-20db ref calibration on an HMC150

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Denwa

Well-known member
I'm used to calibrating cameras to -20db. The HMC150 has a mark for -12db. Any way to calibrate to -20db? Count the dashes?

I think the HVX has a mark for -20db and -12db. Wish the HMC150 did as well. Grumble.
 
I'd say, if you can, calibrate to -12dB on the HMC... the quality difference is very noticeable, especially when you boost levels in post...

Otherwise, get a device that will let you send tone at both -12 and -20, calibrate to -12, and then just see which "blip" on the HMC corresponds to -20...

Cheers,
Barry
 
Well.. tone is actually sent at max levels (setting your levels to 0 on the mixer) so that you then calibrate the camera to -20db so that 0 on the mixer is -20 on the camera. (Assuming the mixer and camera have Vu meters not PPM).

You are calibrating two pieces of electronics together so I'm not sure how to do that and maintain calibration between the two. I just got in from a gig so maybe I need to look at this later after I have woke up a bit.
 
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Each dot on the graph corresponds to 2dB of volume, so count down 4 dots from the 12dB mark and that'll give you -20.
 
Thanks Barry. I thought it might be something like that but sometimes it's hard to know for sure.

I just popped Dean Miles an email. I was taught Location Audio by him 6 or 7 years ago. Didn't know he had a book out. Unfortunately my notes from back then were pre DVX/HVX etc. Most of the audio I have done has been with Wendt's hooked up to shoulder mount cameras.
 
As a post guy, we've had nothing but problems with field audio recorded at -20. Most audio mixers I know are fairly conservative in terms of recording, so they don't let anything go more than 5 - 7 db above 0. So then you're wasting 13 to 15 db of headroom. Our audio mixers are always complaining that the OMF files they get from us are too low and they have to double or triple the tracks to get enough volume. If you do go with -20, make sure your audio guy stays on the plus side of 0 all the time. A lot of mixers have limiters that kick in at 10 or 12 db above 0 so agin, you're still wasting a lot head room.

-20 is definitely the standard for deliverable audio on show masters (with no peaks above -10), but for field audio it's been a pain for us unless our audio guys run everything hot.

Cheers

John Christie
 
jchristie said:
As a post guy, we've had nothing but problems with field audio recorded at -20. Most audio mixers I know are fairly conservative in terms of recording, so they don't let anything go more than 5 - 7 db above 0. So then you're wasting 13 to 15 db of headroom. Our audio mixers are always complaining that the OMF files they get from us are too low and they have to double or triple the tracks to get enough volume. If you do go with -20, make sure your audio guy stays on the plus side of 0 all the time. A lot of mixers have limiters that kick in at 10 or 12 db above 0 so agin, you're still wasting a lot head room.

-20 is definitely the standard for deliverable audio on show masters (with no peaks above -10), but for field audio it's been a pain for us unless our audio guys run everything hot.

Cheers

John Christie
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What field mixers? My experience has been with Wendts, recording dialog at around 0 with the limiter on.

I can't remember at what point the limiter kicks in but I think it's between 0 and +2db?
 
-20 dB is the industry standard, but when you're talking sound it's important to talk in what scale -20 refers to -20dBFS, which is digital. 0 dBFS is the max, and anything at 0 is clipped, and thus destroyed.

On a mixer, most pro gear deals in dBu. It's a measure of voltage through the line. The idea is that you send a the mixer's "Tone" (1kHz at 0dBu) to hit -20 dBFS at the camera. Then setting the limiters on the mixer to kick in around +17dBu (which means that sound can go up to -3 dBFS at the camera, and you've still got 3 dBFS headroom until you really clip, but the limiter prevents that anyway since it's usually a 20:1 ratio, which means that you'd need 60dB of sound BEYOND what you calibrated the system for before you truly clip the sound)

An issue many people confuse is metering the sound in VU vs. Peak. VU is the average sound level, and that should hit roughly 0 on the mixer. The PEAKS are the peaks, so those should usually hit around +10ish or slightly above (if you meter both those settings are about the same).

If you make the mistake of having the peaks at 0dBu, then the levels are WAY too low. (And the difference between peaks and VU I'm talking about is determined by the mixer, a peak meter doesn't show VU and a VU meter doesn't show peaks. Check the manual)

Dialog tends to sit in the final mix around -20 to -25dB VU anyway, so recording like this gives you proper headroom and a great sounding final mix. It's the standard for a reason.
 
Denwa said:
Well.. tone is actually sent at max levels (setting your levels to 0 on the mixer) so that you then calibrate the camera to -20db so that 0 on the mixer is -20 on the camera. (Assuming the mixer and camera have Vu meters not PPM).

