General advice on monitoring sound level
 

Hi, I have a DVC30 with shotgun mic.

I can monior the sound through headphones and adjust the sound input level and monitoring level.

How do you know when you have a good level of sound? I am guessing that loudness isnt appropriate as a guide as you can change the level of sound you hear through the monitoring level but this doesnt change the level of sound you record.

Do you judge sound by adjusting the input level until you dont get distortion? How do you compare the level of sound you get through the headphones monitoring the sound to the level you are recording.

Any guidance appreciated, thanks

You need use the headphones to listen for distortion and use the audio level meter to see if it's loud enough. On the DVC30 you want to see the average sound you want to hear well peak just into the red but not distort or overload.

I've been having recording audio too. The past few projects I've been working on involve recording a festival/showdance/stage performance. The way these events work is that many groups will go up one at a time to perform there thing, with an MC between acts (groups).

Obviously, at the end of a performance (or even during the middle if it's really good), there will be lots of loud clapping, cheering, and screaming. This is where I have problems. Their screaming, etc. makes my audio go way into the red zone and distorts it.

The room is large, fits about 500 people I'd say. And there's even a second floor you can watch the stage from. I usually put a shotgun (though I'm going to change to a hypercardioid or cardioid as I've heard those perform better indoors) near the front of the stage, connected to a Marantaz recorder.

I know that I can manually adjust the audio levels by moving the knob around, but that seems tiresome. The events are usually 1.5-2.5 hours long, and for someone to be constantly looking at the levels and turning the knob down if it gets too loud seems difficult. Not to mention that turning down the audio levels brings everything down. The problem I've had with this is that the audio is so inconsistent throughout the entire event (many parts where the MCs talking is far too soft, or even completely silent because the levels were turned so low to compensate for the cheering, or if a singer is in a middle of a performance and she gets to a loud part, the audio guy turns down the levels causing the song to suddenly change volumes). Furthermore, even if there is a guy monitoring levels constantly, wouldn't he still miss the unexpected loud cheers (at least for the first second or so and he realizes it's too loud and goes to turn down the levels?)

Basically, how do I record such an event without distortion (because it was too loud) and without having the audio volume seem so inconsistent throughout the event?

Also, I am mostly a one-man crew, so I have to film and do audio by myself. That pretty much makes it impossible for me to touch the audio once it starts recording since the camera is several hundred feet away from the Marantz recorder. I also record audio with a camera-mounted mic, but that audio sounds much more distant, often un-intelligible, and usually not as good (unless my main audio distorts, which it often does).

What I do in this instance is use a little mic pre compressor. I have a Joemeek VC3, it's a mic pre and compressor in a half rack space. It requires a 12vDC wall wart, but i'm sure if I had to, I could probably rig up a battery pack for it. But I usually just run an extension cord from the stage. This will control the volume with compression and is much quicker than turning a knob.

Sounds like Alex could use a limiter. The Marantz doesn't
have that?

Michael, could you explain a bit more about how the mic pre-compressor works?

Dave, I think the Marantz actually does (or should, but I'll have to check) have a limiter. So I should just limit the noise next time? But limit it how much? How would I know how much to limit it? What exactly does limiting do anyways? Does it chop off sounds higher than the mic/recorder can take without distortion or does it bring the loud noise down to a bearable level?

I'm still wondering how to manually control levels without having the volume noticeably go up and down (bad continuity) throughout the recording also.

A limiter brings the loud noise down.
Which model of Marantz do you have?

Brings everything down or just the sounds that are going over (into distortion zone) down?

I use the Maranatz Professional PMD670 Portable Solid State Recorder.

Just the sounds that are going over.

What should I set the limiter to?

Well, I'm not familiar with the Marantz but my
understanding is that there are no settings
you can do other than have it either "on" or
"off". I think you have 3 settings: "auto level",
"manual (no limiter) level", or "manual with limiter level".

Can you explain the three?

