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freddy McLennon
dB and gain
on Jan 21, 2011 at 11:54:04 am

Hi,

As an amateurfilmer I got into a discussion regarding dB (decibels) and gain.
As far as my knowledge carries me, this is what I think explains it.

You have decibels (dB) used to measure sound.
0dB is the value that represents sound where an avarage person with normal hearing, hears nothing.
In opposition where 110dB is a lot of sound that could cause hearing damage.
So far so good.
But now the discussion...
In every respectable editing program you will find an audiomixer with UV meters.
These UV meters are standard set to 0dB.
So, as far I'm with the program I assume this setting means that the imported audio is unchanged electronically meaning my sound will be imported with the same strength as it was recorded with my videocamera.
Not that my audio has 0dB if it would be measured with a audio meter.
So I think to know that there's dB to measure sound and there's dB to measure the electrical current to change the strength of the audio wave, as wel for example the videochip.
I know this might seem a stupid question but could someone explain what this is all about.
I get this question once in a while (I give workshops for amateurfilmers)and if I explain this, I want to explain it right.

Thanks,

McLennon


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Ty Ford
Re: dB and gain
on Jan 21, 2011 at 3:58:03 pm

Hello Freddy and welcome to the Cow Audio Forum.

dB may express sound pressure level, voltage (dBu) or power (dBm).

If Mr. Crowley stops by, I think he can give you chapter and verse.

Google is your friend. http://en.wikipedia.org/wiki/DBm

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide





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freddy McLennon
Re: dB and gain
on Jan 21, 2011 at 5:38:01 pm

Tnx Ty for your response.;o)

The things I know about dB and gain comes from that exact wikipedia subject.
But some people keep thinking, because anybody can write on a subject and place this on Wikipedia, that you should not always believe what's written there. (or anywhere on the internet)
That's why I was looking for a scond opinion in a place I KNOW there's people who work with audio and video and also have the knowledge.
And it always sounds better if I can say that I have reference from someone in the field.

So hopefully Mr. Crowley stops by and is able to give a little explanation.
Ofcource I must also admit ... I don't quite understand everything that's written on the Wikipedia page, probably because there talking about a lot of different kinds of dB (confuses me), while I'm just looking for the difference of dB used in sound (sound pressure level) and gain in the editing software.

But tnx anyway.

Greetzzzz,

Freddy McLennon


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Eric Toline
Re: dB and gain
on Jan 22, 2011 at 12:03:54 pm

Meters to measure level in the digital world usually show a range from -40dbfs to +20dbfs.
Digital recorders are calibrated to show a maximum input level of "0" with a standard reference level of -12dbfs or -20dbfs. What you're calling "0" level should correctly be termed -40dbfs and lower.


Eric


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freddy McLennon
Re: dB and gain
on Jan 22, 2011 at 3:14:50 pm

Okey,


Well eric,
now I'm even more confused lol.
But hey, that's why you guys work in the field and I'm a amateurfilmer.

The question ofcource is...
Is the way audio is measured in the world, or the way audio is represented in editing software, the same?
Or is there a difference in Sound Pressure level (what we hear)and the way it's represented in editing software (what we see on the UV meters)
I've been thinking there's a difference and so I was teaching this.
dB for sound pressure level is NOT the same as dB represented on UV meters.
But There've been some occasions where people told me I was telling false information.
That's what got me doubting myself if I was wright or wrong.

When I'm editing, to be honest, I use the UV meters to keep my audio from going into red, all the rest I do with my ears.
But then I believe, when you're teaching people, you should tell them these things correctly.

Greetzzzzzz,

Freddy


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Peter Groom
Re: dB and gain
on Jan 22, 2011 at 6:51:26 pm

In an effort not to recover ground already done, can we have clarity on this use of "UV" meters. I think you mean VU meters. This stands for volume units and shows an average or mean value, therefore is no good for measuring peaks. HOwever I thinmjk it is incorrect to say that all editing software uses this. Indeed Ive never seen it on any editing software.
The full scale DB meter where the top is at o is the normal meter supplied and shows where digital clipping will occur.

