Eventide Eclipse, User Manual

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Eclipse User Manual
Release 4.0.1 Page 55 of 66 Eclipse User Manual
Word Clock Termination
Any cable has a characteristic impedance that should be matched to both the signal source and destination
impedances for best results with high speed sharp edged signals such as Word Clock. This becomes more
important with longer cables.
Most Word Clock signals are generated with 75 Ohm source impedance, whereas the BNC cables used to
connect to them may have 50 or 75 Ohm impedances – clearly the latter are to be preferred.
So, we have covered the source and the cable – what about the destination ? If we only have a single des-
tination it is easy – the destination must also be 75 Ohm – this is ensured by the use of
termination . An
input will be either terminated , meaning its impedance is set to 75 Ohm, or un-terminated meaning it has a
high impedance that will not load the cable.
With multiple destinations only the one at the end of the cable needs to be terminated
– intermediate destinations need not and should not be terminated, as long as the “stub
length” of the cable to them is short. These should be connected by the use of a BNC
“T” connector or splitter attached directly to their inputs so as to ensure the shortest
possible stub length. Using multiple terminations is wrong, both because twice 75
Ohm in parallel gives 37.5 Ohms (wrong !) and also because it increases loading on
the source.
Eclipse is shipped without termination in place meaning that they can be used at one end of a Word Clock
cable without changes. To use them in the middle of a chain the termination should be disconnected by
removing the black jumper on the motherboard directly behind the Word Clock connector.
Eclipse is shipped without internal termination and can be used in the mid-
dle of a Word Clock cable without change. To use it at the end of a chain,
75 Ohm termination must be applied. This can be done in one of two
ways:
1) Internally by removing the cover of the Eclipse (10 screws) and adding a shorting
link to JP1101 on the motherboard (just behind the W/C connector).
2) Externally by using a BNC T piece as shown above and a 75 Ohm terminator as
shown on the right. These may be obtained from one of many suppliers. Connect the
T piece to the Word Clock input, the cable end to one side and the terminator to the
other. This approach requires a modest investment but is recommended as being eas-
ier and more visible.