distortion before the end of scale?

Baluardo

Baluardo

Well-known member
hi all,
reviewing some footage with a decent (but not outstanding) audio equipment, i realized that some distortion happens (i could hear it)even when the VUmeter doesnt touch the end of scale (the little vertical red line).
it's ok until there are 3 red dots (lines), but at the fourth some little distortion shows up.
the mic is an azden sgm-1x. i also used another mic to record the same stuff (music and songs), a handheld kind, a shure C606, and in this case i couldn't hear the same problem.

since i realized that being careful i could hear the same distortion even through the other cheap speaker i have, it's defintely not because of the amp i think.

I guess this is not a normal behaviour, right? distortion should show up just when i hit the end of the scale!
-If you guys confirm this, all signs would lead to the mic. it sounds more like a resonance than a distortion. did anybody experience this? and especially with this mic? also i just realized that one of the 'joints' in the middle of the mic barrel was a little lose. i have tightened the screw now. do u think that could be the cause?
-I am keeping the ALC on all the time. and gain to -50dB.

thanks,
Andrea
 
Andrea,

Not always the case. If the mic input is being overloaded, you can get distortion even without hitting the top of the meter. Hitting the top of the meter is referred to as clipping, and that's a different type of distortion.

Perhaps that microphone is putting out too hot a signal. If you have to have the mic input at -50 to get the right level, that may be the case. You can get an in-line attenuator cable to bring the level down before it hits the mic in, or get a gain control device to dial in the gain before it hits the mic in.

Josh
 
thanks Josh,
i will try settin the gain at -60dB. and see what happens.

I was wondering: would i risk to have the same issue even with a better mic?

Andrea
 
>the 60 is more gain so you will distort more

statements like this make me feel like i'm out of this world. i thought it was all the way around.

anybody else confirm this version?

Andrea

PS I guess i will have to take the time to check this when i'll be home..
 
Baluardo said:
>the 60 is more gain so you will distort more

statements like this make me feel like i'm out of this world. i thought it was all the way around.

anybody else confirm this version?

Andrea

PS I guess i will have to take the time to check this when i'll be home..
Click to expand...


Exile is absolutely correct. The -60 setting will increase the gain applied to the incoming signal. Think of the setting in terms of the expected incoming signal: -60 indicates a lower level than -50 and therefore requires more gain.

-Dan
 
Dude -

With ALC on "all the time" you won't likely EVER see full scale peaks. You'll just hear distortion as the limiter kicks in. Sounds like your gain is too high and you're just banging the limiter a lot. Try setting levels with ALC off, in such a way that the peaks are about where ALC puts 'em (3 blocks below full or so), then use ALC to cover anything unexpected.


99% certain that this is your problem rather than a hot mic. Certainly the first and easiest thing to check!
 
just be careful, the alc is a limiter on the dvx, not an auto level control - people confuse the two sometimes.

i also operate with the alc on when not using a mixer, and its fine, but as ullanta said, thats provided you've set the levels right. the alc is like an untested safety net - it might save you, it might screw you, so lets try not to go there...:)
 
ullanta,
I like your explanation.
I will try your procedure today and will let you know..

thanks a lot,
Andrea
 
timapter:
i was aware of the meaning of the ALC on/off setting, i just thought that setting the audio levels with alc on or off wouldnt make a difference. that's why i kept it on all the time.

but from what you guys are saying, the concept is: if you have ALC "on" and you see more than 2-3 red blocks, chances are you are getting distorted signals. I wasn't aware of that.
I thought that until the end everything was usable..

Andrea
 
Let us know how it works for you!

A limiter is really just a really extreme compressor, where pretty much everything over the compression threshold is going to be VERY heavily compressed. For some things it's ok (very fast transients like some percussion), for most it sounds bad. The compression threshhold on the DVX (sorry , HVX is not here yet... but in the mail, finally!) seems to be about 3 blocks down from full scale. So, anything below that will be pretty much unaffected by the limiter, everything over that will be "distorted". This distortion is much better than clipping, but in most situations best not heard.

I suppose it IS possible to reach full scale with ALC on, but you'd have to be inspecting a running jet engine or something... so 3 blocks down would be the highest you'd see in almost any situation, and you still wouldn't be sure if the limiter is crushing it or not. Hence the idea of setting levels without it!

Good luck,
Barry
 
ok here's an update:
i did more tests. it was a choir concert, so the mic was getting really strong signal. i had to keep the volume of the channel very very low (around the second dot on the knob) in order to get a usable signal.

i can witness it wasn't hard at all to make it hit the end of scale, even with ALC on.

anyway i was getting some distortion, regardless of the ALC on or off. But i've noticed that also it didnt really matter how i would set the volume knob. i would just hear little distorted signal in my headphones on the peaks of the choir. even if the level i would read in the vumeter was below -6dB.
So it seems like the sounds hitting the mic were too loud for it. Obviously i was aware that these devices had limitations, but i was expecting it to be ok for these situations.

and thinking back, the only other situations where i've noticed distortion without hitting the end of scale was for a voice over, where, for being afraid of getting reverb, i placed the mic almost touching the talent's mouth. and she's got a strrong voice. but in that case it might have been because of the ALC too..

