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Old November 21st, 2010, 03:31 PM   #1
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Need help understanding technical aspects of levels

I'm going back to solve the audio level issues that plagued some earlier projects, but am getting lost when it comes to putting it all together.
I'm trying to understand how all the issues of levels fit together, from a sound board to the various inputs on my camera, to the specs of the microphones including my wireless, but am getting mired in the bigger picture. I'm swimming in a sea of DBV, DBu, DBM, ohms, impedance LowZ and HighZ, etc. And the manual for the pd170 mentions kilohms, mV. and even Vrms. --?

I just want to know how these work together and what the levels should be in any given circumstance. Can anyone recommend a clear and concise, simple resource that will help? I am trying to see a picture of it but it's just not clear.

One of the projects that brought this up is described in this thread :
Distortion from a sound board!
I understand the general concept that the board was too hot for the wireless and clipped the signal before it reached my camera, and needed to be turned down. What I don't get still is the bigger picture of all these levels and how they relate to each other.

I've been lucky up until now, except for that fateful project, but luck will only carry you so far.

I"ve been reading up on the various aspects of audio for about five hours this morning, in books, on this and other sites, even the Ken Stone article, and quite frankly it's all beginning to give me a headache.
Would readily appreciate it if someone could simply clear a path through the forest.
Thanks in advance.

Last edited by Kell Smith; November 21st, 2010 at 04:43 PM.
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Old November 21st, 2010, 04:44 PM   #2
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Not sure if any of this helps...

Some thoughts - just a selection, not a complete overview, because what you are asking is worth several books and a lifetime of experience.

Mics: Correctly chosen (type and polar pattern) and placed - neither too far away so that the sound to be recorded is lost in ambient/reflected sound; nor too near and overloaded with high sound pressure levels from a speaker column or drumkit. Mics are Mic level and should not go into a line input.

Avoid on-camera mics and use balanced cables wherever possible. Radio connections are not as reliable as cables. Shotgun mics are NOT an audio equivalent of a telephoto lens.

Direct (line) inputs: MUST be correctly matched. Get a selection of in-line attenuators to sort out the different line level standards. Keep any unbalanced lines as short as possible. Avoid cheapo DI boxes. Don't connect musical instruments directly to the camera. Don't trust somebody else's mix for critical audio. If using a radio link you MUST correctly match the source signal with the transmitter's input settings.

Monitoring your audio: Always, and as late in the chain as you can. Watch the meters if you can but you must listen to it as you record.

Digital recording levels: Avoid AGC (auto gain control) and use manual wherever possible. Levels MUST stay below 0dB peaking at something like -10 dB should keep you safe. Once it's clipped you can't save it. You may need a separate limiter to deal with unexpected peaks in live events. Record a mono signal at two different levels if you can with one lower than you think you should.

If recording unrehearsed/unpredictable live audio is critical to the project, hire an experienced sound person to do it for you. (If they really are experienced, they'll refuse!)

Once captured, hang on to a copy of the original audio until the project is finished. If you are experimenting with audio editing, work on a copy.

I have missed out oodles of stuff that others would probably say was just as or even more important.
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Old November 21st, 2010, 04:59 PM   #3
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I wrote a detailed reply and lost it because I got logged out. I'll try a condensed version. Learn what you can about the variations of audio levels you might encounter on a shoot. Often pro gear is mixed with semi-pro and even consumer gear. Do a forum search and/or google search to learn about the different flavors of levels. Go on line and look at the manuals for the gear you're going to use or interface with. As was stated already listen, listen, and listen. What you're asking about is called gain structure and there are different points in the audio signal path that can be adjusted. If you're not listening and somebody else does something in the path to change the levels you won't know till its too late.
Your question is so broad that its difficult to answer. You need to find out what you don't know so you can ask more specific questions. Proper gain staging is a complex but fundamental element in capturing great audio.
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Old November 21st, 2010, 05:13 PM   #4
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Thanks,
Good advice.
Once I know this area more clearly I may need to take your advice and invest in some extra equipment.

