Word Clock needs a send and return to be able to work properly.
For example: –
Say you have a digital desk and a digital DAT machine both have a digital input and output. Now both the desk and the DAT machine always send out word clock, now if you were to just plug the output of the desk to the input of the DAT machine the desk would never know it was there, it would carry on at it’s own word clock speed and the DAT machine would try to keep up. The DAT machine may say it is locked up to the desk but it wouldn’t be as every so often it could drop out and you would hear horrible clicks.
The way you get over this is by plugging the output of the DAT machine into the digital input of the desk. This way the Desk will see that it has another unit trying to sink up to it and constantly keep checking that they are both in sink.
Also word clock has many languages; you need to make sure that both units are set the same so they can understand each other. If not they will always say unlock!
You also have what is known as slaves and masters. A master would be something like a master word clock unit or a desk or a hard disk recording unit. Things like ProTools Digidesign interfaces prefer it if they are the master as they need to make sure that all the components are in sink with each other. But you could use a DAT machine as the master if you wanted.
Word clock on its own (not using AES-EBU or SPDiff) can only be run a maximum of 2 meters before it breaks up, you can get round this by using a master word clock unit which sends out the signal at a much higher and precise level. The only thing is that they tend to cost allot of money.
So most studios would uses there desk as the master and make everything else their slave.
Word clock comes in many different forms, the normal one would be 75ohm BNC cable word clock, but you also have Word clock that is sent along with SPDiff and AES-EBU.
Because of all these elements, word clock needs to be checked all the time and not just taken for granted. You don’t want to spend time rerecording a song or an album because word clock dropped out during the song but was only noticeable when listening out side of the studio.