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Audio crackling

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Oliver Peters
Audio crackling
on Aug 31, 2012 at 6:54:27 pm

I recently sent audio to my Pro Tools mixer as roles. He complained about static-like crackling in the audio. It was there when I listened to the tracks in FCP X, but not as prominent as on his system in the processing chain. When I compared the same source audio in FCP 7, there was no crackling. This was recorded on a PIX240.

In the same session I had another project that originated on a KiPro Mini. On this job, I also exported the audio to him as roles. No complaints with these files. It would be easy to blame the mics or the PIX240 on the other shoot, but that doesn't explain why it was clean in FCP7. Any thoughts on how this can happen and how to avoid it? My guess is some sort of DC Offset issue, but I really have no idea. Of course, there's no way to fix that in X that I've found.

One of the troubling things in X is that there is no control over sequence bit-depths nor bit-depths on export (except via Compressor).

FWIW - no analysis/enhancement was done to the audio in FCP X.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Michael Gissing
Re: Audio crackling
on Aug 31, 2012 at 11:03:14 pm

I would start by getting one of the Pix files and loading the audio straight into a DAW to check for issues like DC offset. FCP 7 was poor at handling bit depth issues so the fact that the audio plays fine in 7 makes me suspect there is a problem in the mix processing on X.

Try a high pass filter set to 30 hz with a steep roll off to see if that gets rid of crackles on a known clip. It might be low frequencies causing intermodulation distortion. This filter should help if it is DC as well. Sweep it up to higher frequencies in case 30 isn't catching it.


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Oliver Peters
Re: Audio crackling
on Aug 31, 2012 at 11:27:58 pm

[Michael Gissing] "Try a high pass filter set to 30 hz with a steep roll off to see if that gets rid of crackles on a known clip. It might be low frequencies causing intermodulation distortion. "

Thanks for the suggestions. I've exported a number of variations for the Pro Tools mixer to test. That won't be until next week. What I'm starting to think is that it's a combination of two things - levels and stereo. This is split mic audio and I sent it to him as stereo - working under the belief that stereo would be treated simply as a paired set of panned tracks like FCP 7. I think that's in error and it treats stereo in the same way as Premiere. So there's some interaction.

I went back through one of the spots I had sent him and switched all the clips to dual mono and then disabled the channel with the mic I didn't want. Next I went back and set all the levels to -6. Even though nothing is visibly clipping, I don't really trust FCP X's goofy waveforms - not to mention that none of us really know how it's handling the internal floating point processing. In any case, these adjustments seemed to get rid of the crackling. I'll know better next week.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Chris Harlan
Re: Audio crackling
on Aug 31, 2012 at 11:42:47 pm

Mismatched bit-depths and/or sampling rates can cause something that I would describe as crackling sound. I occasionally run into this at a hardware level where sync clock is taking orders from something it shouldn't, but I've also encountered semi-corrupted files that seem to have flags that say they are one thing, when their sampling is in fact something else. When its a file issue, I usually convert to a different format, like .wav, and then reconvert, all the while making sure bit-depths and sampling rates are set correctly.


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Oliver Peters
Re: Audio crackling
on Aug 31, 2012 at 11:57:40 pm

[Chris Harlan] "Mismatched bit-depths and/or sampling rates can cause something that I would describe as crackling sound."

Correct. FCP X doesn't display audio bit depths anywhere that I can see. Only kHz.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Chris Harlan
Re: Audio crackling
on Sep 1, 2012 at 12:02:32 am

[Oliver Peters] "Correct. FCP X doesn't display audio bit depths anywhere that I can see. Only kHz.

- Oliver
"


Sigh. That's sad, but not surprising.


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Steve Connor
Re: Audio crackling
on Sep 1, 2012 at 4:13:14 pm

[Chris Harlan] "[Oliver Peters] "Correct. FCP X doesn't display audio bit depths anywhere that I can see. Only kHz.

- Oliver
"

Sigh. That's sad, but not surprising."


It does but only if it's available in the file metadata, go to Inspector > Info > Audio view drop down at bottom of inspector window.

Steve Connor
'It's just my opinion, with an occasional fact thrown in for good measure"


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Oliver Peters
Re: Audio crackling
on Sep 1, 2012 at 4:32:25 pm

[Steve Connor] "It does but only if it's available in the file metadata, go to Inspector > Info > Audio view drop down at bottom of inspector window."

Yes, I see that for sources. What about projects?

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Oliver Peters
Re: Audio crackling
on Sep 7, 2012 at 12:11:20 am

After a bit of testing back and forth between the Pro Tools mixer and me, this is what I think happened and where the error is.

The production signal flow was as follows. Mics into a Canon C300. SDI from the C300 into a PIX240. The C300 SDI out is only with pulldown, so pulldown was extracted on-the-fly by the PIX to record 23.976fps. The PIX QuickTimes were the edit source. These QuickTimes are the ones that crackle in FCP X, but not in any other application I tested. I can send an OMF generated from these files in FCP 7 to the mixer and it all sounds fine.

The final test today pretty much nailed it. I "ripped" an audio track from one of these camera files using Soundtrack Pro and exported an AIFF. Then I imported the AIFF into X and put it back-to-back with the QuickTime file in an FCP X timeline. Crackling in the camera file and no crackling in the AIFF rip from the exact same file.

I don't know why X is sensitive to this and 7 or STP aren't, but the issue seems to go back to the field configuration. A similar production flow on a different job (using a Sony F3 and KiPro Mini) did not exhibit crackling in X.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Michael Gissing
Re: Audio crackling
on Sep 7, 2012 at 12:17:49 am

That tends to confirm my suspicion that it is a mix engine function within X if an OMF out is clean.

The answer may be in the metatdata of the audio in the quicktime. Sound Devices might be interestedd in this feedback as it may be something they are doing to the audio data when they are passing it through during the on the fly pulldown conversion.


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Oliver Peters
Re: Audio crackling
on Sep 7, 2012 at 12:19:10 am

[Michael Gissing] "Sound Devices might be interestedd in this feedback as it may be something they are doing to the audio data when they are passing it through during the on the fly pulldown conversion"

Good suggestion. I'll try to pass this along to them.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Oliver Peters
Re: Audio crackling
on Sep 12, 2012 at 3:39:31 pm

Coming back to this issue. Apple and Sound Devices tech support are both on the case. I'm not 100% sure, but the issue seems (at least IMHO) to be related to the C300/PIX240 combo used on the original production. The C300 only outputs 59.94i over SDI when shooting 23.98. The PIX can remove the added pulldown to end up back with 23.98 files. I've done some checking online and there seems to be an issue when you do this over SDI as opposed to HDMI. In any case, my files were recorded using SDI. The picture was fine, but the audio crackled (but only in X). Yesterday, I checked some other files recorded using a C300/PIX240 - but via HDMI - and the audio was clean in X. My suspicion (not confirmed by SD or Apple, yet) is that there's some incorrect metadata being written when the path is SDI, probably because the stream isn't correctly detected by the PIX240. This seems to show up as an uncorrectable sample rate mismatch in X, thus the audio artifacts.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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MIke Guidotti
Re: Audio crackling
on Sep 4, 2012 at 8:04:37 pm

In general corrupt audio files could cause crackling like random pops and clicks also referred to as dropouts. This happens when the errors are so large the error correction circuitry of the D-A converter runs out of material to interpolate the missing or bad information from.

You were right to check the source files by having your Pro Tools guy/gal import it directly. If he is worth his salt he can re-import and phase-match the original tracks to the cut ones in tools, then toss the ones that sound like rice krispies…


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