Peter

Post Production Dubbing Mixer


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freddy McLennon
Re: dB and gain
on Jan 22, 2011 at 11:00:03 pm

Hi Peter,

I can feel the red blush on my cheeks.
Ofcourse where talking about VU meters (shame on me)
And yes, I was incorrect about what I wrote earlier about the clipping part.
At least partly...(I think)
I work in Premiere Pro CS5.
In this program you'll find an audiomixer.
The tracks are standard set to '0dB'and can be changed to max 6dB or lower dB values, while the master volume is standard set to '0 dB' but here this is the maximum.
And that's the VU meter I use to avoid clipping.
So that's a very important point you made and so I'm very greatfull for your reply.
I'm working in Premiere for many years now, but I never knew what the difference was between the values in the tracks and the master.
Maybe I should follow some classes myself before teaching others.
But they keep asking me for these things because they like what I do in the amateurclub I'm a member of, and they think I'm on a high level.
Well, I'm on a higher level then they are, but that's because I keep on learning thru tutorials (for example Linda.Com) and thru endless searching on the net. (and to say English isn't my native language
,I learned that all by myself as well... god I love the internet)
Anyway, in compairisan with you guys I'm a newby.
But hey ... I'm learning as we go and I want to get better.

Well, I'm off to bed now.
I'm sure I'll get the answer I'm looking for and in the meanwhile I'm greatfull for the things I learn on the side.

Tnx

Greetzzzz,

Fred



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Peter Groom
Re: dB and gain
on Jan 23, 2011 at 10:55:12 am

HI again

Ofcourse where talking about VU meters.
Thought so

And yes, I was incorrect about what I wrote earlier about the clipping part.At least partly...(I think)

I dont use Premiere Pro and never have (prob never will) but as a rule on any audio if youre driving audio so it hits into the red portion of any meter, you run a real risk of clippimng and distortion. In the old analogue days, you could distort on tape, and the onset of the audible effects was gradual as tape saturated and then had no more headroom, but digital is less forgiving, good, good, good massive distortion and no inbetween.

The tracks are standard set to '0dB'and can be changed to max 6dB or lower dB values, while the master volume is standard set to '0 dB' but here this is the maximum.
And that's the VU meter I use to avoid clipping.

I had a quick google and yes, VERY surprisingly it does use a VU meter (at least in the CS3 video I watched) Totally unsuitable for digital systems in my opinion. As VU is only a mean value, short peaks or transients could easily clip the audio stream, and the meters would never attempt to show it if the following audio were low enough to drag the average down!

Peter

Post Production Dubbing Mixer


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freddy McLennon
Re: dB and gain
on Jan 23, 2011 at 1:16:12 pm

Tnx for your reply Peter,

I'm working in Premiere since version 1.5 and so far I was able to avoid clipping.
But I have a great hearing that compensates my lack of technical knowledge lol :oD.

But I assume on the dB issue that I'm correct in saying that Sound in real live isn't measured in the same way as used in a studio (or audio mixer)or as represented on a videocamera (gain, +3dB, 6dB, etc.)
And that was where I got lost to start with.

Greetz,

Fred


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Richard Crowley
Re: dB and gain
on Jan 25, 2011 at 7:21:22 am

"But I assume on the dB issue that I'm correct in saying that Sound in real live isn't measured in the same way as used in a studio (or audio mixer)..."

It is not clear what you mean by that, but if you mean what I think you mean, then it is not a true statement. "Sound" (whatever you mean by that?) is measured the same way everywhere.

"...or as represented on a videocamera (gain, +3dB, 6dB, etc.)"

That is not a true statement, either. When we talk about gain it is exactly the same for any kind of electrical signal, whether it is audio or video or even RF.


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Ty Ford
Re: dB and gain
on Jan 23, 2011 at 1:26:25 pm

Peter said:

"but as a rule on any audio if youre driving audio so it hits into the red portion of any meter, you run a real risk of clippimng and distortion."

>> The Sound Devices LEDS are red way before they go into distortion. My A/D converters show red 3 dB below clipping.

"digital is less forgiving, good, good, good massive distortion and no inbetween."

>> Peter. I have tracks that have clipped on occasional peaks in Pro Tools and other software DAWs that sound fine.

