High audio lowest level

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Peter Östlund

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High audio lowest level

PostThu Mar 28, 2013 11:04 am

I am using a brand new MKE 600 from Sennheiser with internal battery and professional made cable with a split into camera (to be able to record with different settings).
I all sound well but the audio meter (Alphatrone) shows a constant level that seems to be much higher than what I can hear. It seems to be very low frequency.
has anyone else had the same issue?

thanks


Peter
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Peter Östlund

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Re: High audio lowest level

PostThu Mar 28, 2013 4:51 pm

I am off on a job in SA soon.
I would really be helped if anyone could confirm if they have the same audio meter experience.

Thanks

P
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Zack Wilson

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Re: High audio lowest level

PostThu Mar 28, 2013 6:17 pm

Have you turned on the low cut filter? Does the issue go away?
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Peter Östlund

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Re: High audio lowest level

PostThu Mar 28, 2013 6:46 pm

Yes it is turned on.
The thing is the audio meters are at a high level even when the mic is not attached.
Strange.

Peter
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Robert Rozak

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Re: High audio lowest level

PostThu Mar 28, 2013 7:12 pm

I believe you are observing the DC offset issue. Hopefully that issue, as well as other audio issues, will be address in a future firmware update.

You can learn more on the BMCC DC offset issue (and other issues) in the BMCC Audio Applications Guide video:
http://www.youtube.com/watch?feature=pl ... rXGEUCRS5k
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Peter Östlund

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Re: High audio lowest level

PostThu Mar 28, 2013 8:14 pm

That could be the issue here.

Thanks Robert and Happy Easter

Peter
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Frank Glencairn

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Re: High audio lowest level

PostFri Mar 29, 2013 9:19 am

Robert is right, it's the DC offset - I see that on my TVLogic offboard monitor too.

Sadly there is nothing that you can do in the moment, but to get used to it, fix the offset in post and hope it gets solved in the (hopefully) next firmware update.
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Peter Östlund

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Re: High audio lowest level

PostFri Mar 29, 2013 9:41 am

Superspitze Frank!!

Case closed

Peter
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Peter J. DeCrescenzo

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Re: High audio lowest level

PostFri Mar 29, 2013 5:17 pm

Mic level recordings may be different than line level recordings on the BMCC.

Based on my all-too-brief, hands-on experience using a BMCC, I don't hear DC offset in recordings from a line level source with the camera's audio inputs set to "Line level" @ 75-85% and the internal mic set at 0%.

Here's an example:


Let me know if you hear DC offset in this recording. There may be in fact DC offset there, but maybe my old ears can't hear it?

As noted on the Vimeo page, I used a Sennheiser ME64 mic through a SoundDevices MixPre (original version) mixer. For this test I had both the mixer's & mic's hi-pass filters turned off, and I didn't use the mixer's limiter.
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Robert Rozak

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Re: High audio lowest level

PostFri Mar 29, 2013 6:42 pm

Peter,

DC offsets aren't something that you really "hear".

But, what they do is screw up your metering in production. Since its pushing the AC signal off center, it limits the achievable signal to noise level that you would otherwise be able to get. So, what you would "hear" is poorer SNR.

The are a pain in post production also. You've got to first remove them before you do any normalization, if you are going to be lining up your clips with other audio sources and want to maintain consistency in signal levels.

The DC offsets in the camera at LINE are less than they are at MIC.

But, because of the analog gain structure of the camera, a typical LINE level signal from an ENG mixer will require high gain in the camera at LINE. So, even though the DC offset is smaller, you're taking a hit in SNR because you'll have to use high gain in the camera.

This is kind of the folly when calibrating cameras to the needs of an external mixer. I prefer to find the sweet spot of the camera in terms of SNR (0dB digital gain which is around 20-30% in the BMCC), then calibrate the external meter to the needs of the camera.

In my tests, even with larger DC offsets at MIC level, I can achieve better SNR coming in at MIC level with the camera gain at 20%, versus going in at LINE level with an external ENG mixer tone (where I can't calibrate the meters) where I'm forced to use the camera at high gain. Your clip sounded pretty good for SNR. For some reason, I found that I had to push the gain higher than the 80% you used to calibrate the camera to the LINE tone of the MixPre-D. So, there might be some difference here that I don't have an explanation for.


If the camera had meters, then metering at the SNR sweet spot of the camera is a no-brainer.

Using an external monitor with meters allows you to do the same (the Frank approach) with the camera in the SNR sweet spot.

Using an external meter that has the handles for calibration (like the Riggy-Assist) is another approach to this. The downside is that you're metering BEFORE the camera (like with the ENG mixer). So, if you kick a cable out, or the camera stupidly switches from MIC to LINE, you won't get a visual indication.


I'm going to be releasing another solution to this issue at NAB ...


But hopefully, the camera will get a firmware update fixing its issues, like meters, DC offsets, and the MIC-to-LINE-switch-surprise ...


Robert
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Miriana Marusic

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Re: High audio lowest level

PostSat Mar 30, 2013 5:47 am

How do you 'fix' this DC offset in post. I can see/hear this as soon as I plug into the jack in the camera even if I haven't turned on the juicedlink.
Thanks
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Frank Glencairn

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Re: High audio lowest level

PostSat Mar 30, 2013 10:12 am

Most audio programs come with a DC-Offset plugin or function.
It's usually just one or two mouse clicks.
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Miriana Marusic

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Re: High audio lowest level

PostSat Mar 30, 2013 10:17 am

Thank you.
I will have a look when I get to my computer tomorrow. I'll have a look at audition, I guess, and see what I can find. Much appreciated.
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Miriana Marusic

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Re: High audio lowest level

PostSun Mar 31, 2013 1:22 am

Yes I found it - and fixed it great. Many thanks
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Peter Östlund

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Re: High audio lowest level

PostSun Mar 31, 2013 9:56 am

Good answers on this thread.
By the way, is there a way to adjust the DC-offset in Final Cut Pro 7?

thanks

Peter

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