What's in a Word Clock? - Keeping Perfect Sync Between Digital Audio Devices


We've all experienced audio that's out of sync. When audio doesn't match up with video, it feels like we're watching a poorly dubbed movie. When your studio hardware isn't synced to a good clock, it creates an odd echoey scenario that can be so subtle you may not realize your overall sound is being degraded. So what can be done in order to keep audio in perfect sync? In a word, a word clock. A word clock is a nifty bit of technology that has the sole purpose of keeping perfect time and preventing data errors with digital audio.

Many of today's most widely-used digital audio formats, such as S/PDIF, AES/EBU, ADAT, and TDIF use word clock. A word clock isn't necessarily a device, but rather a data stream that is sent between audio devices so that they stay in sync with a constant, precisely-timed bitrate. The word clock signal doesn't contain any other data. This allows it to do one job, and do it well.

Many modern audio devices, such as analog-to-digital converters, have built-in word clock generators. In this article, we'll focus on a couple of devices where the sole purpose is clock signal generation. Using a dedicated word clock generator ensures a stable, reliable signal; relying on the generator that's built into a multipurpose device can sometimes produce less-than-stellar results.

ART SyncGen

Art SyncGen Compact Signal Generator

The SyncGen is a compact word clock generator made by ART. It's an unassuming and fairly affordable unit. Its small size makes it ideal for installation in even the smallest studio, while the unassuming interface allows even novice users to operate it with confidence. Like all word clock generators, the SyncGen provides a stable, centralized signal to keep all of your linked devices working together, preventing the odd pops and other anomalies that occur when audio devices are out-of-sync.

Say, for example, you're using a Presonus Firebox, a computer audio interface that lacks a dedicated word clock output, but has the ability to sync to an external clock. If you're using any other equipment in your studio that can sync to an external clock (outboard digital effects, a digital preamp like the Focusrite OctoPre MKII, ADAT expanders like the Behringer ADA8000, etc), you can improve your overall sound by making the SyncGen the master clock of your studio.

When you have separate digital components working together in your set up, a master clock makes every 1 and 0 fire at exactly the same time each different device. Imagine each piece of your equipment is a sprinkler on a grassy lawn. Without a master clock, the sprinklers turn in random directions with water splashing everywhere. With a master clock in place, every sprinkler will operate in unison, with every droplet of water landing at exactly the same time. When your equipment is this tightly synchronized, you can hear the difference.

The SyncGen can support all standard sample rates, all the way up to 192kHz. It has four standard BNC outputs and a pair of S/PDIF RCA coaxial jacks, allowing you to connect a wide variety of devices. Front-panel LED lights make it easy to use the SyncGen as a system tester by monitoring the operating status and termination of the BNC outputs. It's not an overly fancy device, but that's a good thing. This is an ideal solution for a mobile or project studio.

Apogee Big Ben

Next up on the list is a bigger, more feature-laden word clock device that's aptly named the Big Ben. Apogee is well-known for their high-quality components and devices, and Big Ben is no exception. Big Ben can work with a large variety of devices, thanks to the AES/EBU, optical and coaxial S/PDIF, and ADAT I/O connectors. It has 6 BNC word clock outputs, as well as word clock and video inputs. Like the SyncGen, Big Ben can support sample rates up to 192kHz. The Big Ben is very commonly used as the master clock in Pro Tools and Logic based studios. There's a reason why the price of admission is so high for a piece of equipment like this. Professionals can hear the results it produces, and know it's a necessary tool to achieve the best sound possible.

At the heart of this rackmount studio unit is Apogee's C777 clock, which uses an entirely digital process, instead of the combination of analog and digital elements that's typically found in many clock devices. The C777 was developed with advanced Direct Digital Synthesis technology and DSP-based filtering. This technology lets Big Ben manipulate and adapt to incoming signals in order to aggressively reduce jitter and produce a highly-accurate master clock signal. Adaptive Loop Filtering technology uses a low-pass filter to intelligently improve incoming signals, no matter what obstacle is thrown at it.

Big Ben Front Panel

Even in the best studios, sometimes a signal can be dropped. In that event, Big Ben has an interesting technology called SureLock, which will compensate for the lost signal by staying locked on the last available frequency sent by the now-missing device in the chain. When the signal is restored, instead of immediately locking onto the new signal, Big Ben will gradually and smoothly re-synchronize to prevent interruption.

Another useful feature offered by Big Ben is real-time digital format conversion, which allows you to connect multiple types of digital devices to the same system, for hassle-free connection of all the devices in your studio.

Big Ben Rear Panel

If you take a close look at the rear panel of Big Ben, you'll notice an expansion slot. This slot makes it easy to expand Big Ben's functionality. By adding an X-Video Expansion Card to Big Ben, you'll be adding a composite video input and 3 composite video outputs, as well as support for PAL, NTSC, and 60Hz B&W standards. The card is controlled from Big Ben's primary interface, and operates as a master black-burst generator and a Genlocking slave device. These features allow the card to re-clock and regenerate a video signal, replacing it with video-black.

X-Video Expansion Card

Final Word

Whether you're working in a small studio for personal projects, or in a big-budget studio producing the latest album or film soundtrack, keeping proper sync between your devices is a critical element for delivering high-quality results. A word clock signal generator such as the SyncGen or Big Ben could be the missing piece between "good" results and "stellar" results.


I now understand the benefit of keeping all your audio devices in sync, but how does this extend to video? If I want to record video from a camera for many hours at a time and record the multitrack audio using an Audio Interface and make sure the audio recorded will be synchronized with the video recorded, do word clocks help here? Do video cameras even have Word Clock Input ports?

I know some people just deal with it in post by trying to sync the multitrack audio to dummy audio recorded by the camera, but is it possible without any post-production work to just have sync'ed data at record-time?

Society of motion picture and telivision engineers provided us with the great SMPTE time code which is univeral between audio and video camera devices.

For many cameras such as higher costing prosumer and professional cameras, there are time code input ports that a audio engineer (from his sound recording device) can synchronize audio from one device (master ) to a another ( secondary). Cameras usually offer input and output for time code to allow for this flexibility. 

For more information, please email provideo@bhphoto.com