From what I understand (let me know if this is correct):

Manual (no limiter) means you have full control - whatever you turn the knob (level) to is how loud the sound will be, but if you set it at a lower level to compensate for loud sounds, the other sounds will be turned lower proportionally.

Manual with limiter means you have most full control, but sounds that are too loud will be lowered so that it does not distort (but this lowering will not be proportional to the other sounds).

Auto means - no clue.

You're right about the two manuals. Auto means
the Marantz turns the level knob for you, constantly
adjusting it. Where you set the knob when in
auto has no effect. What happens in auto is
when the Marantz senses that the sound
is quiet it will turn the knob up to make it
louder. When something is loud it will turn
the knob down. One problem here is that
everything gets set to an average level
when in reality some of the sounds are
suppose to be quiet and others are supposed
to sound loud.

One thing. When the limiter kicks in it will
turn down what is loud so it doesn't
distort but it will turn down all the sounds
not just the one that was loud. But it is
better than distortion.

Wait, so it seems that you just said a contradiction. Does the limiter turn down sounds proportionally or disportionally to the other sounds that are not too loud?

A limiter is like someone standing over your
Marantz and when they hear a loud sound
that it seems like will cause distortion, this
person will quick turn down the level knob,
and then turn the knob back to where it
was once the loud sound has passed.
So, when that guy turns down the
knob to prevent distortion, the loud
sounds and the quiet sounds get turned
down the same amount, so it's proportional.

A limiter is, as the name implies, a circuit which limits the signal passed on to the innards of the recorder. A "hard" limiter is simply a clipper. Any level above the maximum is set to the maximum. Put a nice sin wave of excessive amplitude into it and the top gets lopped off flat and badly distorted. "Soft" limiters don't chop the top of the waveform but rather push it down - still distorting but not so badly. A soft limiter will never put out more than a maximum level.

An AGC (automatic gain control) on the other hand does not distort but simply adjusts the volume automatically to try to keep the average level at a value which loads the A/D converter FM modulator properly. Whereas the penalty for using limiters is distortion on peaks (low level stuff sounds fine) the penalty for using AGC is "pumping", the phemomenon in which you hear the background noise level going up and down in response to the ambient signal level. This is often most noticeable after something like a rifle report (recorded as a thud because even with an AGC it's likely to get clipped). Immediately afterwards there is dead silence as the AGC has turned the gain all the way down (quick attack). Then over the next second or two (slow decay) one hears background noise slowly increase.

To avoid paying either penalty it is necessary to increase system dynamic range. More bits is a good way to go. 24 bit audio has 48 dB more dynamic range than 16 bit (caution: just because the marketing stuff says 24 bits doesn't mean that the performance is really 24 bit). Another scheme is to split the audio into 2 channels set 10 - 20 dB apart in gain. In post you then pick the high gain channel if it's not overloaded and the low gain one if it is.

A soft limiter will clip, it just trys to do it softly.


a compressor and limiter are basically the same thing used differently. A compressor has a ratio that you can set, it has an attack and release and threashold setting. A ratio at say 4:1 means that for every 4 db over the threashold, 1 db will come out.

A limiter has a threashold, it has a ratio generally from 20:1 or above that is fixed as well as the attack and release being fixed.

a compressor is designed (generally) for controlling the average level while a limiter is used for controlling the peak level.

Most of the limiters that are built into say minidisc players and camcorders are a simple limiter. they are brutal and hard. I would say almost as unusable as distortion.

It will take a bit of practice to effectively use a compressor or limiter without having the side effects become noticable.

Basically you want to use the limiter as a last resort and then only sparingly. A compressor can be much more gentle and invisible.

A limiter will turn down the sounds disproportionately - i.e. if you have audio peaking at -3dB, and all of a sudden crowd noise puts it up to +4dB, the limiter will stop it from peaking and put the audio through at the level you select.

I believe most limiters are set to 0dB by default.

