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volume issue after phase cancellation with gain phase left

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Paul Godard
volume issue after phase cancellation with gain phase left
on Mar 24, 2016 at 12:24:53 pm

Thank to JM on Premiere Pro forum... I was happy to find a solution... but as I was on FCPx, I had to update the solution a bit but it worked... BUT

THE BACKGROUND...
I have recorded some interviews with a shotgun Sennheiser connected to my Nikon D800 via an home made XLR to stereo jack... working fine except a very small background noise.
I realised that although the sound plays perfectly well on distant stereo speakers... it becomes bad or silent on smaller device like my cellphone Note 4... then I learn about phase cancellation.
Following the very easy solution that FCPx offers, by ticking phase left or right only in the gain filter... the phase cancellation problem is solved. BUT...

THE NEW PROBLEM
The new problem is that the sound is now by far too low to hear, although the meters seem unchanged. If I export the video, the sound is still too low.
In order to have the correct voice, I need to push up the gain by at least 8... but then the meters often go in the red.
Another issue is that for some voices, doing the phase left and gain at 5-6 seems to reduce the sound quality.

What is the best way to go around that?

Paul Godard - Soul Photography

Paul Godard - Soul Photography


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John Rofrano
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 12:57:57 am

[Paul Godard] "The new problem is that the sound is now by far too low to hear, although the meters seem unchanged. "
It sounds like you might be actually causing a phase problem instead of solving it. If I read what you are saying:

(1) The meters show perfect volume
(2) The sounds coming out of the speakers is low

VU meters don't show phase. They show signal. So you have plenty of signal. But why are your ears not hearing it? (hint: phase cancellation)

What you are describing is the signal canceling itself as it leaves the speakers because it is out of phase which is why your ears perceive it as being low. So I believe you are actually causing phase cancellation while attempting to correct it.

I would use a Phase Meter plug-in to check what the phase of the audio actually is. Don't guess! You can buy one or download the free Voxengo SPAN plug-in and check the Correlation meter. If the meter shows bars extending from the right the phase is fine. If the meter shows bars extending from the left the audio is out of phase (use the gain filter to correct it).

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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Paul Godard
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 5:29:13 am

Thank you John... I will definitely truth phase detector plugin a bit later...

Meanwhile maybe you can listen to the video clip and tell me what you think.

http://www.ticket2utopia.com/Blog_8_Damaraland_Camp.html

What is very strange to me is that only the sound of the first interview is silent on my cellphone, but the ones after that are fine, although all recording was done the same morning with the same equipment.

That video shows another sound issue on my cellphone... very weird.

http://www.ticket2utopia.com/Blog_4_Grootberg_Lodge.html

And of course everything play perfectly well on my macbook pro.

Paul Godard - Soul Photography


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Paul Godard
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 7:43:35 am

I have downloaded Voxengo but honestly I am a bit lost... what do I have to look for in the SPAN window?

I have uploaded a small mp4 that has the problem on cellphone, maybe it will help you to find a solution.

http://paulgodard.com/TestAudio.mp4

I am desperate... I need to find a solution before end of day!

Paul Godard - Soul Photography


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Paul Godard
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 8:11:17 am

If I understand correctly... the sounds was recorded on dual mono with one channel opposite (negative current) to the other. The meters on the camera were correct as the sound in my headset.

Now that I need to cancel one chanel, I get half of the volume (changing from stereo to dual mono in audio/channel config and then unpicking one channel.

So far I understand all this...

In the gain filter when I tick phase left (or right) I get the same volume drop. Why? I thought that the purpose of that filter was to copy the content of one chanel over the other one... so I should not get a reduction in volume?

Is there another way to easily copy one chanel to the other in FCPx?

Paul Godard - Soul Photography


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John Rofrano
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 3:52:02 pm

That test file was a good example. Here is what I did:

I placed Gain before Span so that I can see the effect it will have. In this first shot, Gain is normal and you can see the phase problem:



By swapping the Left channel (or the right but not both) you can see the audio is now in phase again:



You can really head the difference between these two. Hopefully this helps.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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Paul Godard
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 4:22:34 pm

Thank you again John... really appreciate!

That is exactly what I have down and the phase issue is now solved.
The new problem is the sound level... and degradation. Why is the sound lower after applying phase left or right?

In the next example, I thought that the initial recording was good (part 1), but after gain with phase left, the volume of the voice is too low and the background noise too present (part 2), so I apply a gain of 6 and removed the background noise at 75% (part 3) but the sound quality is now degraded.

http://www.paulgodard.com/TestAudio3.mp4

What is your advise to fix that new problem?

And later... but that is not so urgent... how can I avoid the phase issue at the recording with my equipment? What kind of cable or box would solve the problem?

