“I’ve noticed that some of the devices with digital I/O in my studio have a switch next to the I/O that says 75 Ohms. In the manual for my $20,000 Sony mixer it says “set appropriately,” but doesn’t specify what it does. Can you shed some light on this?”
When running very high frequency signals through cables, the impedance of the cable, connectors, and the load at each end of the cable can become very critical, depending on the interface protocol used. For example, SCSI requires a specific type of termination at each end of the cable. When the timing of the signal is critical, as is the case with SCSI and digital audio, this becomes even more important. Without being properly terminated signals can actually reflect up and down the line, which causes errors. S/PDIF specifies 75 ohm cables and termination, and in general most word clock connections follow this paradigm as well. Even if they don’t say it you can assume that you’re dealing with a 75 ohm connection anytime you run across a standard BNC connector, which is what most word clock connections use. These 75 ohm signals require (75 ohm) termination at each end of the cable in a manner similar to SCSI.
In the case of word clock signals, it is common to distribute them throughout a system. This sometimes means the signal may pass through one device on to the next one, and so on. But at some point the signal comes to a termination point. And at that point it needs to be terminated with a 75 ohm resistor. Many devices provide a built in switch to take care of this for you. This is the case with your Sony mixer. The same principle holds true for distributing black burst video signals.
S/PDIF signals are generally different in that they normally are meant to go from one device to another specific device (even if that device is some type of digital patch bay); they are not generally “distributed” the way word clock and other house sync signals are. There is also no such thing as a S/PDIF “bus” like you have in SCSI, so normally S/PDIF connections are terminated at each end right inside the gear in question. The only exception would be if your gear has an option to pass the incoming S/PDIF signal through for the purposes of using it to distribute word clock. Normally a separate word clock signal is used so this would be extremely rare.
So if you are not passing the word clock signal on to another device you should enable the 75 ohm termination.