Omnia Composite Output Impedance Setting or Jumper

Updated 4 months ago by Mark Manolio

Scope

The document explains what the composite output impedance jumper or setting is for and how it should normally be set.

Description/Problem

Q: I just noticed that my Omnia has an output impedance jumper for the composite output. 5 Ohms and 75 Ohms can be selected. Why is the 5 ohm setting the default? I thought the output impedance of every MPX output should be 75 ohms.

Solution

A: The correct impedance setting when feeding virtually any modern device with composite inputs is actually the lower 5 Ohm setting. This is because composite inputs and outputs need to be thought of like unbalanced bridging audio connections and not like RF transmission lines where a matched impedance is needed. So what we want to use in unbalanced bridging connections is a low output impedance feeding a high input impedance.

Often, 75 Ohm coaxial cable is used for composite connections but it is not the impedance of the cable that is important in this application, it is the capacitance per foot. and most good quality 75 Ohm coaxial cable has a relatively low capacitance. But 50 Ohm cable can be used as well as other low-capacitance cables. In fact, I have always used RG-62 93-Ohm coax for my composite connections because of its low capacitance.

The higher 75 Ohm setting has been in all of the Omnias since the beginning because of one particular exciter back in the day which had low impedance inputs.

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