Wait, okay, so some of you are saying limiters turn down sounds proportionally, while others say it turns sounds down disporportionally. Which is it?

Also, how would I split into two channels with different gains?

And hard vs. soft limiters. How do I know which one I have? And is a soft limiter the same as a compressor? One of you said soft limiter does not clip, while another said soft limiters do clip, just gently - which is it?

Limiters don't turn down sound. They implement a non linear relationship between input and output. If you picture a graph of output voltage vs. input voltage as a straight line with 100 mV in giving 100 mV out, 200 mV in giving 200 mV out, -150 mV in giving -150 mV out you are describing a linear (because of the straight line relationship) system with unity (0dB gain). If the line is straight with a steeper slope (100 mV in gives 200 mV out, 200 mV in gives 400 mV out...) you are describing a linear system with a gain of 2 (6 dB). In manual gain control you set the slope of the line. In Automatic Gain Control (AGC) there is circuitry which measures the average output and adjusts the slope to hold the average to value appropriate to the application (e.g. proper loading factor for an A/D converter). Thus AGC may lower gain (compress), raise gain (expand) or do both (compand) but not at the same time. The important things are 1) the transfer curve is linear and 2) the slope of the curve varies with time. Linear systems do not distort.

Limiters, by contrast, have non linear transfer functions that do not vary with time. The transfer function of a limiter may have a linear portion at low inputs but the slope will decrease at higher input levels. If you adjust gamma in images to increase shadow contrast at the expense of highlight detail you will know what the positive voltage half of a limiter curve looks like. Thus in a limiter the gain depends on the amplitude. A change of 100 mV near 0 may result in a change of 100 mV at the output but a change of 100 mV near 1 volt in may result in a change of only 50 or even fewer mV. Because of this low level sounds are not distorted but high level ones are. The amount of distortion depends on the amount of non linearity encountered which depends on the design of the limiter and the total input voltage swing. The most extreme distortion occurs with "hard" limiting in which any positive voltage input is converted to a fixed positive voltage and any negative one to a fixed negative voltage of the same magnitude as the positive limit. More gentle limiters such as the µ-law curve create less dramatic distortions.

The obvious and simplest way to split into 2 channels is to use 2 microphones each going to its own channel on a mixer with the mixer channel gains set as far apart as experiment indicates is necessary. There is another thread here in which a poster suggests doing this by setting the gain on a stero channel pair as required and then panning heavily to one side. Pretty clever. One could split a single microphone's output into two paths but some care would be required with respect to keeping impedances matched, getting phantom power to the mic (if required) and so on.

All devices are ultimately hard limiters and will clip which simply means that no device has infinite input dynamic range.

You need to be aware that semantics gets into the stew here. For example µ-law limiting is often referred to as µ-law compression because the dynamic range of the incoming signal (speech) is effectively compressed into fewer bits than are required with straight PCM. On the other hand what I have described as compression, expansion and companding are in reality AGC functions. It's the same with clipping. Clipping usually means the peaks of the waveforms are clipped off as if with scissors i.e. the tops are flat. In this sense a soft limiter does not clip (unless you apply voltage outside the specified input range it which case it, as any device ultimately must, clips). From this perspective a soft limiter does not clip but rather lowers the height of and rounds peaks.

To determine what you have you could take a recording of a tone (often comes with the SMPTE color bar pattern from various devices) and run it through your DAW's limiters at various settings. Look at the wave form out of the device and familiarlize yourself with the distortions that the various control settings give. Do the same with a speech or music signal. With a little experience you'll be able to tell what has happened to a signal just by looking at it. Also, of course, listen.

Whoa!

A limiter does NOT turn things down. It sets a threshold above which sound level is controlled. The degreee of control depends on the settings of the limiter.

Some limiters do really nasty things to the sound when it crosses the threshold. Others are quite good.

A good limiter allows you to record at higher levels without fear of going into the red. Most AGC circuits on cameras I've heard suck. Literally, sucking up the noise in quieter sections. AGC circuits act more like compressors. Compressors and limiters are different.

