Introduction

Unlike analogue audio connections, when digital audio signals move between devices (such as a digital mixer and a DAW-digital audio workstation) they must be synchronised.

Individual samples in a lone digital signal must be clocked precisely. For example, there are 48,000 samples of amplitude every second in a 48KHz digital audio signal, and if these samples do not occur at regular intervals (precisely every 48,000th of a second) then the correct amplitudes will not occur at the correct time and the audio signal will become distorted. For the most part, individual samples in a lone digital signal will be precisely clocked.

However, when 2 or more separate signals from different sources meet, the clocking of their samples must be synchronised. Like 2 cogs in a mechanical system, they must mesh. Therefore, their sample rates will need to be identical and they will need to arrive "in sync". If the devices are not synchronised, audible clicks and pops will result every time a receiving device fails to correctly anticipate the arrival of samples in an incoming signal.

Here are some of the devices that may be connected together in a digital audio system ...

• DAW / digital audio interface
• A stand alone a to d converter
• Digital mixer
• Digital microphone
• CD player
• Digital multitrack
• A voice channel with a digital output
• An effects processor

NOTE: You may also wish to read Connectors in the recording studio to understand a little more about the variety and uses of digital audio connection technologies.

Master and slave

In a multiple device digital audio system, one device must be the master (supplying a clock timing signal) and all the others will be slave devices synchronising to the master device at its sample rate. It is possible to daisy chain devices together but it is considered better practice to have one dedicated master clock device supplying multiple clocking signals to each slave device individually by way of independent connector leads. However, master clock devices tend to be expensive, too expensive for many project studio owners.

Word Clock

Word Clock is a timing reference for digital audio devices. It is a digital clocking signal running at the sample rate of the audio devices it is synchronising. The Word Clock signal is essentially a square wave at the frequency of the sample rate. Word Clock provides a measurement of the passing of time at the speed of the sample rate. It allows multiple digital audio devices to synchronise with one another.

Word clock may be embedded in a digital audio signal or generated and transmitted separately.

External stand alone master Word Clock device

A stand alone Word Clock device (such as the one shown here) will generate multiple identical Word Clock timing signals and transmit them down individual BNC cables (connector shown) to multiple individual audio devices to ensure the synchronisation of the (seperately transmitted) AES, ADAT, Toslink etc digital audio signals that pass between them.

Using a master clock is arguably the best way to synchronise a system, but many semi-pro digital audio devices do not have separate Word Clock (BNC) input connectors.

Embedded Word Clock

For smaller project studios with devices sporting AES3, S/PDIF/Toslink or ADAT/Toslink connections, a simple and effective solution is to synchronise using the Word Clock embedded in the digital audio signals themselves. One device must be nominated as the master and the rest set as slaves. The devices will be connected together with appropriate digital audio connections (AES3, S/PDIF, Toslink etc) and will have the ability to be switched to either slave or master mode. As an example, my own system has this simple daisy chained system ...

Voice channel (master) AES output
>>to>>
Mixer (slave) S/PDIF input / Mixer (slave) ADAT-Toslink ouptput >>to>>
DAW (slave) PCI ADAT-Toslink interface input

I constructed a special 110ohm resistor enabled AES3 (XLR) to S/PDIF (phono) lead to link my voice channel to my mixer (because of the voltage differences between AES and S/PDIF) and the whole system works perfectly. The only drawbacks are that my voice channel must be on or the mixer won't work, and this system only works because I have a single source (my voice channel).

If you are using embedded Word CLock and your DAW as the master, you will need the means to transmit Word Clock from the DAW to any devices that are sending digital audio signals back into the DAW. This means you will need 2 way connections to your devices, even if a device is only sending audio signals in one direction (such as a voice channel sending a signal from a microphone to the DAW in order to record it).

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