In his April FOH Theory and Practice, Brian Klijanowicz discussed the concept of word clock. If you missed class that day, here is a brief recap: Every digital device employs an internal word clock circuit as a timing mechanism. Word clock determines sample rate, and its accuracy is paramount to audio quality.
Chip Self, president of Logic Systems Sound and Lighting Inc. (St, Louis, MO) elaborates: "Audio is converted from analog to digital by taking thousands of samples of the waveform and converting it to a digital ‘picture' of the waveform for that specific sample. The rate at which it takes these samples is conveniently called the sample rate. In the case of the (Yamaha PM) 5D, it's 96,000 samples per second. What the clock in a digital circuit does is to define the stop/start point for each sample.
"On the other end of the console or processing stream," Self continues, "the clock defines the stop/start point for re-assembling those samples back into an analog audio waveform. Here's where the potential for trouble starts. Within the digital realm everything boils down to math, and the processing time it takes to do that math. Every piece of digital gear has some latency, and they're not all the same. As a result, the specific clock pulse that was used to define the stop and start of the incoming sample is never the same clock pulse that's used to define the stop and start time of the outgoing sample. If those two clock pulses aren't precisely identical, the data from a sample that was 0.00000xxx seconds long, may get played back at 0.00000yyy seconds long. Needless to say, this has an effect on the sound quality.
"The lack of a precision clock causes thousands of microscopic errors, but the impact on a single source is subtle and can be difficult to hear. Further, when listening to a single source, all of the DSP resources of the console are available to that single source, so errors are much less likely because the system isn't taxed. Only when there are many sources running simultaneously, and the resources of the console are taxed, can you easily identify the improvement from a better clock. In my experience, the difference has not been remotely subtle – it sounds like someone took a blanket off the PA. The high end becomes more crisp and detailed, and the low end seems to gain another half octave of range."
What's It Worth?
A master clock is like a fine timepiece and can be very expensive. Last week I mixed on a Yamaha PM5D that was slaved to an Antelope Audio Isochrone OCX master clock. The Isochrone OCX sells at around $1,300. If you think that's expensive, take a look at the sticker on Antelope's Isochrone 10M – around $6,000. The 10M is a Rubidium Atomic Reference Generator with a reference clock reported to be 100,000 times more accurate than the quartz oscillators used in most equipment. It'd better be. In fact, it'd better do my laundry and clean the studio bathroom as well.
Why all the hubbub? Well, as Rich Baker from Oceania Recording Ltd. in New Zealand recently wrote in to FOH: "…I have a digital console with a decent word clock in it (Soundcraft Vi6) and multiple digital equipment downstream of this such as CD burners, digital processing and multi tracks, all of which are connected by either AES or MADI. Is there an advantage to syncing the equipment via an external generator and BNC to the various equipment's external word clock inputs, as opposed to relying on the AES or MADI connections between the console and the equipment to do the syncing? Currently I sync relying on the digital audio connections and have never had any syncing issues. Any thoughts much appreciated."
Put It To the Test
As Chip explains, you can test this for yourself as follows: "With a relatively full console (i.e., a lot of inputs), listen to the PA during sound check with the whole band playing, using the internal clock. Then switch to a good external clock. The difference should be immediately noticeable to anyone. This description has been based on using a single piece of gear, such as a digital console. That said, every point is even more valid, and the effect more dramatic, when it's a series of equipment connected digitally.
For example, imagine a system where the sources are converted to digital with different converters, some in the console and some using external converters. Then the output of the console is sent digitally to an external system processor, where the signals are crossed over and output to analog. In this case there are at least three different units, all running on different internal clocks of varying quality. The potential for conversion errors is exponentially higher than with a single piece of gear. In this case, clocking all three units to a single external clock would vastly improve audio quality."
[It would also preclude any possibility of synchronization issues. When a single master word clock is distributed to multiple devices, they run at the same sample rate and they take the same steps at the same time – which is a critical requirement for exchange of data. When two digital devices are not locked to a common clock and attempt to share audio, you'll hear intermittent clicks and pops or possibly loud static hash noises – even when they are set to the same sample rate. -ed.]
I've run into a lot of engineers who agree with Chip on the audible improvements gained through use of a premium clock. Anybody out there listening? Drop us a line and let us know what rocks your clock!
