Why is my audio drifting when I slave the audio in my computer to SMPTE? (Why does my audio drift?)

Whenever slaving audio in the computer to an external source such as SMPTE, you must have Word Clock (Time Base) as well as address (SMPTE or MTC). The Word Clock tells the audio in the computer exactly how fast to go. Without Word Clock, the rate of the audio in the computer will never match the speed of the audio outside of the computer. The symptom would be that your audio will drift over a period of time.

The solution is to use an MTP AV or a device such as a BRC. The MTP AV will convert the incoming LTC (or SMPTE) and generate MTC (the Address) for the computer as well generate Word Clock (the Time Base) for your Audio Hardware. If you have Audio Hardware with a Word Clock input (to connect the Word Clock to) such as MOTU Audio Hardware, then the Word Clock will keep the audio in the computer running at the same rate as your external audio for phase accurate sync.

Note: Some software programs offer a feature that may be called "Sync Audio to Time Code". While this featue may work for short sound files it is not the proper solution. This method works be actually pushing or pulling the audio according to the incoming address since it has no Word Clock as a reference. This method could also cause artifacts in audio playback.

For further information you may want read the tech note titled Word Clock. Also, if you use a MOTU FireWire audio interface or a MOTU PCI-424 system, check out the tech note titled Slaving directly to SMPTE.