I've been doing pretty well with most of that - although a few of those things I've learned the hard way. Like monitoring your audio.
Matching it to the board threw me. Didn't even see it coming.
I've never thought of hooking up a musical instrument to the camera...but good to know beforehand...
I am getting clearer on mic patterns and placement and did quite a bit of reading on that this morning, also.

I'm pretty good about riding the levels in the camera and usually record between -20 db and -10 db.
But if I had to tell an audio engineer how far to turn the level down for a line out to my camera or wireless - as in the studio situation - not as clear about that. Once I am out of the comfort zone of going through the motions of doing the right thing - hooking the mic to the camera, keeping the level below zero - once you bring in other equipment and start throwing around terms, I"m lost.

For instance, in looking at the input specs for the PD170, it says, "
-60dbu:3kilohms, +4dbu:10 kilohms, (0dbu=0.775Vrms)."

For the RCA jack it says, "327 MV(at output impedance more than 47 kilohms),
Output impedance with less than 2.2 kilohms
Input impedance more than 47 kilohms."

Greek.
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Old November 21st, 2010, 05:20 PM   #5
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"what the levels should be in any given circumstance."

Fundamentally, you want the signal as high as possible without hitting the upper limit. The reason you want it as high as possible is to get it "out of the mud" of the noise and distortion at low levels. And the reason you don't want to hit the upper limit is to avoid major distortion. At virtually every point in the audio chain you will need to optimize the levels, aka. "setting the gain structure".

It starts with selecting a microphone appropriate for the application. Some mics are very low self-noise and are very sensitive so that they work well in situations of low sound levels or distant pickup, etc. At the other end of the spectrum, there are certain microphones that are favored for use in high sound pressure level (SPL) situations such as recording gunshots or explosions or jets taking off, etc.

It continues with using the highest-quality microphone preamp you have access to. This often means using an external mic preamp with or without a mixer and/or recorder, etc. Note that most video camcorders and ALL DSLRs have poor audio recording capabilities because that is how they save money to make the pictures nice.

And setting levels, particularly in the mic preamp and recording device (whatever it is) is one of the most critical aspects in the entire process. Ideally, you would have a dedicated sound person who adjusts levels during rehearsal so that maximum levels are safely below the maximum. We try to make dialog peak at around 10-12dB below full-scale, depending on the nature of the production. Clearly, people yelling at a football game require a different margin of safety than people whispering and hiding in a closet, etc.

You have asked an overly-broad question and it is impossible to answer here. I don't even know of any comprehensive but concise web references. Your question is the topics of whole books Speaking of which, the books on audio production and post-production by Jay Rose are highly regarded and recommended...

Producing Great Sound for Film and Video ISBN-13: 978-0240809700
Audio Postproduction for Film and Video ISBN-13: 978-0240809717
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Old November 21st, 2010, 05:21 PM   #6
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Gain structure.
I'll do some research on that term and maybe will have some clearer results.
The thing that is most confusing to me at this point is all the different terms and how they fit together. If I had a clear scale or line that would bring it all together and allow me to visualize where all these terms and proper levels fall along it, that would be very helpful.

And..I suspect...if I understood the full picture of what this really meant with respect to my camera inputs, mics and any levels going into the input:

For instance, in looking at the input specs for the PD170, it says, "
-60dbu:3kilohms, +4dbu:10 kilohms, (0dbu=0.775Vrms)."

For the RCA jack it says, "327 MV(at output impedance more than 47 kilohms),
Output impedance with less than 2.2 kilohms
Input impedance more than 47 kilohms."

...a large portion of my question would be answered.
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Old November 21st, 2010, 05:40 PM   #7
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Quote:
Originally Posted by Kell Smith View Post
for the PD170, it says,
-60dbu:3kilohms, +4dbu:10 kilohms, (0dbu=0.775Vrms)."
-60dB is "mic level", and +4dB is "pro line level"
It is important to keep levels aligned (mic-level source to mic-level input, etc.)
It isn't that often that you run into impedance problems, once the levels are sorted out. Going from a line level (pro or consumer) to mic-level can be handled with a passive attenuator. Where going from mic level to line level requires an amplifier, typically a mic preamp.

