Rick's Articles

Audio Technology - Volume 1 : Issue 3
Timing Is Everything Bzzzz... I wake up, turn off my alarm clock and roll over the day. The red glow of my quartz-locked digital clock reads 10:00am. Timing, as they say, is everything, and my life is simple. If I get up in the next five minutes, spend 10 minutes in the shower, 10 minutes fussing, and get to the train station by 10:30am, I will get to work at 11:04am, more or less.

The real problems start when I remember that my girlfriend is the kind of person who sets clocks 10 minutes fast, so she can be 10 minutes early where ever she goes. I have to consider the possibility that she might have adjusted my alarm clock, so I always check my mobile phone, which also has a clock in it and tells the correct time - unless I've forgotten to charge it up. On the way to the kitchen every morning I check the video recorder but that always tells the time in 24 hour mode, so I also check the time on the microwave. Every day it astonishes me by learning a new language, this morning it showed the time in Egyptian hieroglyphics. Inevitably, I confer with one of my 12 or so analogue wristwatches, which can never agree but usually they think it is about the same time as my clock radio which, as I said, may or may not be fast.

None of this really matters because the train is never ever early so every day I sit and wait for it to arrive. My recording session cannot start until I arrive, so me and time get along fine enough.

Any sane person would reason that if I set all the clocks in the house to the same time then they would all read the correct time and I could save myself a big hassle. But have you ever tried to set all your clocks to the same time? It's too hard, and even if you did, how long would they stay that way? Every clock in the world runs at a different speed. Clocks just ain't clocks.

An analogue wind-up wrist watch operates in movements of about 18,000 per hour, or five per second. An expensive Rolex works at about double that speed. A quartz crystal wristwatch makes about 35,000 oscillations per second, and an electric quartz clock about 45,000 per second. The best analogue watches are good to a couple of seconds per year, while the average digital clock is good to one second every 10 years. The best clocks in the world are atomic clocks, which run on atoms of caesium and such stuff and are encased in huge vacuum tubes. One of those will see you on time with more than nine billion vibrations per second, and your timing is good for a thousand years or so, give or take a second.

So what has all this got to do with audio? Well, virtually every digital box in the world relies on a quartz crystal clock to regulate its entire operation. In a box with a sampling frequency of 44.1kHz there is a little quartz clock pulsing away, resetting the time for every sample and making sure your audio is sampled and played back at regular intervals at around 44,100 times a second. And as we've just discussed, that's about the same speed as an electric quartz watch, you know, the kind you buy for 20 bucks. They keep real good time...

The problem for me is not with the design of digital audio, but with its real world application. Every manufacturer has to use a digital clock to regulate every thing digital. So what is this clock? Well it's basically a chunk of rock, cut and shaped to a particular size and weight. They glue some wires to it and stick a voltage across it, and if it doesn't crack, shatter or just plain burn up, then it vibrates at it’s resonate frequency - which is exactly what it's supposed to do. Cool, huh? Providing nothing changes as it shakes like a little whippet hound at around 44,100 times per second, this piece of rock controls the sampling and playback intervals. Add a little decoding and we get audio. It's magic, really. Digital designers got it pretty good; they even allowed a little room for error. Add a little bit of correction, and the heated chunk of quartz rock ain't a bad timepiece.

Now imagine a $20 watch from Bali, its heart is made of the same stuff - probably from the same manufacturers - as your new digital sampler's master clock. In the Bali watch, the quartz rock doesn't have to work that hard, it just moves a second hand once a second, no big deal. But in your digital audio clock it has to synchronise 44,100 times harder than the watch and it has to be 44,100 times more accurate or something will give. It's no surprise that every single second a sample or three falls in-between the oscillations of that precision cut chunk of rock. These timing errors are called jitter. Now it seems that no one can decide how much jitter is bad, but when it is real bad you get clicks (you know those unexplained clicks that just appear?). When it is not so bad, digital sounds grainy and harsh. When they get it right (low jitter), then digital sounds good, and when digital sounds good everybody is happy. Low jitter = happy people, easy isn't it?

The real world problems begin with the quality of your clock. How accurate is it? Has it changed over time? That little piece of rock gets a pretty hard life, and that life is not infinite. But the biggest problem the rock/clock faces is the age old problem that every clock faces - they are all different. When you plug two digital devices together there are now two clocks that have to agree on the time, and they have to be exactly synchronised. Add a third box and you've got three clocks that have to be exactly synchronised, etc.

Synchronising digital audio is relatively easy if your box has an external word sync input. Just hook all your digital devices together, loop the same clock pulse into all of them and everything will clock to the one master clock - that is, if you have a master clock and if you have external sync inputs. Most devices don't have external sync inputs, most studios don't have a master clock, and it seems to me that everybody has problems.

If your studio has the occasional digital click or low level mutes, weird problems with digital transfers, unexplained changes in sound, etc, digital clocks are a great place to start looking.

Fortunately, things are changing fast. Several manufacturers are offering master sync clocks, more manufacturers are paying attention to clock quality, and external sync inputs are becoming common. I've had a master clock for years and I don't understand how you could integrate your digital systems without one. You don't have to spend the ridiculous amount that I spent on one of the first systems, several manufacturers offer sync boxes really cheap, and these things will solve, or rather 'resolve', most of the timing problems you didn't even know you had.

If you can go into your digital desk, onto your hard disk editor, out to your DAT and then burn a CD digitally, without ever getting those mysterious clicks, I bet you have solved your clocking problems. If not, I guess it's time to face up to the reality of clocks.

As for me, it's Friday night and all I have to do is figure out how to get out of my studio on time to meet my girl, because I never seem to time it properly and timing, as they say, is everything.

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