You are calibrating two pieces of electronics together so I'm not sure how to do that and maintain calibration between the two. I just got in from a gig so maybe I need to look at this later after I have woke up a bit.
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Hmmm... you can go with the Green Barry's 2/dB per blip estimate and not be too far wrong, but I wouldn't trust the HMC meters (or any camera's) to be so linear throughout the range... meters are rarely designed that way. (I think the 2dB/block is good for the -12 to 0 range of the HMC, I'm mainly leery about the range below -12dB). So, here's a procedure you can use in general - in any cae, it never hurts to verify this type of information!:

If your mixer doesn't have adjustable tone-level, then either
a) Feed your mixer from another steady sound source (tone from another mixer?), and use it to adjust levels; or
b) Feed the tone from your mixer into another device (another mixer?) that lets you adjust levels fairly precisely.

1) Feed the camera a signal at the nominal level you want to be -12dBFS on the camera (e.g., +8 if you want 0 to be -20dBFS on the camera)

2) Adjust the gains on the camera so that this level lines up with the HMC's -12dB marker

3) Drop the level of the signal you're feeding to the camera by 8dB

4) Mark the level now indicated by the camera as -20dB (or a reasonable approximation).


Make sure you carefully moniti=or your recordings. In my experience with 2 HMCs (posted on 'User somewhere), the HMC has a pretty low headroom on the input signal, and introduces distortion REGARDLESS OF THE METER READINGS when the input signal passes that threshold (in my tests, around +10 dBU) (see http://www.dvxuser.com/V6/showpost.php?p=1622274&postcount=181)... by setting a nominal level of -20dBFS, you have a greater risk of this problem rearing its head. I don't know if all HMCs have such a low input clipping point (none of the HVXs or DVXs I've used do), but I's say you should definitely experiement to see if your particular HMC is "safe" for operation when configured that way... on mine, e.g., I've determined that one dot below 12 o'clock on the gain dial is the lowest safe gain setting on the camera I can use to be sure that the meters will indicate clipping and the limiters will have the desired effect.... at that level of camera gain, I have to calibrate my mixer quite differently (+10 dbu on the mixer needs to be below 0dBFS, so 0 dBu at the mixer would be -10dBFS on the camera; -8dBu at the mixer would be -20dBFS on the camera. You can see how calibrating 0dbu as -20dBFS would NOT WORK for at least my particular 2 HMCs...
 
Yes, you mentioned your input problem before. I haven't noticed it on mine but I haven't specifically been looking for it and this cam so far hasn't been connected to a mixer very often.

What did Panasonic have to say about the input clipping?
 
Don't think I heard any response from Panny.... and since it's a fairly easy workaround for me, I haven't followed up on it too much...
 
Gohanto and ullanta, great posts. I'll be sending these to the audio recordists we work with. 8 times out of 10 they record too low and our post pro-tools guys get on my case.
 
I think it really depends on the mixer. High Quality mixers have a really low noise floor so something like a Wendt or Sound Devices, etc (have not used an SD yet myself) should be clean enough that it will not be a problem in post.

I'm currently looking into cheaper mixers as I don't have the budget for a wendt X4 at the moment. Any decent ones in the $500 range that are 3 or 4 channel with pads, High pass and CLEAN audio? Preferably with tone.

Not using a locations mixer when I am shooting leaves me feeling like I'm driving down a mountainside with bad brakes!.
 
I like the Sound Device models that are a grand. The 500 dollar jobers will work, but sometimes with less than desired results.. Like the Shure, does the job but not the best unit out there.
 
Denwa said:
I think it really depends on the mixer. High Quality mixers have a really low noise floor so something like a Wendt or Sound Devices, etc (have not used an SD yet myself) should be clean enough that it will not be a problem in post.
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...well, the self-noise of the CAMERA electronics are an issue here more so than any mixer. Cameras are electronically noisy environments for audio, and don't tend to have great circuitry. I regularly record double system to the camera and a dedicated recorder, and I can tell you that even a super-cheap dedicated recorder (e.g., MicroTrack) will sound significantly better than the camera when fed the same signal.

Add to that the audio compression on the HMC, and it's even worse. That's why I suggested -12db (or even higher in situations where you can get away with it) as a better reference point on the HMC than -20dB. When you boost the levels in post, camera audio (especially HMC audio) gets yucky.
 
Hmm.. true. I guess I was used to Shoulder mount cams. They always seemed to have decent audio. Bad panny!

Someone suggested a marantz PMD661. They are not much more than a zoom and don't have that preamp issue with XLR input.

http://www.youtube.com/watch?v=6LebyZbFd8k

Damn, I might pick one of those up and get a mixer later. It does have adjustable preamps and LCD VU meter. Oh wait. I don't think it has a limiter.
 
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