Arcade Monitor Interface Circuits

An arcade monitor is a colour video monitor in it's simplest form. Feed it analog red, green and blue video and they are amplified and drive the CRT. Feed it horizontal and vertical sync pulses, they are used to synchronise the video to the scan circuits that draw the raster. Some have more features, but that's pretty much it.

Arcade monitors make excellent monitors for arcade games (obviously), video game consoles and emulated arcade and console games on a PC (MAME cabs). Here are some characteristics of these video 'standards'.

Arcade video (standard resolution)
Video amplitude: 2-5Vpp
Video DC offset: yes, as much as a volt or two
Video input impedance: 1k-10k
separate horizontal and vertical sync (most of the time)
sync polarity negative (most of the time)

PC video (VGA, running special software)
Video amplitude: 0.7Vpp
Video DC offset: yes
Video input impedance: 75
separate horizontal and vertical sync
sync polarity changes (can be set in software)

SCART video (RGB SCART)
Video amplitude: 0.7Vpp
Video DC offset: no DC offset, AC coupled
Video input impedance: 75
sync signals stripped from Composite video (CVBS)
sync polarity negative

The output of many VGA cards will rise to ~1.4Vpp in the absence of a 75 ohm termination resistor. This may be enough to drive a sensitive arcade monitor without any adaptation (so try it first!). If the picture is lacking in contrast then build the Direct Coupled Video Amplifier to amplify the RGB video signals.  The sync signals can be set to the correct polarity in software or by using this Sync Fix Circuit.

One problem that comes up when connecting a PC to an arcade monitor (or any low frequency monitor) is that unpredictable things can happen if the monitor is connected while the special software hasn't been loaded, esp. during the time it takes for the computer to boot. In the worst case this can result in damage to the monitor. My next circuit, Intelligent Direct Coupled Video Amplifier, overcomes this problem by monitoring the signals from the video card and blocking them if they are unsuitable for the monitor.

The third and most complicated circuit is for anyone wanting to connect RGB SCART devices such as video game consoles, DVD players and 80's microcomputers to an arcade monitor. The SCART video differs from the PC video in that it is AC coupled, meaning that there is no DC component in the video signal. If this AC coupled video was to simply be amplified and fed to the input of the monitor then the resulting picture brightness would vary with the picture content. To fix this the signal is clamped to a fixed reference voltage before it's amplified. The way the sync signals are obtained is different too. In this case the sync signals have to be stripped off a composite video signal before being separated into horizontal and vertical TTL compatible sync signals (that the monitor wants). Behold the AC Coupled Video Amplifier.

enough waffle, on to...The Circuits

DC Coupled Video Amp
Equivalents: if no BC548 then use BC549, PN100, 2N3904 or 2SC1815 (i.e. pretty much any general purpose NPN transistor with a beta > 100).

Input should be 0.7Vpp 75ohm video. Output is 4Vpp with about half a volt of DC offset added. This amplifier is can be built out of parts from the junk box. Even if you have to buy the parts, they shouldn't cost any more than $3 in total. Parts List

This amplifier has a very low output impedance, just a few ohms. This can cause problems (excessive contrast mainly) with some monitors that expect to be connected to high impedance sources. To fix this, simply add a resistor in series with each colour output and experiment until you find a suitable value (270 ohms is a good start).

There's three amplifiers, one for each colour. Each amplifier consists of two stages. The first stage is a common base configuration with a gain of about 6 and an input impedance of 75 ohms. The second stage is a buffer, needed because of the first stage's high output resistance. The two diodes in the buffer stage are there to drop the DC offset to an acceptable level.

Intelligent DC Copuled Video Amp
Equivalents: IC1 can be either a 12C508 or 12C509, see previous circuit info.

This circuit employs a microcontroller to monitor the frequency of the horizontal sync. If it gets any higher than ~17khz both the horizontal and vertical sync signals will be blocked (via IC2) and the video will be blanked (Q1, Q3, Q5 are biased into saturation). Led indicates status (on = signal blocked). Other than this it's the same as the previous circuit.

