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Discussion Starter · #1 ·
Hi all,


Here is an interesting question for the more knowledgeable than I. I have a Mitsubishi VS-1281E CRT projector using RGBHV input from my HTPC.


The input that I am using has little slide switches under each BNC connector that allow you to choose between 75ohm termination & 'high' termination.


Today I thought that I would switch these little switches to the 'high' position to see what happened (being the addicted tweaker that I am). Switching to the 'high' setting increased the overall brightness of the projector by about 30%. It is quite an incredible brightness boost and colours seem much more vivid.


Now - the question. Will this increased light output damage the tubes? In all other regards the projector is setup very normal with brightness at -15 (-50 to +50 scale) & contrast at 0 (-50 to +50 scale). Black levels are fine (if not better than before) by Video Essentials calibration.


So - any thoughts? This is really a remarkable brightness boost (bright scenes light up my lounge room walls and wash out shadow details in the scene) and I would rather not have my tubes burnt out prematurely.


Regards

Cameron
 

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Hi Cameron, by removing the termination you've created an effect that is no different than turning the contrast way up on the projector. On top of that though, now you've got 75ohm termination on one end of your cable and infinite resistance (high Z) on the other end and it will cause ringing and ghosting. Unfortunately, you gotta switch it back. You *can* increase contrast using the PJ controls to make the image more pleasing to you (but be nice to your tubes and don't bloom).


Cary
 

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Discussion Starter · #3 ·
Thanks for the reply.


I thought that might be the case - yet I asked because there has been no noticeable degradation in the image quality (no ringing or ghosting whatsoever) In fact, it looks to me as though the image is somewhat sharper than on the 75ohm termination. Using the Video Essentials test patters for blooming it does not show any.


It is really quite odd.


Cameron
 

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Hmm. Is there any chance that you have other termination elsewhere? You could check by removing one of the RGB cables at the source and with everything turned off, measure the resistance between the BNC center conductor and the outside - you should see 75ohms.


Cary
 

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Discussion Starter · #5 ·
Quote:
Originally posted by jcmccorm
Hmm. Is there any chance that you have other termination elsewhere? You could check by removing one of the RGB cables at the source and with everything turned off, measure the resistance between the BNC center conductor and the outside - you should see 75ohms.


Cary
Cary,


I am not quite sure what you mean here. I am using a DB15 -> 5 x BNC cable. Should I unplug the lead from the graphics card and measure the resistance between the correct pins on the cable (that match the r/g/b centre & shield conductors), or unplug from the projector and measure the resistance on the cable there?


Thanks for you help again!


Cameron
 

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The increased light output is because of the increased voltage after removing the terminator. This is called reflection.


The signal travels to the cable and at the end it is reflected back because of the low impedance. This signal is double backing, doubling the voltage of the signal.


This is like putting a rope on the floor, then with your arms making an up/down move, the wave on the rope will travel away from you to the end and the end the rope will jump up high.


In electrical systems, this is mostly bad, altough PCI actually uses this reflection to boost the voltage on the bus.
 

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Good explanation! But for something that's analog it's still not a good thing (PCI is synchronous so the signals can do what they want between clock edges. Plus, it's digital levels and only have to pass through thresholds.).


If you unplug the DB15 and measure the resistance from either R, G, or B (pins 1, 2, or 3) to GND (pin 5), with the other end plugged into the projector, you should measure 75ohms which is the projector's termination (you might have a few ohms more than that due to cable resistance). If you only unplug the projector's end and measure, you should measure the source's termination of 75ohms. (each conductor will be terminated to GND through 75ohms at each end of the cable).


Cary
 

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Oh and this reflection is on the HV signals too, as projectors like Sony 12xx don't terminate these signals, so you should do it for them. (spliter + terminator).


The reason for the projector to not terminate HV signals is probably for synchronising other equipment, something like genlock. Almost nobody terminated these HV signals as it is not sensitive for reflections.


On the source side (VGA), the HV signals are not terminated either, but as most people use VGAtoBNC5 cables you can't terminate this side.


The reason for VGA to not have HV signals terminated as these are TTL level signals and are not carries over coax inside a VGA cable.


I should put coax between the projector and my source, but I don't have the money at this moment.
 
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