[ This article was first published in the April, 2009, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]
Bart Weiss sent this question in:
I have a question about editing sound. When working on a documentary we, of course, usually cut down the audio track. STP is great for this, especially with [ its ability to edit on the] zero point crossing. That would be the best way to cut the synch track down in Final Cut, but if I round trip from FCP to STP and cut tracks using the zero-crossing point, the picture comes back out of synch. How can I used the zero point editing and get back to FCP in sync.
Larry replies: Bart, thanks for writing.
First, the short answer: you can’t.
Now a bit longer explanation. When we edit in Final Cut Pro, all our edits MUST occur where the video frame changes. For example, in NTSC, we can only edit 30 times in one second. (For PAL, we can only edit 25 times in one second.) To keep this illustration simple, let us ignore all other potential frame rates.
Because it is not possible to edit on the half-frame, quarter-frame or any other partial-frame, all edits must occur on frame boundaries. Again, in NTSC, that means every 30th of a second.
However, audio is not recorded using a video frame rate. It is recorded using individual samples — 48,000 of them a second. When we edit audio in Soundtrack Pro, we can edit between any two samples — an amazing amount of precision!
Here’s an audio clip loaded into a Soundtrack Pro Audio File project. I’ve zoomed in so we can see the waveform more clearly. As this illustrates, audio is a wave — areas of high pressure (up) and low pressure (down). Or, in electrical terms, the wave measures the change in voltage of an audio signal. Positive voltage is up and negative voltage is down.
But notice that heavy gray line in the center, where the cursor is pointing. That’s called the “zero-crossing line.” This is the point in a waveform where the audio retains all its frequency (pitch) information, but has absolutely no volume.
If I were to make an edit as indicated by this selected area, I would be editing from an area of high pressure/voltage immediately to an area of low pressure/voltage. This will always cause a pop or click in the audio at the point of the edit. This is often the case when you edit in FCP because our audio edits MUST occur on the video frame boundary. The only way to fix this in FCP is to put a quick audio dissolve across the edit. It isn’t ideal, but it will minimize the click.
A much better way to work is to edit at the zero crossing.
Soundtrack provides a menu choice that makes that easy. Select Edit > Adjust Selection to Zero Crossing. My recommendation is to generally extend it outward. This avoids any possibility of a noise intruding into your edit.
Notice what happens to the selected area when we adjust the edit point? It has expanded both sides of the edit. Now, if you were to delete that selected area, the edit would be absolutely soundless.
This would be an ideal way to edit — EXCEPT that there is no relation between the zero crossing line and frame boundaries. It is entirely possible that the zero crossing line is more than a frame away from the selected edit.
Consequently, while it would be great to edit all video this way, when you are working with sync sound, you are best editing inside Final Cut, then trimming your edits in STP during the final audio mix. That way, everything stays in sync and you are able to clean up any noisy edits without getting out of sync.
I do almost all of my mixes this way and hope to create a video tutorial that illustrates this in the next few weeks.
For editing synced audio and video, turn snapping on and make sure that View > Snap To is set to Frames, rather than the default of Ruler ticks.
Also, you can use Edit > Move Selection > To Next Video Frame – or – To Previous Video Frame to keep everything in alignment.
UPDATE – April 2, 2009
Bob Demers adds:
FCP does support subframe editing for audio, which is handy for fixing zero crossing error. The resolution is 1/100 of a frame. Basically, it involves a shift-drag [of the In or Out in the Viewer] to navigate the timeline in the player audio tab. A search of the latest manual for “subframe” will lead to a couple pages on the topic.
Larry replies: Bob, you are correct. And you can set audio keyframes within a frame as well. However, the problem I have with sub-frame editing is that it is very easy to move this too far. At which point the audio will be out of sync, but FCP does not display any red flags. I’ve never been a fan of this technique.