Yes, analog VU meters with needles are too heavy (and therefore too slow) to catch peaks, but who uses them anymore anyway?

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide





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Peter Groom
Re: dB and gain
on Jan 25, 2011 at 4:32:25 pm

HI Ty
To address the points you raised 1 by 1.
I was really meaning the red or "danger" area in DAW / NLE meters, not field recorders etc, but i wasnt specific enough. Sorry.

distortion.
i was reallly suggesting that digital doesnt have a saturation / steady onset of distortion in the same was as old analogue tape used to have. So when clipping occurrs, it is when all the digitas have become 1's There is no more level available, and this usually represents itsself in my experience as a very noticable and objectionable crackke (mis spelt to beat profanity filter)

Vu meters. Who uses them. Well apparently the original poster does in his NLE. That was where we started, and i googled the app and yes it seems to use VU. I agree theyre a wast of space.

Peter


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Ty Ford
Re: dB and gain
on Jan 25, 2011 at 7:44:16 pm

Freddy, and Peter,

What DAW has analog meters?!

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide





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freddy McLennon
Re: dB and gain
on Jan 25, 2011 at 7:53:19 pm

That I don't know.
I even had to google te word DAW lol.
Anyway, I don't have equipment with analog meters.

greetzzzz,

Fred


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Peter Groom
Re: dB and gain
on Jan 26, 2011 at 10:30:28 am

I looked online and the software appeared to Use VU meters. Obviously not an analogue version of it, but an emulated digital form at least inits weighting and on screen representation. very odd i thought.
Peter

Peter


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Ty Ford
Re: dB and gain
on Jan 26, 2011 at 2:36:37 pm

Freddy,

What they look like on a computer screen has absolutely no bearing on how they display transient peaks, which is what matters.

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide





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Jordan Wolf
Re: dB and gain
on Jan 22, 2011 at 8:45:35 pm

Read this for some audio insight.

Decibels always describe a ratio between two values (in our case, it's voltage level compared to time). More to come later...nap time now. :-)

Wolf
<><


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freddy McLennon
Re: dB and gain
on Jan 22, 2011 at 11:06:59 pm

Hi Jordan,

I read the site following your link but to be honest ...
this reads like chinese for me lol ;o)
(I'm terrible with mathematics, formulas and numbers)
So I'll waite for the rest to come.
In the meantime, have a great nap.

greeetzzz,

Fred


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Jean-Christophe Boulay
Re: dB and gain
on Jan 24, 2011 at 8:25:10 pm

dB are maths. You can't really understand what a decibel ever represents if you don't understand the maths behind them. That explains why so many audio pros don't really understand them either. A Bell is a logarithmic unit of measurement that expresses a unit's value relative to a given baseline of the same unit. It's not just for sound, and even for sound it applies differently wether you're talking about sound propagation through air of electrical propagation through a conductor. It's not meant to be easy.

You can work with dBs without understanding them, many do, but if you really want to understand the deciBell, it's all about maths, formulas and numbers.

JC Boulay
Technical Director
Audio Z
Montreal, Canada
http://www.audioz.com


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freddy McLennon
Re: dB and gain
on Jan 24, 2011 at 8:47:13 pm

Hi Jean-Christophe,

I'm terrible at math.
The thing ofcourse is that I'm working with audio, although I don't understand the maths behind it.
But I'm thinking now...
If I don't understand all of it, then they won't understand it also.
So I'll try to find a very neutral answer if someone asks this question to me again lol.
Something like "google it" or "search for it on Wikipedia".
So I do understand there are different ways you can use dB, but it is not neccesary to know alle the maths behind it to be able to work with audio.
As I told before... I'm not an audio technician but meanly I use my ears when recording or editing audio.

So I wanna thank all people here for there reply, because it made me a little wiser.

Greetzzzz

Fred


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Jordan Wolf
Re: dB and gain
on Jan 24, 2011 at 10:43:51 pm

The key to using dB as an effective reference is making it meaningful. When I simply say "6dB", there is no indication of what I am talking about - am I talking about Sound Pressure Level (how "loud" something is) or am I referring to the difference in level between two audio clips as shown on the VU meters?