Ty Ford

Okay, all that technical stuff vaguely made sense to me. Could someone explain to me how practically I could record audio without distortion when there are definitely going to be softer/quiet/near silent parts as well as extremely loud cheering/shouting/clapping? Or even if a singer has a soft part and then pumps her volume up for dramatic effect during the song? I want the audio to capture all this without distortion and increases/decreases in volume to be proportional (linear?) with reality, except in the cases of cheering/etc. where it doesn't matter (I actually prefer to bring cheering down and not get distortion).

Sure, hire a sound guy/gal who know what to do. That's what the pros do.

Ty Ford

Uhhh...that doesn't help much. In fact, that is completely out of the question as first, it does not teach me anything, second, it costs a load of money that neither I nor the club have, and third, there is no time to get them to the event.

You've already let us know you don't really know what to do to fix the problem. You've already let us know you don't understand how a limiter works. In order to get what you want, you have learn it, get it or pay someone else to do it. That's pretty simple.

I don't know how to do Calculus or my taxes. Tried, didn't get it. I pay someone to do my taxes. If it needed Caluclus, I'd pay someone.

Executive producers, directors and (sometimes) shooters don't really understand audio. Audio is NOT trivial. You have discovered that. Take the time to learn, or get someone else to do it and hope you make the right choice in getting someone who actually DOES know what they are doing. Those are your two options.

Regards,

Ty Ford

Probably the simplest solution is to use the AGC in the camera. They generally have fast attack thus protecting you from sudden loud noise but slow decay which leads to the annoying pumping. The slowness of the decay may be on your side as it is slow enough that you should be able to comensate for it to some extent in post. Another simple approach might be to use a pair of cardioid mikes with one aimed towards the performers (or more pertinantly with the low gain part of the pattern aimed at the audience) and the other aimed into the crowd. Adjust the gain on the one so the performer can't overload and the gain on the other so the crowd can't overload.

With either approach you will have to experiment to get the settings right. This is where the learning comes in.

As I have mentioned many times in the past. I have yet to hear a camera AGC circuit that does more good than harm. An AGC is not a replacement for a good limiter.


Ty Ford

How about this? Bring along a second recorder and record some of the audience applause. Record the performance normally using whatever method you prefer and don't worry if the applause is distorted. In post, edit out the distorted audience reaction and replace it with the undistorted audience reaction from the separate recording.

Compressors and limiters are different only in the sense of how they react. They basically do the same thing. I can take a compressor and turn it into a limiter with a few adjustments. Where as dedicated limiters usually don't include the controls to use it as a compressor. expanders, gates, companders, agc, de'ssers and limiters are just different ways to impliment a compressor. They are all doing the same things. How a designer decides to impliment the parameters of a compressor will dictate how you can use it. cost is usually the deciding factor.

Hiring a sound guy is usually not as expensive as you would think. In NYC, there are probably thousands of fresh out of school audio students who would be more than happy to man the controls for next to nothing. You could even contact an audio school in your area and request an intern who is interested in audio for film. Heck, they may even have their own gear.

I'm not myself a big fan of AGC. In fact I make my living running around yelling "Turn off the @#*&^ AGC and set the gain so the noise floor is at the LSB" and while the signals are not audio the principals still apply. OTOH I would never ever use a limiter because distortion (and this may be colored by my professional experience which is far more extensive than my audio experienc) is totally unacceptable to me. The need for either AGC or limiting is indicative of poor gain management. All that said if I'm out in the field by myself I can't manage zoom, focus, shutter speed, color balance and audio gain so I let the AGC handle the latter. Even if I wanted to manually ride the gain I can't reach the controls while the camera is on my shoulder. The results (XL series cameras) have always been excellent but then I've never tried to shoot a rock concert.

