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[karambos]

I apologise in advance for my ignorance but what does a "Word Clock" actually do?

I'm told this is a good one.

But I don't know what it does?

[bobkatz]

karambos wrote on Fri, 06 August 2004 08:15

I apologise in advance for my ignorance but what does a "Word Clock" actually do? I'm told this is a good one. But I don't know what it does?

Oftentimes, a "word clock generator" is a solution to a non-problem, but we can talk about that at another time.

If you decide that you need a word clock generator, then what happens is a very stable crystal oscillator (or a phase-locked-loop locked to some video source) generates a square-wave signal. Usually at TTL level, but, well, that's another subject. A good treatise on "what is a proper word clock level voltage" can be found in the manual for the RME ADI-8-DD, downloadable from the RME website.

This signal is then distributed to multiple isolated outputs. In a long signal run, at the other end the signal should be terminated in 75 ohms, and you have to find out if the word clock receiver has this termination built in. Word clock is at the base sample rate, by the way, it is a signal at 44.1 kHz if that is your sample rate. The word clock receiver (inside your A/D or D/A converter or DAW or processor) then receives this signal with a simple PLL. The output of this PLL is loosely slaved to the incoming wordclock, and generates all the signals that the receiving box needs, such as a high frequency bit clock, another wordclock, and so on.

The stability of that last PLL determines the quality of any converters that are run from it. For example, if you feed a wordclock to a Digidesign box, its converters will slave to it, and the sound quality of those converters will be affected. But that's another thread.

That's it in a nutshell.

[John Klett]

simple answer is that it provides a timing reference for digital audio players recorders, mixers, consoles and so on...  these things can all run on their own but in some situations they all have to operate at exactly the same sample rate in order to make all digital signals in a studio or facility synchronize, line up and move without glitches from device to device

so...  a CD runs at a sample rate of 44100 Hz and maybe you want an entire studio running at that rate.  The are 44100 samples per second that get turned into an analog audio output OR they may go out digitally to another device...  this is where word clock come in handy

when you have a large studio with a number of digital things like consoles and so on you may have something like a console where you have multiple different digital signals coming in from multiple digital devices or sources.  Word Clock (WCK) is used to as a common "clock" so the sources and the destinations line up.

Say something is a little off when running on it's own...  say by a tenth of a Hz so instead of running at 44100 Hz it is going at 44000.1 Hz.  At that rate it is outputting an extra sample every ten seconds and, like two note coming out of a synth or whatever that are a little off from each other there is a continual phase difference that caused a 360 phase shift every 10 seconds.  So this phase shift in digital transmissions really comes out to be a timing difference and at some point the timing error between the source and destination will result in a tick or a pop or a bang depending on the gear involved.

Word Clock does another thing...  it's a square wave and for stereo the high and low halves of the square wave tell the various devices how to line up the signal so that the left and right channel samples contained in each digital "word" or "frame" also line up.  

Word Clock is a single ended signal so it is normally not possible to reverse the phase and exchange the highs and lows of the square wave but ONCE I saw a situation where a user decided to run the word clock down a balanced wire and he had put baluns or transformers on both ends AND wire one xlr out of phase...  so THAT flipped the phase of the word clock and caused some major confusion.

the digital AES signal is balanced but flipping phase on that does not matter because in this case the receiver is only looking at the timing between highs and lows...  which is the same way SMPTE works.  This is, I think, called "Manchester coding" but that really has no practical bearing on anything.

So...  Word clock is a common clock that synchronized all the digital stuff so it all runs on speed...  

next question, I guess, would be what's a sample rate converter... thats a thing that does allow one to move dissimilar sample rate signals from on thing to another without glitches.  So a lot of consumer "prosumer" gear - consoles with digital inputs especially have sample rate converters on the digital inputs to make them work all the time no matter what...  though these do have an effect on signal quality - sometimes a LOT and not in a good way.