Audio and Video Progressively Out of Sync

Symptom #1: Clips Go Rapidly Out of Sync and Exhibit Audio Artifacts

"My clips go rapidly out of sync in the Viewer or sequence, in addition to hearing the beeping sound saying FCP wants to render the audio. I may also hear some crunching artifacting noises."

Background

Using the correct sample rates in your capture settings is critical for avoiding out-of-sync audio. FCP and QuickTime use the number of samples per second to determine the timing of audio in playback. If an audio clip is identified as having been sampled 48,000 times per second (48 kHz), FCP plays it back at 48,000 samples per second. If the initial identification of the sample rate is wrong, FCP will be busy performing a sample rate conversion so that the file's playback rate matches your sequence sample rate.

Solution

The only way to guarantee that you don't encounter this problem is to make sure that your capture settings match the sample rate of the tape. With DV tape, this will be a default sample rate of either 48K or 32K, depending on how the camera was set. On DV cameras, these two rates are identified as 16-bit and 12-bit, respectively. Unless you have a DV tape that was recorded in 32K, you should always use 48K sample rates.

Symptom #2: Capture Settings Are Correct, But Clips Go Slightly Out of Sync After Several Minutes, and a Red or Green Line Appears in the Timeline Above the Clip

"My settings are right, but my video and audio still go slightly out of sync after playing back for several minutes."

"I know my settings are correct, but sometimes when I put my 48K clips in a 48K sequence, I still get a 'resample on-the-fly' green or 'render' red line on the audio clip in the sequence."

Background

One of the biggest problems with DV audio as a format is that sample rates aren't always standard. Some professional DV cameras and decks offer what is called locked audio, whereas the majority of prosumer DV cameras use a nonstandard unlocked audio sampling method.

With locked audio, the audio digitizer is extremely precise, and an audio sample rate of 32 kHz or 48 kHz will have exactly that number of samples per second. Audio sync is virtually guaranteed, because the ratio between the number of samples and number of video frames over a specific (short) time period are always the same. For instance, with locked 48 kHz audio, there are always exactly 8,008 audio samples for every five frames of NTSC 29.97 video (1,601.6 samples per frame).

Theoretically, all 48 kHz and 32 kHz recording devices use locked audio. Unfortunately, many aren't that precise. Some cameras are worse than others, and the irregularity of these devices can be very frustrating. Prior to Final Cut Pro 4, FCP included a Sync Adjust Movie feature that calculated these variations so that audio always stayed in sync.

With FCP 4 and later versions, sample rate calculation happens under the hood at the end of each capture. Every clip you capture is analyzed, and if the sample rate is off, FCP makes a slight adjustment to the clip's sample rate setting so that it plays correctly. That's why sometimes you put a 48 kHz clip in a 48 kHz sequence and you get a red or green line in the clip telling you that it isn't matched to the sequence. FCP may have detected it as being a 48,048 Hz clip rather than standard 48 kHz and recalculated it accordingly. Whenever FCP has to resample clips on-the-fly in a sequence, you will get that red or green line.

With unlocked audio, one of the most common non-standard sample rates you will encounter is 44.1 KHzkH. Because the number of frames required to find a common denominator between 44,100 KHz and video's frame rates is so large, the sample rate's relationship with time (and frame rate) usually fluctuates. Your track may start out on time and end on time, but during the course of playback, it may speed up or slow down to fudge the numbers and bring it into sync.

Solution

If you must use audio CD tracks with a native sample rate of 44.1 kHz, make sure you convert the sample rate of the track you are importing prior to editing the clip into the sequence. (For more information on sound file imports, see Lesson 13.)

Another reason you might end up with 44.1K clips is if you used the onboard mic or mic input of a Macintosh to record voice-overs in the Voice Over tool. As of this writing, the Macintosh uses a 16-bit 44.1 kHz sample engine for its onboard mike input, so unless you upgrade to another audio digitizing tool or use a DV camera, your Voice Over tool recordings will be in 44.1 kHz. Upgrading your recording device and doing voice-overs at 48 kHz doesn't have to be expensive. There are several decent USB audio adapters on the market, and you can use a DV camera or deck to do voice-over recordings.