This could turn into a AGC vs limiter *&%%ing match which might as well be a MAC/PC discussion for the value it would add. It's important to recognize that taste and experience lead us to the opinions we hold, to remember that "de gustibus non disputandem est", and to encourage Alex to try any or all of the suggestions being made here to see what works for him.

hahaha, well the mac/pc debate I think has ended. Mac is the winner being that they can run both operating systems now.

Correct me if I'm wrong, but I don't believe that limiters clip audio. Their transfer functions are not applied to instantaneous voltage, but rather to a sample of signal.

You can confirm this with any good audio editing program. Produce a sinewave tone at about -3 dB then put a curved envelope on it so that it tapers to zero at the beginning and end. Now apply limiting to clamp the output at - 10 dB. You'll find no distortion.

As with any active audio device, the signal can be clipped at the input of a limiter if its amplitude is out of range.

depends on the design. Analog limiters can and will clip. digital look ahead limiters can react faster and while not technically clipping the result is similar. They try to round off of the tops to a certain degree depending on how hard you hit them. If it's gentle, then they will be more rounded, if it's hitting hard, they will be more flat topped.

I'm afraid I'm not convinced. Why would anyone design an audio limiter to clip? That's pretty nasty, since we buy them to prevent clipping. What would a clipping limiter be for?

Not sure who you are responding to, here, but I'm
in agreement with you. Limiters are designed
to prevent clipping. Even soft limiters most times
prevent it. If your limiter is clipping you should adjust
it, if possible, so that it doesn't clip.

right.

Ty Ford

Just a few simple thoughts.
Firstly try to get the mic closer to the sound source you want and further from what you don't want.
If the levels of the general program is set to peak to say -6dB FS then the limiter if used should be mostly hitting the applause and even a bit of harm done to that isn't going to matter.
To record another track at lower level get a XLR splitter cable. Feed one output directly into one channel and the other to the other channel via a 10dB pad, these are available from most pro audio houses. That's assuming you don't have individual gain controlls on each channel, if you do just leave the pad out and set levels so one channel is good for most of all but the loudest program and set the other 10dB lower. Select best channel in post as needed.

Nope, limiters aren't designed to prevent clipping. They are designed to restrain the level of a signal. They were designed for media that also was analog and therefore a tad more forgiving. This of coarse can be used to try to prevent A/D clipping. But with any analog design, there is a reaction time. This reaction time works well for radio, vinyl, tape, but not so well for Digital. I won't get into what happens on the D/A, but you can get as much as 6db of clipping if you pin your meters and have no overs converting it back from digital to analog. If you convert it to any compressed format like mp3, mp4 etc... then there is even more clipping occuring. Digital look ahead limiters can prevent clipping in the digital domain, but that doesn't really help with capturing analog and recording it to digital. And only then, there are only a few digital limiters that can guestimate overshooting and compensate, But we're talking in the $10,000 range and they of coarse loose sync do to the fact that they have to delay the signal in order to react quicker.

This is why compressors are used before the limiter. The compressor controlls the signal before it reaches the limiter. not only does it sound more natural, but does a much better job on rms levels (sign wave or vocals) where limiters do a better job on peak levels (spike like a snare or gunshot).
The very nature of using these non linear process's is distortion. The waveform is not the same, the definition of distortion. It's a tradeoff of one distortion for another. in the case of some camcorders built in limiters, the tradeoff doesn't seem worth it.

So in order to record a very dynamic signal that involves varying rms levels as well as dynamic peaks to a medium that is finite like a digital camcorder or any digital format, you have to make sure you don't push it to the limit. just because something says limiter, doesn't mean it's a free lunch and it doesn't mean you aren't clipping the signal or that the signal won't clip down the line.

Strictly speaking you could say that's true, and there are other applications for limiting output level. For videographers though, the reason is almost invariably to prevent clipping, and the limiters marketed to us for that purpose do not themselves clip, flatten, fold, bend, staple or mutilate audio signals to any significant degree, including pure sine waves, when operated within their design range. That would be absurd. That's the door I came in.