Paul Godard - Soul Photography


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John Rofrano
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 6:32:13 pm

I'm not sure why you have a phase problem in the first place. This is just dialog. It should be a mono signal. Just change the audio from Stereo to Dual Mono and turn off the second mono channel. Phase problem solved without the need for plug-ins. When you render, FCP X will feed the mono signal into both stereo channels so the final render will be a copy of the mono signal in each stereo channel. I believe this what you are trying to achieve.
[Paul Godard] "I apply a gain of 6 and removed the background noise at 75% (part 3) but the sound quality is now degraded."
The sound is degraded from too much noise reduction. Sometimes it's better to use less reduction in multiple passes. I would remove the noise first and increase the gain later but FCP X doesn't allow you to do this because it's noise reduction is placed after other plug-ins. You would need to use a 3rd party noise reduction like CrumplePop AudioDenoise, SoundSoap, or iZotope RX DeNoise (all good btw).
[Paul Godard] "And later... but that is not so urgent... how can I avoid the phase issue at the recording with my equipment? What kind of cable or box would solve the problem?"
You'll need to explain how you are recording to figure that one out. How was this recorded that it turned into a stereo file? One mic? Two mics? Stereo Mic?

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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Paul Godard
Re: volume issue after phase cancellation with gain phase left
on Mar 25, 2016 at 10:38:49 pm

"I'm not sure why you have a phase problem in the first place."
Well according to the correlation meters.... I had a phase problem.

"Just change the audio from Stereo to Dual Mono and turn off the second mono channel"
I did try that but again the volume goes down a lot although the meter stay at the same level and weird still on both L & R, although mono2 is no ticked!



"When you render, FCP X will feed the mono signal into both stereo channels so the final render will be a copy of the mono signal in each stereo channel."


I am trying that... and it kind of works IMHO... as when I re-imported the clip into FCPx, both channels are now filled up.... but the volume is still too low... so I have to use gain again.

"You'll need to explain how you are recording to figure that one out. How was this recorded that it turned into a stereo file? One mic? Two mics? Stereo Mic?"

As I said before, I am using a shotgun Sennheiser microphone (mono I presume) connected to my Nikon D800 via a home made XLR to stereo jack (I will need to check which connection I used as I can not find it right now) ... working fine except a very small background noise. What kind of cable or box would solve the problem?

Paul Godard - Soul Photography


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Paul Godard
Re: volume issue after phase cancellation with gain phase left
on Mar 26, 2016 at 5:34:42 pm

The best way I have found so far to deal with this problem is to export the voice over track to Audacity, remove the background noise, copy one chanel to the other and level... then export as best quality and reimport into FCPx... not ideal workflow but it definitely gives the best results!

So what should I do in the future to avoid background noise and phase cancellation?

Paul Godard - Soul Photography


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John Rofrano
Re: volume issue after phase cancellation with gain phase left
on Mar 27, 2016 at 1:38:06 am

[Paul Godard] "So what should I do in the future to avoid background noise and phase cancellation?"
I would get rid of the "home made XLR to stereo jack". It seems like that's what's causing the problem. Do yourself a favor and pick up a proper Beachtek DXA-2T Universal Compact Camcorder Audio Adapter. This will give you the XLR capabilities you are looking for with none of the nasty phase issues.

As for background noise... mic placement is the most important thing. If you are doing interviews with the mic attached to your camera, get it off of there and on a boom closer to the subject. Nothing fixes background noise better than mic proximity to the source.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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John Rofrano
Re: volume issue after phase cancellation with gain phase left
on Mar 27, 2016 at 1:53:04 am

[Paul Godard] "I did try that but again the volume goes down a lot although the meter stay at the same level and weird still on both L & R, although mono2 is no ticked!"
Mono has no concept of left and right. Left and right are stereo concepts. Mono audio, when feed into a stereo bus will play equally out of both speakers. You can, of course, pan a mono signal to the left or right once placed in a stereo signal path, but mono all by itself plays through all speakers equally if you don't change it's path otherwise.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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Paul Godard
Re: volume issue after phase cancellation with gain phase left
on Mar 27, 2016 at 9:44:02 am

Thank you John.

I will definitely buy an adapter box next time I shop at amazon.

Ideally I would like to find one that can power the mike (phantom) from the Nikon DSLR to avoid another batteries... but it does not seem to be a standard item... Any idea?

Paul Godard - Soul Photography


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John Rofrano
Re: volume issue after phase cancellation with gain phase left
on Mar 27, 2016 at 12:26:38 pm

[Paul Godard] "Ideally I would like to find one that can power the mike (phantom) from the Nikon DSLR to avoid another batteries... but it does not seem to be a standard item... Any idea?"
Yea, if you are really concerned with getting great audio, you might want to pick up the Tascam DR-60DmkII 4-Channel Portable Recorder for DSLR. This will not only give you phantom power but will probably record higher quality audio than your camera. I have one and it works great. It attaches right to the bottom of your camera between the camera and your tripod. You can link the audio with the video using FCP X built-in Synchronize Clips function.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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