Audacity export doesn't synch with video

Audacity Help Forum macOS
1

I am recording video (1080p60 AVHCD format) to the SD card in my Panasonic HCX920 camcorder. I am recording the audio from a microphone output separately from the video in two ways. The audio from a wireless mic output is fed into my Mac Pro via an M-Audio Profire 610 firewire input. Here it is recorded with Audacity 2.0.3. I have the Audacity preferences set to 48000 Hz sample rate and 24-bit sample format. I’ve been exporting the audio as M4A (AAC) format. I am simultaneously sending this mic signal from the output ports of the Profire to a USB audio interface that is connected to a Mac Mini. On the Mac Mini I am recording the audio using the Wirecast app. The encoder is set to record the audio with the 24-bit Integer encoder with the rate set at 48.000 kHz and the bits per sample set at 16-bit.

The audio recorded on the Mac Mini is in exact synchrony with the video recorded on the SD card. But the audio recorded by Audacity is out of synch with the video. By the end of a 45 minute recording, the audio lags the video by several seconds.

There must be a simple explanation (and fix) for this that I am overlooking.
Your help would be greatly appreciated.
John

2

The explanation is simple but the solution is expensive.
The simple explanation is that the three devices that you are recording on are not all running at exactly the same speed.

The solution is to synchronise all devices to a single “word clock”. Note that it tends to be only the more expensive “pro” equipment that support an external word clock (I believe that the Profire 610 has word clock sync ability via its S/PDIF connector but you would need to check the Profire manual for details).

There is an explanation about word clocks here: http://www.sweetwater.com/forums/showthread.php?49165-What-is-Word-Clock