75 ohms vs High Mode

[Jack Burton]

I've been using this old Hitachi Monitor for gaming and my desktop use for a while now.  It supports 240p through 120hz modes, so it's pretty good for arcade gaming. 

However, when I compare games on it side by side with my dreamcast on a small JVC production monitor I own the image on my Hitachi seems rather dull and flat. 

On the back of my monitor are three switches underneath of each BNC connector.  When I press them in the image on my monitor becomes much brighter, and there is a huge amount of red color bleed on my whites.  If I turn the contrast down a bit the color bleed goes away and the image is much brighter and more colorful, similar to the JVC.

These aren't labeled, but from my experience these switches are 75 ohm to High Mode switches used when you are looping video between multiple monitors.  Am I correct in this assumption?  This monitor doesn't have a video out, so I'm not really sure.

Will it harm my monitor to run it in High Mode often? 

Also, there is another switch underneath of the sync cable.  It seems to do nothing.  What is it for? 

[ed12]

hi
1= is for +/- sync which is not used with bnc
2=yes the 75 ohm to 10k ohm will raise the brightness alot
just re adjust the brightness and contrast

ed

[MonMotha]

In general, the proper setting is going to be 75 ohms.  Here's why:

When used with a PC source and mini-coax breakout cable, the 75ohm termination matches the characteristics of the cable and back-termination on the PC.  This will reduce ghosting and similar due to transmission line effects and yield proper signal levels.  The same goes for arcade boards with similar outputs (some boards with HD15 outputs, especially the ones with support for 31kHz mode).

When used with conventional JAMMA arcade boards, the 75ohm input will load down the board's output and dim it.  This sounds bad, but arcade boards like this conventionally output much higher signal levels than the monitor expects (~3.5V vs. 0.7V).  Many arcade boards were designed deliberately do have proper signal levels for both arcade monitors with Hi-Z inputs as well as PC type monitors with 75 ohm inputs due to careful component selection.  If the video is REALLY dim (this will happen with some boards, such as MK2, depending on how they were built), go ahead and try the Hi-Z setting.


You can use this as a crude brightness/contrast switch, but you shouldn't.  You probably won't break anything if you have it set wrong, though.  The increased intensity may cause premature tube wear (burn/dimming), at the extreme end.

If you can't get the monitor "bright enough" on the 75ohm setting when being driven with a PC, you may need to adjust the color gains/cutoffs in service mode.  Also consider that it may just not actually be intended to be that bright (see the burning/long-term dimming comment above).  Many people have been conditioned by years of "brightness wars" on TVs as well as modern contrast tricks and excessively bright backlights on LCD/plasma to expect super bright and contrasty displays with overly saturated color (this will combined to look "vibrant").  I guess it's personal preference, but the "proper" (photorealistic looking) setting is generally much "flatter" and overall "dimmer".  I tend to set my monitors to this setting, with the side effect of making them last longer, but again it's your monitor in the end, and you should make it look the way you want it to.  Many people who have me adjust their monitors for them initially comment that it's "too dim" but quickly decide it actually looks better.

The setting could be used for pass-through video, as you note, as well as for using passive splitters.  This will ruin the beneficial termination effect, but it at least keeps things from getting excessively dim when this is done which was sometimes a worthwhile trade-off in a commercial environment where these monitors often needed to "just work" on a field setup.

The switch under the sync input is probably a similar function.  It usually won't matter for TTL sync as used by PCs and arcade games.  Some professional video applications had sync that was different electrically and either needed or benefited from (on long lines) the 75ohm termination.


As a note, some arcade monitors have a similar switch.  On them, switching to the "Hi-Z" setting not only changes the input termination but usually also adjusts things to expect the higher signal levels, so on them you want the Hi-Z setting when used with JAMMA boards and the 75ohm setting for use with a PC.  Confusing, I know.

[Jack Burton]

Pretty much confirmed what I was already thinking/knew. 

I had a monitor similar to this one in the past that supported 15khz that I tried hooking up to a Mortal Kombat 1 pcb and encountered a very dim image.  With the HIGH mode setting I was able to get a serviceable picture, so that is one potential use.

I think that the damage it can cause is actually going to be pretty quick.  Just playing around with it today I noticed the brightness of the tube flickering a bit while I was playing a game.  And the white levels seem kind of "burnt" anyway (not blown out).  It also caused quite a bit of ghosting in higher resolutions. 

You are right about the conditioning for an image that "pops".  However, I think I have been around enough monitors and calibrated enough of them myself to not fall victim to it.  People usually tell me the exact same thing (too dim) when I adjust their displays for them. 

 Rather, I think in this case it is that the computer monitor was simply designed to give off less light overall when it is properly set compared to the JVC pro monitor or any standard TV.