This post focuses on digital sync between PCM digital audio devices.
Digital has been marketed and is generally understood to be bit-for-bit perfect, resilient to noise, and able to be copied infinitely without any reduction of quality. This is true… to a point. Digital audio is essentially data, reinterpreted as audible information. But, underneath all the simplicity of the digital medium lies quite an elaborate system of error correction and synchronization. Did you know that the hard drive in your computer routinely has data errors? There’s software running in your computer that looks for these errors and corrects them as the drive is accessed. Digital is a balancing act and the goal is to have it all balance out to zero errors.
All things digital operate on what’s known as a clock. It’s just like a clock you’d find in the physical world, it keeps time in a regular manner. But these clocks are all a little different. Some run a little fast, some a little slow. If you’ve ever noticed that some of your clocks get out of sync after a while, it’s because the clocks aren’t part of the same system. The same thing happens in digital audio. If you hook up two digital audio devices together and have them talk digital to each other without sync, there’s a chance that one of the devices is running a little fast at 44105 ticks per second, while the other is running a little slow at 44095 ticks per second. That’s a 10 tick difference where audio is dropped, which can produce pops or clicks in your material. Not cool.
What can we do to fix this? Make the two clocks part of the same system! This involves using digital sync. Synchronization is like dancing whereby one device leads while the other follows. With most modern digital formats such as SDPIF and AES/EBU, digital sync is essentially piggybacking on the audio signal. Usually all that needs to be done is to fish around in the menu system of one of the devices and set it to ‘master’, then do the same on the other and set it to ‘slave’ or something along those lines. If both devices are set to ‘master’ (probably the default), they’ll ignore the sync signal altogether and remain out of sync.
Some things to consider…
- Set the higher quality of the two devices to master. One of the things that sets high-end digital devices apart from cheap devices is rock-solid clock. Some boutique devices such as Apogee’s Big Ben are designed only to do one thing: keep sync locked down. I dunno if it’s worth that kind of money, but sync is certainly one of the most fundamental things that can affect your audio quality – so if you’re shooting for the best, why not?
- Make sure you run both devices at the same data rate!