Sound sync problem

Question

I have a bizarre sound synchronization problem which I am really puzzled with.

I recently recorded an interview with the video camera capturing the sound of the interviewed and an external sound sound capture for the interviewer.

When editing it with Adobe Premiere CS6 I started by synchronizing the start of the interview since both recordings started a few seconds apart. So far so good.

Then, one of the recordings was lagging and at the end of the recording (lasting around 25 minutes) where I found a delay of more than one second on one of the sound tracks. So I found a sync point and used the rate stretch tool to adjust one of the tracks to match the other and since beginning and ending were synchronized (I checked) all was fine... Or at least so I thought.

When I started editing further I realised that towards the middle of the recordings there was further de-synchronization. It is like the lagging and speed-up of sound is non linear; like an accordion...

Has anyone experienced this before? What causes it? How can it be corrected other than cutting everything in smaller chunks and adjusting them one by one?

Thanks in advance for any help.

PS: I have also posted this same question on Sound Design beta since I laster realised it relates more to sound than to video.

Answer 1

The most common cause of this kind of asymmetric drift is dropped frames. With older equipment, there were sometimes inconsistencies in the internal clocks that would result in issues that time didn't flow at a consistent rate between samples, but most decent modern hardware, while it may have slightly different rates, is inherently stable at that rate.

Dropped frames, however, upset this balance. With a dropped frame, data is missing or delayed for an instance of time and that gap has to be absorbed. Frequently, that is absorbed by simply pretending the frame didn't occur. When this happens however, sync is lost with any other source that did not also drop a frame at that same time.

It can be quite a pain to deal with since you don't necessarily know where exactly the frames were dropped and need to resync on either side of a dropped frame. This is why it is best to ensure your recording equipment has fast enough cards and processing to avoid dropped frames in the first place. They are a major nuisance and even one dropped frame is often enough to cause a shot to be re-captured or re-shot if detected at a time when it is possible.