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Analog audio has very little structure in terms of time - it starts, it continues for a while, then it ends. Obviously music is normally highly structured, but the analog audio signal it is turned into is temporally very simple indeed.
Not so digital audio. Digital audio is broken up into 44,100 samples every second, and the timing has to be precise. Other common sample rates are 48,000 and 96,000 samples per second, but let's stick with the CD-standard 44.1 kHz sampling rate for now.
Let's suppose that in some fit of madness, you decided to connect the digital output of your keyboard to the digital input of your soundcard or audio interface. Firstly, you would have to make sure the two were working to the same sample rate, because if they are not then strange things will happen. You might get almighty glitches, you might get the occasional glitch now and then, you might get no audio at all, or the audio might be at the wrong pitch. It might - just might, mind you - be perfect because there is an automatic sample rate converter in the system. Vietnamese pot-bellied pigs might repair the Hubble space telescope!
So once you have set the two devices to the same sample rate, you can just hook up and set to work. Right?
No, not right. You have to make sure that the sending device is set to run from its internal clock - also known as wordclock - generator, and the receiving device is set to external sync. Then it will all work surely?
Yes it will. And don't call me Shirley.
One of the clever things about digital audio that does actually work without you having to do anything is that the signal carries the wordclock pulse. So if you connect via the normal S/PDIF connection, or the AES/EBU connection if you have higher-end equipment, then both the audio and sync connections are made simultaneously with the same piece of wire.
This works with two devices in the system. But if you want to go beyond that - and let's face it, who doesn't? - then it suddenly becomes more complex. Which piece of equipment supplies wordclock, and will it always be the source, or will you sometimes want it to receive signals?
Beyond two pieces of equipment, the best solution is to have a master wordclock source and set everything else to external clock. This is how broadcasters do it, and there's nothing that broadcasters don't do to as close to 100% professionalism as you can get.
If you have a digital mixing console, then this will probably have a wordclock output that you can use to feed all your other digital devices. However, although this might be OK for broadcasters with their teams of technical experts on hand, the lone recordist will find everything a whole lot easier just to send signals to the digital mixing console or soundcard in the analog domain.
Digital voodoo? Stay clear - use digital equipment where it has advantages, but for simple hooking up, you can't beat the simplicity of analog.