Whenever you’re recording audio from a different device than your camera and without the help of a timecode generator, it’s distinctly possible that you experience audio drift. That is, your camera’s audio output and your recording device’s output start in sync but drift apart along the way. It’s very frustrating and — except when one of your devices is broken, of course — very easy to solve. It all comes down to checking your settings.
The problem with an external audio recording drifting in relation to the in-camera recording lies with your equipment lacking timecode locking capabilities. With timecode capability in both devices, your audio cannot drift and if it does, it’s immediately obvious that you’ve made an error somewhere along the way. Without timecode, that is not so obvious.
A good example is when you’re recording a video presentation using a GoPro HERO with the intention of replacing the HERO’s sound with a recording made on your desktop computer in post-production. The reasons for doing are plentiful. You might have a PC or Mac audio interface with vastly superior sound capabilities than any camera can offer, for example.
I tried it the other day using Logic Pro X as the recording software, a HERO 8 as the camera and Final Cut Pro X. This led to the same audio drift several times over. Now, as a professional reviewer I have a whole collection of apps on my system that most people usually only have one or two of. The advantage is that I can try out many different ways to fix the problem.
Well, in this case, I could just as well have had zero software on my Mac as the problem wouldn’t go away. I tried using Revoice Pro 4 and PluralEyes, both to no avail. I also tried using different recording software such as Audio Hijack instead of Logic Pro X and even Pro Tools Ultimate, which I am trying out right now for two upcoming articles. Nothing helped.
Drifting isn’t a normal phenomenon when using different recording devices. Synchronisation problems at the start of a recording are normal if you can’t synchronise the devices — for example, by lack of your camera sending a start signal to your recorder. Those problems are easily solved by using a clapper (or your hands) or software like Revoice Pro, VocAlign or PluralEyes. With Revoice Pro 4 you first load the audio in your DAW or directly in the app itself and then play with both pitch and clip length if you’re using it for synchronisation. However, synchronising drift appeared to be very hard if not practically impossible and the reason became clear after taking a good, hard look at Final Cut Pro X’s project settings I was using.
You see, GoPro devices will tell you they’re recording at 30fps in their menus and in the user guide, but instead, they’re actually recording at 29.97fps. You can check this by creating a clip, downloading it to your Mac and opening it in Telestream’s Switch, QuickTime Player or an open source videoclip editor/inspector.
The difference in frame rate accounts for the drift you’re bound to experience. You think you need to set up your project with a frame rate of 30fps, but in reality you should be using 29.97fps.