This is why there are various dB scales (dBu, dBV, dBSPL, dBFS, etc.). They allow us to refine what exactly we are talking about.

For example, one that may be particularly useful to you is dBFS (or Full Scale). Basically, this tells you how much headroom - in dB - you have left before clipping occurs. You can use if for any piece of equipment, per se, but it is normally seen when digital conversion/processing is employed. If an analog mixer can output a maximum of +22dBu, then THAT is its "0dBFS". In order to keep short-duration signals from clipping the signal, it is best to have around 12dB-20dB of headroom. -18dBFS would indicate that you have 18dB of headroom before clipping will occur.

The dBu specification has a somewhat-interesting history involving the telephone company, but the spec itself can be used for comparing like pieces of gear and calibrating levels between them. (It is very helpful to know the math behind it, but that can be put on the back burner for now.) If you have a field mixer with a built-in tone generator, you can "send tone" so that your mixer's levels match those of the recording device(s) down the line. This will ensure ample headroom so that your signal remains clean and clear.

I'm glad that you are looking into the resources you have been given. Please continue to ask questions here; we will be happy to answer them as we are able to. :-)

Wolf
<><


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Richard Crowley
Re: dB and gain
on Jan 25, 2011 at 5:25:11 am

"You have decibels (dB) used to measure sound."

That is true for both physical (SPL = Sound Pressure Level) and electrical (voltage, power, etc.) measurements.

"0dB is the value that represents sound where an avarage person with normal hearing, hears nothing. In opposition where 110dB is a lot of sound that could cause hearing damage. So far so good."

That is a generalization of how dB is used for SPL. Note that it has no direct correlation with how the term "dB" is used for electrical signals or for mixing, etc.
http://en.wikipedia.org/wiki/Decibel#Acoustics_2

"In every respectable editing program you will find an audiomixer with UV meters. These UV meters are standard set to 0dB.
So, as far I'm with the program I assume this setting means that the imported audio is unchanged electronically meaning my sound will be imported with the same strength as it was recorded with my videocamera."


VU meters in the digital world are labeled "0dB" at the very top. That is known as dBFS (deciBels Full Scale) It means that "0dB" is all the bits on, the very maximum voltage you can possible handle. "When you're out of bits, you're out of beer" to paraphrase the old advert.

When we are "tracking" in a music studio or recording original dialog on a video/film set, we use some amount of "headroom" between our average recording peaks and 0dBFS. Because 0dBFS is the absolute maximum, there is no "grace area" (as there was back in the analog age). Some popular headroom offset values are -12dBFS or -20dBFS. The higher value (-12) is more commonly found in more casual, 16-bit production (as with semi-pro equipment, or for "run-n-gun" things like the evening news.) The -20 value is more commonly found in 24-bit sound recording for more formal and controlled productions like feature video/film production, etc.

"Not that my audio has 0dB if it would be measured with a audio meter."
That is rather an ambiguous statement because several of the words (like "audio" and "measured") have several different meanings in different contexts.

"So I think to know that there's dB to measure sound and there's dB to measure the electrical current to change the strength of the audio wave, as wel.."

Yes, you are on the right track there. And there is also dB used other things, as well like measuring mixing ratios, gain, attenuation, etc.

dB is fundamentally a RATIO. It is not an absolute measurement like voltage or weight or height. It is use most commonly as a ratio when we talk about how much gain you need to boost microphone level up to line level (40-60dB). Or how much attenuation you might get through a long length of cable, or how much high-frequency reduction you would get from hanging thick curtains in your studio, etc.

When people use "db" as a MEASUREMENT, it is meaningless without a REFERENCE. Unfortunately, this reference is frequently omitted. For example, in modern digital use when you see the statement that something has "-10dB" output level, they are probably mean "dBV" where the zero reference point is 1 volt.

http://en.wikipedia.org/wiki/Decibel#Voltage

I didn't know I was so popular. I am traveling in South Africa (a grueling location scouting trip for a large concert tour. It is definitely NOT a vacation.) Currently in CapeTown and back to Joberg this morning and then 30 hours back to home in Portland. NOT looking forward to that, either.


