If you record audio via an external recorder in order to replace the audio your camera captures, you’ll usually need some sort of a synchronisation tool. The simplest is using a clapper, with the second-best using Red Giant’s PluralEyes. But what if the audio your recorder captures drifts out of sync during recording? Why does that happen and how can you avoid it?
If you’re used to recording audio externally when shooting video, and you’re not using a device that supports timecode, like the Zoom F4, the Sound Devices MixPre-3 (and better), or an Apogee Duet iOS with the optional Ultrasync Blue Bluetooth timecode protocol module, you’ll risk your audio signal drifting out of sync when the recording exceeds a couple of minutes.
This happens because and means that you’re recording audio at a different framerate than the camera is running at. You can easily see this yourself by starting a recording of, let’s say, four minutes and generate a short, dry and loud sound every 30 seconds or so. Synchronising audio that’s drifting out of sync is much harder – if not impossible because of the difficulty that simultaneously arises with lip-syncing – than just synchronising the start of a recording that runs in sync for the rest of the time.
In short, you should avoid it from happening. Timecode supporting equipment is usually quite expensive. A non-timecode-capable Zoom H4n Pro costs $219 whereas a Zoom F4 is $549 and a MixPre-3 costs $649. In addition, your camera must be able to output – through a cable or an add-on like the Ultrasync Blue – its timecode so the two devices can synchronise.
If your main output channel is Youtube, the expense of these two components might be just a bit too expensive. So, how can you avoid drifting then, without buying a timecode device and possibly a wireless timecode generator?
For starters, every decent recorder does support beats-per-minute. Let’s say you’re going to record a 30fps movie shot on a GoPro Hero 5,6 or 7 and line up the sound with a recorder that is set to 120bpm. We don’t have to do anything as 120bpm divides perfectly by 30 and no drifting is likely to occur since the length of the external recording will obviously match or exceed that of the camera. No change to the tempo is needed and all we need to do is line up the two audio signals.
Things start going wrong when the 30fps isn’t really 30 frames per second but 29.97fps. This is what happens when you record with a GoPro Hero action cam set in NTSC mode.
In this case, those 29.97fps translate into 1798.2 frames per minute, and that’s not dividable by 120. You’ll lose a bit of synchronisation per minute and after a few minutes, you’ll start seeing the typical lip-sync error.
When you’re in the comfort of your studio, you can easily compensate for drift. Supposing you’re using Logic Pro X, the startup screen lets you set a “frames per second” option. With GoPro cameras, for example, you should choose 29.97fps, even if you read 30fps in the user guide. If you’re using a different camera and have experienced drift in the past, it pays to try to set Logic Pro X’s framerate to the NTSC ones or the others that aren’t rounded figures (like 23.97fps).
When you’re using Audio Hijack from Rogue Amoeba, there’s a sync block that you can put in-between the microphone and the format block. However, that one measures in milliseconds, so that isn’t very useful. And when you’re using a non-timecode-capable device or an app like iZotope’s RX 7 Advanced, Sound Forge Pro for Mac by Magix or others as well, you can only avoid drift if you set the camera to one of the rounded framerate figures.
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