"Extract Audio"-What Does It Really Do?

Question

Level 1

6 points

"Extract Audio"-What Does It Really Do?

Does "Extract Audio" extract audio at the 48KHz 16 bit optimum that is suggested by Apple, or does it simply copy the audio at whatever the original sample rate of the Audio track was?

The reason I am asking is because "Extract Audio" used to eliminate the "audio sync problem" for older video tapes when making DVD-Videos in iDVD. Doesn't seem to do the trick anymore.

I suppose I could try to "Export" the audio at the 48KHz, 16 bit setting, then reimport.

As a side, is there anyway to find the audio track sample rate?

Any and all input is greatly appreciated.

Powerbook G4 1GHz, Mac OS X (10.4.5)

Posted on Mar 17, 2006 5:00 PM

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Answer 1

Mar 17, 2006 5:41 PM in response to Pablo Kent

Pablo,

I've always assumed that when you extract audio, all iMovie does is to copy the audio, place it on one of the audio tracks and set the volume level to zero in the original video track.

I think it just copies what's there without any changes.

I know when you choose Get Info in iTunes you can see the sample rate. Perhaps you could use it to examine your track.

I'm just shooting from the hip here. Hopefully others more knowledgeable than I will also chime in.

Matt

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Answer 2

Mar 18, 2006 12:25 AM in response to Matthew Morgan

Hi Matthew

I don't think the "extract audio" just copies because once you have "extracted audio" you can't get it back. The volume control in the video track is muted forever.

Also in past versions of iMovie once the audio was extracted one could not use the facility to pull clips out, I don't know what this is called as I never use it.

The iMovie sound is a law unto itself.

Karl is the expert in this area.

Jane

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Answer 3

Mar 18, 2006 8:46 AM in response to Jane Thorpe

Jane,

"The volume control in the video track is muted forever."

Not true. You can just raise it again with the clip volume controller. Try it!

After a bit more research. It appears that when you extract audio, iMovie converts it to an AIFF file.

In my case, it also seems to preserve the 16 bit rate that the audio was originally recorded with.

Matt

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Answer 4

Pablo Kent Author

Level 1

6 points

Mar 18, 2006 10:19 AM in response to Matthew Morgan

Matt,

Your hint about iTunes "Get Info" has allowed me to answer my own question. Thank you. The audio is extracted at 16 bit 48 KHz. This is true even if the original audio was analog 8mm, as with my old 8mm 2 hour tapes that I imported to iMovie with a Sony Digital 8 Camcorder. The extracted audio is COPIED to another track, and the original audio is muted. It is not deleted.

Of course, my original mystery still reamins: Why is the audio "out of sync" on my iDVD 6 projects, when this process seemed to remedy the problem in iDVD 4?

Part of the problem, I believe, has been alluded to in other posts. Becuase iMovie 6 now imports up to 1 hour clips at a time (as opposed to the old 9 minute plus limit), and "Extract Audio" extracts audio from the entire 1 hour clip, not the clips that are user "Split At Playhead", there is some sort of performance penalty. This penalty may cause the end problem of "audio out of sync" on iDVD 6. I know "Extract Audio" from these old analog 2 hour tapes now takes hours instead of minutes. I am using a external drive, but this didn't cause any problem in the past.

At least this part of the puzzle has been answereed. Thanks again.

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Answer 5

Mar 20, 2006 1:24 AM in response to Pablo Kent

Hi Pabio

Re your DVD out of sync problem. I had this problem when I first authored a long project in iDVD. At the end the sound went out of sync. I had joined up several 20 minute projects and this was the problem. The clips were too long and the sound and the pictures parted company.

In professional DVD programmes one is able to put in compression marks to keep the two together. Every cut made the compression marks are added automatically.

I think it was Karl who suggested that I put in a few cuts especially at the end.

This worked a charm. Try it.

Jane

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Answer 6

Mar 20, 2006 1:34 AM in response to Matthew Morgan

Hi Matthew

Yes you are absolutely right! I never realized that! I though extract sound meant extract sound! Silly me.

But this does open up opportunities when using a clip mike, which only comes out of one speaker. If one extractred the sound and then pushed up the video line sound might it come out of two? it certainly pushed the volume up.

I must give it a whirl, as they say.

Thank you

Jane

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Answer 7

Mar 20, 2006 2:47 AM in response to Jane Thorpe

Jane,

All the audio tracks in iMovie are actually pairs of left and right stereo tracks.

So to deal with audio that was only recorded on the left or right channel of the stereo pair requires venturing outside of iMovie.

Here's a recent thread that deals with it:

http://discussions.apple.com/message.jspa?messageID=1847852#1847852

Matt

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Answer 8

Mar 21, 2006 12:02 AM in response to Matthew Morgan

Thanks Matthew

I solved the problem with Final Cut Pro!

Things have to be really easy for me otherwise they do not get done. I am afraid my audience has just to put up with the sound only coming out of one speaker.

Obviously this does not happen now!

Thanks to you all for being such a help. Anything I want to know I get an answer here.

I have got better at it over the four years but I have still a lot to learn.

I just enjoy it. I think it is amazing that one can do this at home! I always wanted to make TV programmes but was not allowed to do so. Women were personna non gratia in my day. Still are some extent. When I attend Apple presentations I can count the women on the fingers of one hand here in NZ and yet editing seems such a good job for women.

Again thanks for helping us all out.

Jane

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Answer 9

Mar 21, 2006 8:07 AM in response to Jane Thorpe

Jane,

I overlooked an easier solution which Sue points out in this thread.

Matt

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