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freddy McLennon
Re: dB and gain
on Jan 25, 2011 at 7:08:32 pm

Hi Richard,

Tnx for this reply.
The big mistake I made was indeed assuming that dB was used in the same way as for example KG, or Volt or Watt and so on ...
The only difference between me and the people who gave me the reaction that I was telling wrongfull things about dB was that the maths behind it are used different for different apllications, while they believe it is completely the same.
So to be honest I still don't understand all of it.
But tnx to all of you guys I do have a better understanding now and it gives me a little more insight on how to use my audio and what my VU meters mean instead of simply avoiding going into red because that might cause clipping.
For example, when shooting video I mostly use 2 shotgun mics.
I have 2 XLR inputs on my camera and mostly I use them to generate a kind of stereo sound. (for theater and stuff, when I make shortfilm I use one mic, mostly on a boom)
Today I filmed an animator who brought a short (45minutes) show for kids, mostly for birthday parties, but today he hired an indoor location with the purpose of shooting the whole gig so we can make a promotional video.
Since there where 3 camera's, I didn't realy needed 2 mics, but I still used both of them.
I mostly don't use AGC and set the gain equally for both mic's.
Today I set one mic lower, and the ohter higher.
In the past it happened (with or without AGC) that I had clipping when the audience starts to applaud.
So I set one mic lower to avoid clipping when that happens.
The other mic was set higher so the animator was understandable.
(the animator had a headset mic that was send to speakers in the room)
So I imported the footage into Premiere and for the first time I was able to mix my audio in such a way that there was no clipping when loud but the artist was clearly understandable all the time.
I don't know if that's te good way to go about it.
But after reading all your reply's, read your links, did some googeling and Youtubing, this was what I came up with in an attempt to improve my skills as an amateurfilmer.

And so far (at least today) it worked great.

So, tnx again.

Greetzzzz,

Fred


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Dave Haynie
Re: dB and gain
on Mar 4, 2011 at 3:21:39 pm

You have to keep in mind that "dB" itself is a dimensionless ratio between two different logarithmic quantities. You only get something akin to an actual unit (eg, like volts), which the basis for your ratio is a known quantity.

For example, in my day job I do digital radio design. We usually work in dBm, the "m" being shorthand for milliwatt. That means my "zero" is at one milliwatt... 10dBm is thus 10mW, 20dBm is thus 100mW, 30dBm 1000mW = 1W, etc. (if you didn't understand logarithmic scales, hope that makes it a little clearer).

For digital VU meters, you set the 0 point at full scale, whether that's going to be a numeric reading of 65,535 or 16,777,215 or whatever depends on your bit resolution. But these numbers get large and meaningless, the log scale is a far more useful scale, and lets you concentrate on relative signal strength, regardless of the resolutions involved.

For sound level meters, you probably measure in dB(spl), spl = sound pressure level. If you do recording, you'll find ratings on your microphones in db(spl)... you probably want something in the 120-140dB(spl) range on that kick drum. Technically, on this scale, the zero is set at 0.0002 microbar of sound pressure, which is more or less the threshold of human hearing.

The reason we use log measurements for hearing is simple: the human ear has about a trillion to one dynamic range. Rather than dealing in large numbers of crazy and meaningless precision, we can do the same thing in dB(spl) from 0 (threshold of hearing) to 120 ("The Raconteurs" in concert, also, where permanent hearing damage can occur).

-Dave


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Ty Ford
Re: dB and gain
on Mar 4, 2011 at 7:13:03 pm

Dave,

On behalf of The Cow, thanks for stopping by and offering such an articulate explanation.

BTW, what guitar is that in you shot?

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide





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Dave Haynie
Re: dB and gain
on Mar 5, 2011 at 4:33:24 pm

Hi-
I've been hanging out in other areas of the Cow for years, but doing more audio these days, I decided to poke around here. Hope that explanation was useful.

The guitar is the Martin "Backpacker", designed by my friend Bob McNally for Martin. I keep this one in my computer room for quick access, and do actually take it backpacking on occasion.

-Dave


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Ty Ford
Re: dB and gain
on Mar 5, 2011 at 4:57:24 pm

I know the backpacker.

Nice little piece for exactly the reasons you say.

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide





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