clipping question

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dv416

Well-known member
i've had several instances recently where the audio meters indicate clean sound, as the level never goes into the red.

however, upon hearing it playback, the audio is distorted.

at first i thought this was a mic/line issue, but i double checked and it was not.

is there another means to calibrate a mic's input so that the meters give accurate information? i feel like i have heard of -10 and -20 db settings on some cameras / mic setups. not sure what the deal with that is though.

any info would be appreciated.

update: most recently i was using an ME66 shotgun mic, hand held, for interviews. obviously, this mic is not designed for this use specifically... so, could it be clipping in the mic?
 
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There are many different types of distortion, and digital clipping is only one of them. VU and peak meters do not report the presence or absence of distortion, as they're only concerned with the signal level. Naturally, if the meters are pinning, then you're likely going to have distortion on your hands, but there are plenty of ways to introduce nasty distortion without nudging the meters out of the normal range.

You could be distorting on the mic, if the sound you're recording is particularly loud. While the ME66 isn't designed for handheld use, it can certainly be used that way.

The only audio meters you should ever trust are the ones on either side of your head. Always monitor your recordings with headphones, and use a reference tone to ensure that your headphone levels are consistant.
 
What camera are you using, is its audio set to AGC? How are you hearing the clipping, headphones or through your NLE?

Cheers.
 
i was shooting with a sony V1u.
not sure about AGC, but the level controls next to the XLR ins were set to manual, and i was lowering them periodically, as i was also monitoring thorugh a pair of iso headphones. it sounded a tiny bit distorted thorugh the headphones, so i knew i was gonna have some issues like this...

though after looking at the raw signal in FCP, i notice some clipping of the raw file, even though the raw file was well under 0db. its either clipping in the mic (not screaming, but host had to talk over music and ambience) or its some camera setting that I am unfamiliar with.

seems just lowering the levels on the cam wasn't enough...
 
Could be the ME66, doesn't have a pad to drop the input level. Only 125dB max input, you're probably hearing its limit. Do tests and back off the voice each time, note distances.

Cheers.
 
Hold the me66 further away. Like 1.5 feet from the mouth. Better yet get a hand held dynamic for interview style. Sure SM58 is a good cheap dynamic. But also you should be using a mixer like the Sound Devices MixPre. It has limiters on the input and out put. Sounds awesome. 66db of gain. Line level.
 
interesting. if the clipping is from the me-66 hitting its db limit, would a mixer w/limiter really kill the distortion? seems to me the signal straight out of the mic was causing the problem.
 
one last update:

i did a few tests. one on a DVX. i was getting clipping on the mic setting even when i lowered the input level so it did not go in the red. however, when i switched it to Line input, the clipping went away.

i just did another test with the V1u and i think i got similar results. i also noticed a "mic trim" setting in the menu on the v1u. is this a pre-amp attenuator of sorts? is this a similar effect to switching from mic to line? is that just an adjustment to the cameras pre-amp?

if this is the case, than it seems my distortion problems were caused by the cam's pre-amp blowing out and not the me-66.

thoughts?
 
dv416

If you are plugging the mic straight into the camera's XLR - Set the input to "Mic". If you are going from a mixer to the XLR - set the inputs to "Line".

You have a trim! Great. It works differently on different cameras, but it is most likely the trim setting that is off, and causing distortion. Turn your trim down. The trim pumps up your input when you have a quiet source. That way your don't turn up your levels and add noise. Try setting your record levels to 50% - 75% (remember: more level ads more hiss so use your ears). Start at 50%, then go adjust the trim, maybe down a bit.

The ME66 is a sensitive mic not designed to be addressed straight on in interview style. So for normal use your trim might be at a good setting, but if you are going to use this mic for interviews you need to pull back the trim.

Also I recommend testing your trim settings for different things. Like getting birds or nature in a quiet setting - turn the trim up. If you are at a road race, turn it down (and use a dynamic mic). Be sure and read your manual on Trim. Sometimes the way it is implemented is confusing, as on the EX1 - they use -db numbers and it seems like you are turning it down, but you are turning it up. Also check for AGC in the manual, and make sure it is off for all situations except for maybe when using the built-in mic run & gun style. Otherwise it will compress/distort your nice mic signal.

I have the ME66, and find it pretty brittle sounding, and also shotguns are meant to be used outside. If you were interviewing in a room close to a wall, you will be getting odd reflections off the walls. I'm hoping to get a Rode NTG-3, which is comparable to a Senn 416 at half the price.
 
at this point the last question i have is whether or not the "trim" is processing the raw signal from the mic or if it only attenuates the sound after the pre-amp. if its after, the whole trim funciton is sortof useless.

though the me66 is not the ideal mic for this suituation, i like to think it was a pre-amp issue and not a mic issue.
 
Try adjusting the INPUT TRIM!

Try adjusting the INPUT TRIM!

dv416 said:
at this point the last question i have is whether or not the "trim" is processing the raw signal from the mic or if it only attenuates the sound after the pre-amp. if its after, the whole trim funciton is sortof useless.

though the me66 is not the ideal mic for this suituation, i like to think it was a pre-amp issue and not a mic issue.
Click to expand...

