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Old 09 July 2009, 23:46   #1
Rabbit80
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Increasing voltage on RGB lines to 1V

We (A friend of mine) have followed the instructions on http://members.iinet.net.au/~davem2/overclock/a520.html to make my A520 into s-video port, but the colours are washed out as it needs 1V for maximum saturation on the RGB lines.

Can somebody point me to a simple circuit to increase the voltage from 0.7 to 1 volt? (maybe using an op-amp)

Cheers

Rabbit
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Old 09 July 2009, 23:49   #2
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Use a transistor and a pull up.

Pretty sure that the analysis of why the colours are washed out is wrong. Very unlikely that a part would require 1v when the industry standard is 0.7v
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Old 09 July 2009, 23:53   #3
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No - the MC1377 in the A520 was designed to work with 1V for maximum saturation..
http://www.datasheetcatalog.org/data...a/MC1377DW.pdf

Quote:
R 3 Red signal input. Presents 10 kW impedance to input. 1.0 Vpp required for 100% saturation.
G 4 Green signal input. Presents 10 kW impedance to input. 1.0 Vpp required for 100% saturation.
B 5 Blue signal Input. Presents 10 kW impedance to input. 1.0 Vpp required for 100% saturation.
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Old 09 July 2009, 23:55   #4
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I know that is what it says in the article but I find it very hard to believe. The industry standard is 0.7v so it would be pretty crap for interfacing to anything.
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Old 09 July 2009, 23:56   #5
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Its in the datasheet for the encoder as well...
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Old 09 July 2009, 23:58   #6
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Such a crazy part! Almost zero uses in real world TV electronics without additional components.
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Old 10 July 2009, 00:00   #7
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OK - so i need a circuit diagram for a transistor pull up - if possible please
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Old 10 July 2009, 15:08   #8
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Anyone?
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Old 10 July 2009, 21:31   #9
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Old 10 July 2009, 22:15   #10
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Google is your friend....
Google is my enemy I don't have a clue how to do it
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Old 10 July 2009, 22:15   #11
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Hi,

Find the three 75 ohm terminating resistors on the RGB inputs.
Change these to 150 ohms and you will have RGB levels of around 940mV peak to peak.

Ian
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Old 11 July 2009, 00:02   #12
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I've been playing with Yenka - going to use a 324 op-amp to do this...
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Old 11 July 2009, 00:39   #13
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@Rabbit80,

An (LM)324 is most definitely not suitable for video, it is too slow. If you use it your picture will be fuzzy and colours poorly defined.

To amplify video signals, you need an amplifier with a minimum slew rate of 15 V/us, the LM324 only has a slew rate of 0.5V/us. This is calculated based on the fact that for PAL/NTSC resolutions, a single pixel is 68ns. You need an Op-amp that can go from 0 to 1V in 68ns or better. The LM324 can get to 34mV in 68ns. To calculate this, take the slew rate in V/us and divide by 100 then multiply by 68.

I can't remember the part number of the amplifiers we use at work for video circuits, I can check tomorrow when in the office.

Try my earlier suggestion, locate the termination resistors in the A520 and change them and let Ohm's law do the work. A video signal is generated at 2x the required amplitude. The transmitter and cable medium have an impedance of 75 ohms, at the receiver there is a 75 ohm terminator. Using a derivation of Ohm's law and allowing for a 1.4V video signal from the Amiga (I have verified this) you get, Vo = Rterm/(Rin+Rterm) * Vi where
Rin is the input resistance (Amiga and cable) and Rterm is the termination resistor in the A520. When both resistors are equal you get 0.5x Vin as your output. Changing Rterm to 150 ohms gives 0.93x Vin of 930mV. I chose 150 ohms as it is a standard E12 series resistor value.

Ian
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Old 11 July 2009, 00:47   #14
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OK - thanks... Which resistors are they?

How about the TL074 slew 13v/us? is that close enough?

Last edited by Rabbit80; 11 July 2009 at 02:25.
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Old 11 July 2009, 19:23   #15
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Good but not enough!

Check an old thread of mine where you see what I used to output YUV (video component) from the Amiga.

The better part I found was the AD818, excellent slew rate, good price, "normal" voltage requirements (+5 & -5V).

Last edited by rkauer; 11 July 2009 at 19:37.
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Old 11 July 2009, 20:01   #16
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OK - thanks... Which resistors are they?
Looking at http://members.iinet.net.au/~davem2/overclock/a520a.gif, the terminating resistors are probably the three mounted vertically, towards the top left of the picture (in between a hole and a row of three capacitors).
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Old 11 July 2009, 20:34   #17
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Whip them out and replace them with wires and see if that helps

I wonder if it were as simple as that would "Dave" not have realised instead of modding his A500?
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Old 12 July 2009, 16:30   #18
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@Rabbit80

Don't waste your time trying to boost the input level to the A520, Commodore have already done it.

I found an A520 in my Amiga junk box, opened it up and traced the input circuitry, the termination network provided 1V into the MC1377, using the same method as I suggest. The input termination network is a potential divider with 330 ohms and 1K ohms to ground. At the centre point you will get 0.75x Vin. Vin is 1.4V peak to peak for an unterminated video signal, so you should get 1.05V for peak white.

If I get the time, I will verify this with my oscilloscope.

I have started working on a video interfacing guide for the Amiga, covering RGB to SCART and will also look at Y/C adapters and general video issues with LCD TVs. It is not ready yet. The guide will be on my website when complete.

Bye,

Ian
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Old 01 August 2009, 17:51   #19
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@Stedy:
Good initiative!
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Old 01 August 2009, 23:07   #20
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Quote:
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I have started working on a video interfacing guide for the Amiga, covering RGB to SCART and will also look at Y/C adapters and general video issues with LCD TVs. It is not ready yet. The guide will be on my website when complete.

Bye,

Ian
Hi Ian,
could you specify (for those of us who don't remember it) your website address?
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