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Old August 25th, 2005, 05:15 PM   #1
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48kHz-12bit-4 channel audio

I have 2 miniDV tapes recorded in 12 bit mode with 4 channel audio (I think recorded from an XL1 cam). When I captured the footage into FCP 5.0 a warning came up telling me the sample rates are not the same on the clip with the source tape. I looked to change the setting but I can't seem to find anything that lets me choose 12bit. Is it not possible to edit 12 bit mode in FCP? And if not, any suggestions to make this work to where I don't loose audio sync with the video? Any thoughts will be much appreciated!
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Old August 25th, 2005, 05:20 PM   #2
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Jay, try setting your audio to 32KHz instead of 48KHz.
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Old August 25th, 2005, 05:55 PM   #3
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Duh. I was so focused searching for the "12bit" that i didn't look at other options. Thanks for your quick response Nate! Definately what I needed.

I am guessing I need to do two different captures of the same video clip to get the channel 3-4 audio tracks into FCP correct? Seems kind of odd, but oh well.
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Old August 25th, 2005, 06:59 PM   #4
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I think somewhere in the capture settings you can choose to capture all four audio channels.
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Old August 25th, 2005, 08:39 PM   #5
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I now captured a 45 minute clip and the audio is not synced with the video. The tape was recorded on an XL1 camera with 12 bit audio ch1-4. I brought the clip as 32Khz setting in FCP 5.0...any ideas why the audio is out of sync?
Not doing too good today...
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Old August 25th, 2005, 10:06 PM   #6
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I'm not sure it is possible. Think about it...12bit audio is not even supported by your soundcard. Sound cards support 8, 16, 24...but no 12bit. It may be interpolating the audio and as a result, skewing its time. I remember having trouble trying to work with 12bit audio in Adobe Audition. It was actually being time stretched to fit into a 16bit audio stream because it wouldn't allow me to record it in 12bit.
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Old August 26th, 2005, 03:57 AM   #7
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"Interpolation" doesn't occur when converting 12-bit data to 16-bit data.
The 12-bit are just packed into the 12 highest bits of the 16-bit word.
This is a level operation and doesn't affect time alignment.
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Old August 26th, 2005, 04:36 AM   #8
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Jay,

If this was the original XL-1 shooting those tapes, there is a known issue with the audio not running at precisely 48khz and 32khz. This will lead to the sync issue you are experiencing. The workaround is to capture shorter clips. Try recapturing that way. Or, you can adjust the audio to be in sync by adjusting it on the timeline. Turning on the waveform display for the audio on the timeline will help you sync up picture with sound.

In general, the mini-DV format is prone to sync issues on longer clips because it doesn't use 'locked audio' like other formats such as DVCAM do.

Stephen,

The Mac platform doesn't work with audio the same way PC's do. In fact, I don't know that you can even put a sound card into a Mac. In any case, I think Gints is correct in the way 12bit is mapped into 16bit audio.


regards,

-gb-
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Old August 26th, 2005, 08:21 AM   #9
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Thanks for all your help.
Mental note for future...no 12bit option! I guess we live and learn...sometimes the hard way.

I have captured a longer clip with my Sony VX2100 camera and it was just fine. So this issue must be with the XL1 and being 12 bit.
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Old August 26th, 2005, 02:17 PM   #10
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what greg was referring to is how the audio was recorded on the tape... so it shouldn't matter what playback deck is used for capturing a digital file, the data on the hard drive should still be the same.

if i remember correctly, there is only one way to set up 4-channel audio on the xl1... i don't think that the dv standard allows for any choices in that respect.
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Old August 26th, 2005, 02:51 PM   #11
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I don't understand all the implications but just found out yesterday that the XL1 (and presumably the XL2 as well) record audio at 48.009 ksps in 16 bit mode and 32kHz + a similar percentage offset in 12 bit mode. This was apparently revealed to the world by an Apple engineer working on an earlier version of FCP. If this is indeed the case (I don't always believe everything I read on the net) FCP must resample the incoming audio in order to keep it locked to the video and this it is supposed to do. Thus FCP needs to be told or automatically detect that the source is a Canon camera so that it knows that it must do this resampling.

I don't capture with FCP preferring to use BTV pro. I have noted that FCP renders all audio so imported (always wondered why it did this and this is a clue) and am experimenting at this time to see if I can detect the removal of sync slippage when this rendering is done. This may be related somehow to the problem being described here.
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Old August 27th, 2005, 02:12 PM   #12
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some early xl1's did indeed have an issue with the frequency being off very slightly... but that situation was fixed years ago, the xl1s and xl2 have never had that problem.

i think that one of the old versions of premiere addressed the frequency issue of the xl1 without having to render the audio, but it would make sense that you could re-render to the exact 32khz/48khz... that is a seperate issue from capturing the audio to the hard drive, tho, because the data on the hard drive is exactly the same as the data on the dv tape... it will not change based on just the playback deck.
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