From Sony's user manual, it would appear that the "TRIM" settings do indeed affect the sensitivity of the mic inputs. It would most certainly be worth the few minutes it would take to do the experiment with the "TRIM" at different settings to see if you can duplicate the clipping situation.

I agree that it is unlikely to be a microphone problem. It seems much more likely to be a mic preamp sensitivity problem.


From page 69 of the Operating Guide:
"The INPUT TRIM function adjusts the input level from an external microphone. when using a highly sensitive microphone or recording loud sound, set this to the minus side When using a less sensitive microphone or recording quiet sound, set it to the plus side.

"When recording loud sound, the sound may be distorted at either the input point or the recording point. If it is distorted at the input point, adjust the sound using the INPUT TRIM function. If it is distorted at the recording point, lower the total volume level manually.

"If you set INPUT TRIM too far to the minus side, the microphone volume becomes too low, resulting in a poor signal to noise ratio.

"Test the effect of the INPUT TRIM function according to the microphone used or the sound field of the recording site before actual recording."

As much as I hate auto-level, especially for music recording, using the automatic function for speech is quite acceptable IMHO, especially when shooting by yourself (without a sound person) where you can't concentrate on riding audio levels. Been there, done that, become more pragmatic in my old age.
 
Richard if you use a mixer with limiters (like the MicPre) you can set it and forget it on speech. I've been using it for 3-4 years now and have yet to get any distortion on my audio. $665.00 for distortion free audio is a small price to pay. AGC is always going to overprocess the signal, and it will pump up the noise when nobody is talking. Or if one does not want to opt for the quality of a mixer, one can record the camera audio low enough to account for loud speech. Just have your speaker talk loud or yell when you set your levels. That way you have the option to turn it up in post, and you have not squashed the life out of the audio track with the AGC. You can always compress/limit in post. A mixer though, gives you loads of gain, you can use the line level inputs on your camera, which knock down hiss considerably, and you have a clip free - 2 stage limiting system to protect you.

Or not if audio quality isn't your goal. But remember: Garbage in - garbage out. You can't fix distortion in post.
 
Another note about blowing out the inputs:

Sometimes even the trim won't save you! Every sound person should have a couple of these for situations like our friend with the distorted speech.

Audio-Technica AT8202 Attenuator with 10, 20 or 30 db of Selectable Attenuation
http://www.bhphotovideo.com/c/product/68085-REG/Audio_Technica_AT8202_AT8202_In_Line_Attenuator.html

With this you can turn down the mic cleanly BEFORE it hits the camera/mixer input. Even with my Sound Devices MixPre, I use these when recording a loud band. Mostly I have only needed to attenuate by -10db. But there is -20 or -30db for more extreme scenarios. If DV416 has this put in-line after the mic and before the camera input, there would have been no clipping.

I suggest every camera operator have one or 2 of these.
 
Chadfish said:
Richard if you use a mixer with limiters (like the MicPre) you can set it and forget it on speech. I've been using it for 3-4 years now and have yet to get any distortion on my audio.
...
You can't fix distortion in post.
Click to expand...

I was assuming that the OP was not using a mixer since he didn't mention one.

My concept of "distortion" is not limited to clipping. It includes pumping and other audible artifacts of auto-level. But of course it depends on what you are recording (and how). As I said auto-level is sometimes the lesser of the alternate evils.

Some amount of clipping can be repaired in post. I have used both manual methods and the automated feature of Adobe Audition to repair flat-top waveforms. But it is most certainly not a general work-around for setting improper levels during original tracking.

Completely agree that a few inline attenuators is a very handy thing to have knocking around in the bottom of the bag. The commercial ones are vastly overpriced, IMHO. My budget solution:

XLR-female to XLR-male "adapters" $3.25 from www.audiopile.net (wired as phase reversers, which is also useful)
Re-wire for H-pads per instructions here: http://www.uneeda-audio.com/pads/
 
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dv416 said:
interesting. if the clipping is from the me-66 hitting its db limit, would a mixer w/limiter really kill the distortion? seems to me the signal straight out of the mic was causing the problem.
Click to expand...

I'm sure that the distortion was not coming from the ME66. For one thing, clipping of the ME66 mic would require very loud, constant yelling close to the mic to create the regularly occurring clipping that you saw on the Final Cut waveform display. Also, since you mentioned that setting the input to line level made the distortion go away, this proves that the distortion did not come from the mic.

The ME66 is a fairly hot mic, though, so if whatever you are plugging it into (camera, mixer, or recorder) has an input sensitivity adjustment (switchable pad or adjustable trim), it may need to b e set towards the lower end of the scale.

Also very important: If you are plugging the mic into a mixer, and then plugging the mixer into a camera/recorder, then the input setting should match the mixer output (line-line, mic-mic, etc).

When using a camera mounted mic, and when going into situations where sound levels can range form low to high without warning, AGC can do a good job keeping levels reasonable without overload.

Glen Trew
 
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