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Sound Advice

Staying In Synch – Part I: Word Clock Explained by Bob Snelgrove

Everyone using more than one piece of digital audio equipment should be concerned about the quality of their studio’s Word Clock. This article will explain the critical role that Word Clock quality and distribution plays in the digital audio environment and the audible effects that poor clocking has on digital music systems. Examples of common mistakes and suggestions for proper hook-up will be made.

I need to start off by saying that when it comes down to quality of sound, I tend to approach the wonders of digital audio with a healthy degree of caution. I love my CD player and I personally own and enjoy lots of other digital audio toys. Nevertheless there are still many problems with the digital representation of audio that have yet to be solved. All of these problems relate to digital audio’s sonic accuracy and transparency.
At best, well executed word clock generation, synchronization and distribution will make your studio sound audibly better and allow you to become a better artist, engineer or producer. At worst it will simply eliminate countless technical gremlins that will make you inefficient, ruin your mixes and drive you crazy.

Timing
Despite the many similarities, compared to analog audio, digital audio has a couple of unique and major differences. The first is that an analog audio signal is a continuously varying signal, which is represented digitally by a limited number of discrete numerical values. The second is that these numerical values represent the analog signal only at specific points in time or sampling instants rather than continuously at every moment in time. Sampling instants are determined by various devices and processes, the most critical being Analog to Digital and Digital to Analog conversion. Converters are responsible for transforming an analog signal into a digital representation and back again, this is where it all starts and this is where it all ends.

A sample clock determines when these sample instants occur. All digital audio devices have some form of a sample clock to control their internal sample rate or sampling frequency. In a studio where we integrate many different pieces of equipment that all depend on their own clocks to function we will invariably have sample instants taking place at different times unless we synchronize all the clocks in each piece of equipment and tie the timing of these events together.

This synchronous timing is required because unlike analog audio, digital has a discrete time structure consisting of individual samples. Successful communication between different digital audio devices or the mixing of different digital audio signals together will fail if each device is not producing their bits of data in precise co-ordination with each other.

Poor quality timing between multiple digital audio devices or the improper distribution of those timing signals will result in non-synchronous operation and the result will be the creation of random and highly audible artefacts, often described as clicks, pops or glitches.

There are two standard timing signals used to synchronize the internal sample clocks of digital audio equipment. The first, commonly used in large post-production and broadcast facilities is the AES3 Digital Audio Reference Signal or DARS for short. This bi-phase signal’s carrier is exactly the same as a balanced AES3 signal but no audio data (digital zeros) in the data stream. An XLR sync input can usually be found on pieces of high-end audio equipment and workstations. DARS is distributed the same way as AES3, via balanced 110-ohm digital audio cable and AES3 XLR type connectors.

There are two things that make the DARS or Audio Black signal particularly attractive. The first is its high frequency of operation, which is between 2 and 3 Mbits per second. The second is the fact that the professional AES3 interface is balanced and ground isolated making it relatively immune to induced noise, which can be a major source of jitter.

The most common clock distribution method however is Word Clock. The Word Clock waveform is a simple unbalanced square wave. Word Clock is designed to be distributed on 75-ohm, unbalanced coax cable terminated with BNC connectors. In order for synchronous operation to take place all digital audio devices must be fed one of these timing signals from a master reference Word Clock time base. These are typically referred to as Word Clock generators and sometimes as synchronizers depending on the functionality they provide. Great care must be taken to carefully distribute these Word Clock signals to each piece of digital equipment in the studio or the timing signals will be degraded and audio quality will suffer.

Word Clock Generation
The most basic requirement for a Word Clock generator is that it must be able to produce high quality stable square waveforms. The square waveform will need to be at one of two frequencies of either 44.1 or 48 kHz also referred to base frequency or Fs. The generator must also be able to produce industry standard multiples of either of these two base frequencies yielding Fsx1 thru Fsx4. This x1 thru x4 multiplication generates the common sample frequency sets we are all familiar with in digital audio and generates Word Clock frequencies in the range of 44.1 to 192 kHz.

A special case of declining interest and usefulness is Digidesign’s proprietary SuperClock at FsX256 or a frequency between 11.289 and 12.288 mHz. Because different manufacturers design their products to accept different multiples of Word Clock base frequencies, all multiples from one to four must be supported, and the master word clock generator must be able to produce different Word Clock frequencies from different outputs simultaneously.

It is important to note that not all BNC word clock outputs are created equal. When distributing Word Clock signals, isolation and proper source impedance from each output BNC is important. If the outputs are simply fed from one low impedance source then it will be impossible to correctly terminate a line and a single bad cable or poor connection will reflect back and compromise the performance of every other line. This is one condition that would also create jitter.

Join us next issue, where we continue are look at synchronization in the digital world, tackling the topic of jitter.

Bob Snelgrove is the President of GerrAudio Distribution and the Canadian Product Specialist for Audio Precision test instrumentation.

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