If this 'intelligent' function is required and you're scared of microcontrollers then see other methods of achieving the same result here.

Parts List
Binary (hex) to program pic microcontroller.
asm source code

Here is an ultra crude (but fully functional) sync combiner circuit that can be used in many places (including the above circuit) where composite sync is required. The only requirement is that both the horizontal and vertical sync signals must be negative.
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And finaly, the all powerful...

AC Coupled Video Amplifier (SCART to arcade monitor adaptor)
This one's got everything! Video amplifier, video clamp, sync stripper and sync seperator. What more could you want??? Basicaly it does just what it says: facilitates the connection of any RGB SCART (or any video device that outputs RS-170/0.7Vpp RGB video) er..thing, especialy video game consoles to an arcade monitor. The most importaint difference between this circuit and the previous one is the inclusion of a video clamp (aka DC restoration circuit). This will clamp the video signal to a specified voltage during the horizontal retrace period through the use of a cmos analog switch. If this isn't done then the video brightness would fluctuate whenever the video signal changes.

VR1 is there to adjust the bias of the video amplifiers and add or reduce any DC offset in the output siginal (variable between +2v and -1v). The gain of the video amplifiers is fixed at about 5. This can be modified, if necessary, by changing ratio of R2 to R3 (and equivalent resistors in the other amps). IC3 and associated components (C7,C8,R17,R18) can be omitted if a horizontal sync output isn't required. One-shot 3A has a duration of 48ÁS and 3B goes for 5ÁS. Electro caps are 16v unless otherwise stated.

And, as a special treat, here's a power supply to go with it...

Power Supply for AC Coupled Video Amp
T1 is a small, cheapo >3VA (at least 130mA) 24V power transformer with a centre tap on the secondry winding. It's a simple linear design. The video amp doesn't draw much current so the regulators should be ok without heatsinks.

The video amp can also be powered from a standard computer/jamma power supply. Just put a couple of diodes in series with the 12v line to knock it down a bit.

acpcb.jpg (7453 bytes)
I've designed a PCB for this circuit, power supply included. Download...
PCB artwork in PNG format - Print at 600 DPI
PCB atrwork in Protel Autotrax format
PCB overlay diagram
Photograph of my prototype
Photo of the solder side of my protoype - I had to fix a couple of routing errors.

Resources

Special Software for adjusting video timings to suit arcade monitors (15.7khz horiz, 50-60Hz vert)
Advance Projects - home of AdvanceMAME (arcade emulator) & AdvanceVGA/VBE (dos tsr)
PC2JAMMA - home of ArcadeOS and Vantage (another arcade emulator)
EnTech's Powerstrip for use with microsoft windows
VGATV dos tsr
SciTech Display Doctor and UNIVBE - both are now free to download.
Mon-ARC - another vga > 15khz TSR (works very well with a supported video card)
Raine - Another multi arcade game emulator

Other
Game Console SCART Cable Diagrams - Construction of game console SCART leads.
Game Station X - game console stuff...with forums and stuff
Ultimarc - they sell video cards, etc. see their Monitor FAQ
Monitor Tube Swap - why not
Get A Grip On Clamps, Bias and AC-Coupled Video Signals - Maxim app. note

Changelog
1/9/06 - Added PCB design for AC coupled video amp and slightly modified the circuit diagrams (nothing major, just changed the values for a few filter caps)
14/6/06 - Fixed an error in the IDCVA circuit (pins 7 and 14 of IC2 were swapped arround). Thanks Marton for (re)pointing that out.
30/12/05 - AC coupled amp finaly published along with a power supply to suit. The people rejoice.
19/6/05 - DC coupled amp circuits are revised...AC coupled amp circuit still pending.
1/6/05 - Page is created.

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