:09

Quote:

Originally Posted by AmmoJammo

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

Quote:

Originally Posted by AmmoJammo

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/data...ogy/lt1253.pdf
Pinout on page 2!

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!

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:

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

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...

dJOS · 04 January 2013, 09:11

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

demolition · 04 January 2013, 09:46

Quote:

Originally Posted by AmmoJammo

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

demolition · 03 February 2013, 13:09

The gain is set the same way for current FB opamps as it is for voltage FB opamps, however you have to add a series resistor (typically around 100-500 ohms, look in the data sheet of the opamp for examples) to the input pin. Any capacitance on an input pin will cause it to oscillate, practically disabling it.

AmmoJammo · 03 February 2013, 23:41

pin 3 and pin 5?

AmmoJammo · 04 February 2013, 05:39

R22/23 were able to be adjusted to removed the red hue, but by the time the red hue is gone, so is most of the red

demolition · 04 February 2013, 09:55

Which schematic are you using?

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04 November 2012, 13:18	#41
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	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

22 November 2012, 02:48	#42
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	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. 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

22 November 2012, 03:15	#43
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	Should I be adjusting R18 and R19?

22 November 2012, 23:03	#45
Stedy Registered User Join Date: Jan 2008 Location: United Kingdom Age: 46 Posts: 733	@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

23 November 2012, 00:10	#46
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	The circuit appears to have been assembled as per the schematic... should I perhaps replace R6/R7 with a potentiometer?

24 November 2012, 04:47	#48
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	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...

24 November 2012, 05:08	#49
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	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/data...ogy/lt1253.pdf Pinout on page 2! 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! 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: 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?!?

24 November 2012, 05:41	#50
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	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...

24 November 2012, 07:14	#51
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	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...

04 January 2013, 09:11	#52
dJOS Registered User Join Date: Aug 2012 Location: Melbourne, Australia Posts: 888	How's your adapter going AJ, any more progress?

11 January 2013, 23:36	#54
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	Havent touched it. One day I'll get around to redesigning it for these different opamps.

03 February 2013, 03:49	#55
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	I dont suppose anyone could offer some input as to how to calculate the correct resistors? :P

03 February 2013, 05:45	#56
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	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

03 February 2013, 13:09	#57
demolition Unregistered User Join Date: Sep 2012 Location: Copenhagen / DK Age: 43 Posts: 4,190	The gain is set the same way for current FB opamps as it is for voltage FB opamps, however you have to add a series resistor (typically around 100-500 ohms, look in the data sheet of the opamp for examples) to the input pin. Any capacitance on an input pin will cause it to oscillate, practically disabling it.

03 February 2013, 23:41	#58
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	pin 3 and pin 5?

04 February 2013, 05:39	#59
AmmoJammo Registered User Join Date: Nov 2012 Location: Adelaide, Australia Posts: 57	R22/23 were able to be adjusted to removed the red hue, but by the time the red hue is gone, so is most of the red

04 February 2013, 09:55	#60
demolition Unregistered User Join Date: Sep 2012 Location: Copenhagen / DK Age: 43 Posts: 4,190	Which schematic are you using?

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