Digitally interfacing the Ultracurve to consumer equipment
Input
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Use a good quality, well screened 75 Ohm Coaxial Cable. The
previously mentioned CT-125 is a good choice, better if an additional
shield is fitted.
Use a high quality RCA Plug suited for the cable, ideally one that
offers close to 75 Ohm Charateristic Impedance. Using a 75 Ohm BNC
Plug and socket is preferrable. Terminate the other side of the cable
in an XLR plug as shown in the Behringer Manual, HOWEVER also apply a
an added 220 Ohm termination Resistor in the XLR plug, located as close as to
the section of the cable that conforms still to the original geometry
and with as short leads as possible.
Resistor quality matters, Allan Bradly Carbon Comp's or non inductive
Metal Films (Vishay/Caddock) should be used.
Output
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The output conforms to the AES/EBU Data Format and is also
electrically very different from the S/P-DIF interface. Thus two
conditions must be met to use the output from the Ultracurve
directly.
The DAC the UC is being connected to requires a receiver chip that is
transparent to the format differences between S/P-DIF and AES/EBU,
this is certainly the fact with all Crystal/Cirrus Logic Parts, like
CS8412,8414,8420 etc.... It should also hold true for the Yamaha
Receiver chip, but there I'm not sure. Secondly, we must adapt drive
impedances and levels to match the S/P-DIF Format.
It is IMHO advantageous to keep the output and cable balanced up to
directly ahead of the receiving unit. Thus us a pair of high qualiy
50 Ohm Coax Cable, both screens tied together and to pin 1 on the XLR
Plug and with one of the centers each tied to pin 2 and 3
respectively.
On the receiving end a little box will be required to hold all our
components. Make sure the two Coax Cables are solidly strain
reliefed. Again, connect the screens together, but do not connect
them to anything else yet. They can be OPTIONALLY tied to the DAC's
chassis, but in most cases this will not be desirable. Then connect a
150 Ohm resistor directly across the two center conductors. Two
further 150 Ohm resistors connect to the center and screen of a high
quality 75 Ohm coax "Tail", this "Tail" should be quite flexible and
as short as feasible. A 91 Ohm Resistor is then connected across the
center and screen to complete the impedance matching.
The remarks WRT resistor quality hold equally as above. This
circuit/Cable will match levels and impedances between the AES/EBU
Output and any S/P-DIF Input. The resultant cables should provide the
lowest possible jitter levels in system. A drawing illustrating the interface circuit is here.