Anyway, the lesson is that a simple vu-meter doesnt' tell you all the truth.
But im still wondering if in the same situation a more expensive mic would behave better.

Andrea

PS i ended up recording the concert using a handheld kind of mic on one channel and the internal mic on the other. i already see some of you going to throw up in the bathroom :))
 
Baluardo said:
PS i ended up recording the concert using a handheld kind of mic on one channel and the internal mic on the other. i already see some of you going to throw up in the bathroom :))
Click to expand...

Perhaps they won't be throwing up!:)

Dynamic mics accept a much higher level of sound than if your were using a higher quality condenser mic. True though the quality of the quieter parts wouldn't have been as good quality.


trying to record screams and fireworks, No matter how many tries, with a condenser, I found the dynamic to be a much hardier choice.
 
A lot of cheaper and expensive mics have a -10db pad for working in high SPL environments. Oktava comes with one and it is cheeeeeap. :D

The meters on the DVX/HVX tell you the truth but they are not VU meters but dbfs peak meters. You are hearing analog distortion before the camera and the cam is just showing you where the peak levels are hitting as the A/D does it's work.

Best
 
Andrea,
It sure sounds to me that you ran into the problem Josh described at the beginning of this thread. I've had it happen to me. It's real easy to overload the front end of the preamps on the DVX with a hot mic signal. Once this happens, adjusting the gain pots won't help and the ALC won't save you. They are both downstream of where the distortion is occurring. You need to reduce the signal coming to the DVX. Some mics have a -10db pad built-in that you can switch on. Or you can buy an inline pad or an attenuating cable. I seem to recall that Jay Rose's book has directions for making your own pads if you're handy with a soldering iron. The DVX could really use a -35db setting.
-Dan
 
well i own Jay Rose's book, but never read it too carefully i guess:)

these 'pads' you guys are talking about are electrical circuitry or mechanical sound attenuators?
So is the overloading problem electrical (too high levels for preamps) or "mechanical" (mic just can't accept well such strong sound)?

Anyway thanks everybody for the suggestions/opinions!

Andrea
 
It's a simple passive electrical circuit. 'Attenuator' would probably be a more descriptive term. Here's a link to an extract from the book that talks about pads and how to make your own if you wish. This is directly from Jay Rose's website:

http://www.dplay.com/book/pgs2e/pads.pdf

You don't have to make your own though. For example:

http://www.bhphotovideo.com/bnh/con...s&Q=&sku=135091&is=REG&addedTroughType=search

btw, did you say which mic you were using when you got the distortion. Was it the Azden? I took a quick look and it's max SPL seemed a bit on the low side(110db) but I would think it would have to be a really rockin' choir for that to be a problem (overloading the mic itself).

-Dan
 
Yeah, where were the mics placed for the choir? I find it hard to imagine a choir that loud!

We are assuming you're plugging the mic directly into the camera, right? Is there any equipment in-between?

And can you confirm that this only happens with one mic (the Azden, I guess)? That requires a battery, right? You sure the battery's good?

All very interesting!
 
Baluardo said:
and thinking back, the only other situations where i've noticed distortion without hitting the end of scale was for a voice over, where, for being afraid of getting reverb, i placed the mic almost touching the talent's mouth. and she's got a strrong voice. but in that case it might have been because of the ALC too..
Click to expand...


Andrea, this sounds like you may be talking about wind noise (often called "pops" when produced by close vocals). It may perhaps have been the issue with the choir as well? Mics can be extremely sensitive to air motion, which usually produces a rumbling sound. Actual wind (or even drafts in some buildings), air from people's mouths as they talk/sing, people moving quickly near the mic, even the motion of the mic, can cause this. It will interact interestingly with ALC, too! Check and see if this is the case, and maybe invest in windscreens and a pop filter if it turns out to be the case!

If you've had more success with the handheld mic, it may be because they are specially designed for motion and for being held close to the mouth!
 
i found the attenuator part on Jay Rose's book. it's one of those things easy to do but better to get done by some manufacturer i guess...

i would say it's not defintely the kind of noise of the popped p's or wind noise.
for those who are interested, i posted a small sample:
http://www.baluardoproductions.com/distortion.wav

you can hear the distortion on the words 'how far'. this was recorded with the mic about 1 inch from the speaker's mouth. and she has a strong voice.
the same kind of distortion i was getting with the choir.

Something is getting overloaded. either the preamps, or the mic itself. this happened just with the azden sgm 1-x mic and not with the others. Since i dont have an attenuator, i was wondering whether i could do a quick test to check whether it's the preamp, switching the input settings to 'line', or maybe it would get too little sensitive like that?

with the choir, the mic was on a stand about 3-4 meters from the closest performers. and yes, it was a loud choir. the battery was brand new.

thanks everybody for your help!
Andrea
 
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