Tricky audio problem

We recently had a problem with mastering a digital beta tape using FCP 3.1.

The final audio mix was brought in as a wav file. Playing the file through quicktime, worked fine, however there were very specific sound FX that would not play back through final cut or our system. I even exported the clip from FCP and made a movie in the Black Magic Decklink codec. That file played the auido just fine from the desk top, however when I brought it back into FCP, and tried to lay it off to tape, the same SFX dropped out of the sound track.

So we isolated the sound that wasn't working, and brought it in. The same problem persisted. We then exported the sound file so that instead of having audio in only one channel of a dual channel audio file, we just made it a monochannel sound, and put it in one channel only of our time line.. and THIS WORKED!

So I have a fix, but its the problem that we dont understand. We need to make sure that this sort of thing doesnt lead to larger problems in the future and would really like to get to the bottom of it. If anyone could help us out with this please do!


-A

AFAIK FCP doesn't play well with .wav files, it likes AIFF files. That could be part of the problem. Also by the sound of your work around it could be that you had a mono file on 2 tracks which could be causing a phase problem and they were cancelling each other out. So when you have just a single file on one track it works.

Nolan's right. Whoever contributed the effects track did it as a mono track shared between left and right, but did it with one of the two electrically out of phase with the other. As long as you listen to the show in full stereo, the effects track will be there although if you listen carefully with headphones, it will sound funny and "like it's coming from behind you."

The minute you mix the show to mono, it electrically adds the left and right sound to each other. Since one of the effects tracks (left) is mirror-image of the other (right), it vanishes.

If the effects are part of the completed sound mix, you're kind of dead in the water. There's no way to fix this in post. If the show is ever broadcast to my house, I won't hear the effects track at all because I listen on a mono sound TV.

One of the classic ways this happens is an error in the way an original analog effects recording was digitized. Broadcast analog audio is easy to mess up this way and is one of the reasons large sound houses have meters that detect this. Sound editing programs can fix this easily, but I don't know if FCP has the ability to flip one track. You still can't fix it once it's part of a healthy mix. You can only fix it if you have the efftects track available by itself.

Koz