How to Synchronise Audio with Video

Recording the same event with more than one device is common practice during video and film productions. You might make a separate sound recording to go with your video or perhaps use more than one camera. The real challenge in doing this is getting everything in synch when you get it back into your edit suite. The problem is that every device has its own clock. Just like digital watches which, if once set to exactly the same time, slowly diverge over time. To solve this problem, one of your pieces of footage has to be played faster or slower than it was recorded. It is always better to change the speed of a soundtrack instead of changing the speed of a video clip so that you don't have to drop video frames.

I've been using Adobe Premiere Pro CS3 recently and found that the tools that can be used to synchronise sound with video are quite imprecise. It was impossible to get two pieces of footage to match exactly, so I developed a technique which allows you to synch up an audio track with any piece of footage even if your editing program doesn't allow speed changes. This technique is not limited to Adobe Premiere, it will work with any respectable editing package.

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How to Synchronise Your Clips

Choose a Sample Clip
Find the longest continuous clip on your tape which was recorded with the soundtrack and drop them both on your timeline.
Choose Start Synch Points
Find a point near the start of the clip which you can synch both tracks together. The ideal point would be the clap of a clapper board but sometimes it's not appropriate to use one of these. I like to use 'S' sounds in speech. I mute all but one audio track I'm interested in and advance the play needle one frame at a time until I hear the first hiss of the 'S'.
Find the same synch point on both tracks.
Line Up In Points of your Tracks
Take your synch points from the last step and trim the clips so that they both start at their respective synch points. Then drag them to the start of your timeline. When you play back your timeline now, the two clips should be in synch, at least for a little while. If they are not, try to find a different synch point.
Choose End Synch Points
Find a point towards the end of each clip that you can synchronise with in the same way as step 2. This time, trim the clips so that both clips end at their respective points but don't line them up.
Measure the Lengths of the Clips
Now take your play needle and place it exactly at the end of the shorter clip. Notice what time this point occurs in the timeline and note it down, naming it lengthshort. Do the same for the longer clip and name it lengthlong.
Calculate the Stretch Factor
Now you can calculate the stretch factor by dividing the lengths found in the previous step. This stretch factor can now be used in a sound editing program to change the speed of your entire audio track (not just the sample clip).

I have used Adobe Soundbooth for this and I prefer to set the duration, so I divide entire durationoriginal of the audio track by the factor if the audio track is too long or multiply if it is too short to figure out how long it will now be. I call this durationnew. Soundbooth has a feature that it can retain the original pitch of the recording which would otherwise change with the playback speed. This is very useful since if the pitch does change, you will not be able to mix this audio track with any other piece of audio or it will sound out of tune.
Save and Reimport the Audio Recording
Now that your audio recording has been retimed, reimport it into your editing program. You can now align the recording with any piece of video shot with the same camera at the same time by finding only one synch point; The audio should never diverge from the video.