General Termination Resistance Question

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Cosford

Well-known member
Hi Guys,

This is unrelated to Teensy; it's actually a project regarding a raspberry pi, but I digress.

What I'm looking to do is connect a 75 ohm output impedance (composite video), to 4 displays. Of course, in order to maintain correct (or there abouts) image, I need to match the output impedance of the signal to the input impedance of the displays (tv's in this case).
Now, assuming the signal from the output is split and connected via 75 ohm leads to each tv, we have this scenario:

Code: 
--R---A-----R--input1
          |---R--input2
          |---R--input3
          |---R--input4

Where in this case R is 75 ohms. This would mean that at point A, the output can see an equivalent series resistance of 17.5 ohms, therefore, I need to increase the resistance of each line to 300 ohms (300 / 4 parallel lines = 75 ohms per line).

Code: 
--75---------225--75--input1
           |---225--75--input2
           |---225--75--input3
           |---225--75--input4

Is that correct, or have I misunderstood what I've read regarding this?
 
Cosford said:
Hi Guys,

This is unrelated to Teensy; it's actually a project regarding a raspberry pi, but I digress.

What I'm looking to do is connect a 75 ohm output impedance (composite video), to 4 displays. Of course, in order to maintain correct (or there abouts) image, I need to match the output impedance of the signal to the input impedance of the displays (tv's in this case).
Now, assuming the signal from the output is split and connected via 75 ohm leads to each tv, we have this scenario:

Code: 
--R---A-----R--input1
          |---R--input2
          |---R--input3
          |---R--input4

Where in this case R is 75 ohms. This would mean that at point A, the output can see an equivalent series resistance of 17.5 ohms, therefore, I need to increase the resistance of each line to 300 ohms (300 / 4 parallel lines = 75 ohms per line).

Code: 
--75---------225--75--input1
           |---225--75--input2
           |---225--75--input3
           |---225--75--input4

Is that correct, or have I misunderstood what I've read regarding this?
Click to expand...

I'm not sure this works.

IMO, what you need is a 1:4 transformer
e.g.
http://www.highfrequencyelectronics.com/Feb04/HFE0204_Sevick.pdf
shows and explains not only the classical guanella balun (balanced to unbalanced) but also an unun (unbalanced to unbalanced)
 
By using just resistors to match the thing, you'll reduce the signal amplitude for each monitor by 75% which will result in a very dark image and most probably sync problems on top of that.
4 Monitors will draw 4 times more current from the output than one, thus I'm not sure that the 75Ohm rated output will be able to drive all 4 monitors at the same time. I'd suggest using a separate buffer amp (voltage follower with high input impedance) for each monitor or at least each pair of monitors. Look here
 
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