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Discussion Starter · #1 ·
I've seen in the sony 1272 manual that RGB are

terminated with 75ohm while the HV is high impedance.


As I remember from electronic engineering is that if you use 75Ohm coax on these inputs that you will need to terminate them my self (spliter, terminator).


What I wonder though is, I use a VGA output which has 5 coax cables connected to it. Does VGA have simular characteristics as RGB is 75Ohm and HV is high impedance, as you need to terminate that side as well?
 

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Takev,

If I am reading you correctly, VGA output is RGBHV. If you search on this forum you will finnd the pinout for your dB15 VGA connector. The VGA output should have the proper characteristics for the RGBHV input into the 1272.


Terry
 

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Discussion Starter · #3 ·
I just read www.spanorama.net/documents/pc/vga_bd15.html


It says only RGB are terminated with 75Ohm, the VH are high impedance TTL output.


So RGB from VGA to projector, need not to be terminated with 75Ohm as they are terminated by the equipment.


The HV from VGA to projector however, will need to be terminated with 75Ohm, with a splitter and a terminator. As the input and output are high impenance and the coax cable is 75Ohm which would make reflections on both ends, if the cable is not terminated correctly.
 

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I dont know? I have always ran 5 RG6 coax cables and never had a problem with the Horozontal and vertical syncs?


Most factory bought RGBHV cable is simply just 5 RG59 coax's in a shrink wrapped sheath.


what about if you bought a factory VGA computer cable and got a breakout to hook up to the projector? Same Same right? all the conductors are the exact same.


I can see being sure your video cables are true 75 ohm capable of carrying full video bandwidth but the sync signals are just a low volt electrical signal, does it really matter if you use 75 Ohm for those? I dont really think so?


I may be way off here?
 

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Discussion Starter · #5 ·
both video and sync signals are low voltage. Except TTL output on sync is probably 5V, while video is 1V.


You would think video signal is high bandwidth and sync low, but remember the sync is a block wave (whish consumes a lot of bandwith) and should begin and end at the same time resolution as a video signal.


This means that it is indeed desireable to have sync on coax cable, to keep the block shaped as a block and to have the same propagation speed as your video signal.


What happens when you not terminate this sync signal is that the reflection on the cable causes a small spike upwards just behind the rise of the signal. And a small spike downward just behind the fall of the signal.


This probably doesn't mather for the projector as it is syncing on the rise of the signal and ignores the spike.


But if you're hooking up a projector as we do, should we not connect it the right way, even if it doesn't mather in image quality? A spliter and a terminator doesn't cost to much as you compare it to the cables you bought for it


Indeed the VGA to 5BNC breakout cables aren't correctly terminated either, but running a length of VGA cable is less desirable anyway.

What you want is a very short breakout cable, say 2 to 3 inches, you can terminate the cable correctly then and run the rest of the length to your projector in coax.


Or even better modify your equipment to have 5BNC output.
 
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