Quote:
For the RCA jack it says, "327 MV(at output impedance more than 47 kilohms),
Output impedance with less than 2.2 kilohms.
That sounds like a definition of "consumer line level" It is not as high as "pro line level" but way higher than mic-level"

Quote:
Input impedance more than 47 kilohms.
That is a typical consumer line-level input. Note that output impedances are generally much lower than input impedances. So you don't typically need to worry about that.
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Old November 21st, 2010, 05:48 PM   #8
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I think part of the confusion here is that, other than db, I don't know the terms.
I can picture a scale or line with Odb in the middle somewhere, which is where a project peaks.
But don't have a clear sense of what a MV or a kilohm or Vrms is and how it relates to that.

What's so frustrating here is that I don't know how to convey the question.

I'm quite certain that when I've taken sound out of a sound board, directly into the XLR on the camera, I've had it set to line and yet it peaked. So if either they turned the signal down from the board, or if I had something in between that turned it down, what would that level (before it reaches my camera) have to come down to?
If it peaks at 0 db on digital sound, then where does the +4db come in?

And along the same line, I'm still not clear on when to use the +48v switch on my camera, or for that matter the mic att switch, or more importantly, why.

I have the general concepts down: line to line, mic to mic, generally how to get sound and keep it below 0db, when in doubt ask the sound guy etc. But I want to be able to look at the Greek in the specs and know what it's saying. I want to understand it in technical terms. And I don't.

Last edited by Kell Smith; November 21st, 2010 at 06:33 PM.
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Old November 21st, 2010, 06:52 PM   #9
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Apologies if this question sounds as dumb as a bucket of rocks. But please, if that's the case, enlighten me. I know there's some basic concept in here I'm missing that would bring it all together.
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Old November 21st, 2010, 07:17 PM   #10
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mV = millivolt = 1/1000 of a volt, the unit of electrical potential
Vrms = Volts Root Mean Square = the average voltage of a sine wave alternating current
kOhm = kilohms = 1000 ohms, a unit of electical resistance and impedance

dB expresses the ratio between two voltages where one voltage is compared to another reference voltage. dB can be positive or negative - if positive the measured voltage is stronger than the reference while if it's negative the measured voltage is smaller than the reference. The reference voltage is defined as 0. So +4dB is stronger than the reference voltage. In the analog world, professional equipment manufacturers chooses to call 0.775 Vrms 0dBu and the normal operating line level is +4dBu = 1.23Vrms. Consumer equipment manufacturers, OTOH, call 1 Vrms 0dBv and the standard line level is 0.316mV, or -10dBv. OTOH, the digital world chooses to call the biggest voltage a system can handle without clipping zero, so a signal that is weaker than that will be something -XXdB.
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Old November 21st, 2010, 07:21 PM   #11
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Quote:
Originally Posted by Kell Smith View Post
I'm quite certain that when I've taken sound out of a sound board, directly into the XLR on the camera, I've had it set to line and yet it peaked. So if either they turned the signal down from the board, or if I had something in between that turned it down, what would that level (before it reaches my camera) have to come down to?
Remember that in addition to the mic/line setting on the XLR input, there is also the record level controls. And, indeed, many sound boards put out even more than +4dBu. Some as high as +22dBu

Quote:
If it peaks at 0 db on digital sound, then where does the +4db come in?
The deciBel is a ratio. It is the measurement of something relative to some reference, point. When we talk about the voltage levels in a cable going into an input or coming from an output, we are typically talking about the level relative to 1V ("dBV") or "1mW into 600 ohms" ("dBu") which is a measurement that dates back around 100 years to the origin of the telephone system.

But when we talk about RECORDING LEVELS we mean "dBFS" even if the "FS" part is understood and not written. "FS" means Full Scale. 0dBFS is by definition, the absolute maximum that is possible to record. This is completely independent of the voltage level coming in. Whether you have a -60dBv signal from a microphone, or a +22dBv signal coming from a mixing board, the recording level can't go over 0dBFS without incurring massive distortion.

Quote:
And along the same line, I'm still not clear on when to use the +48v switch on my camera
You use the +48V (phantom power) switch when the microphone requires phantom power. Microphones that require this won't work without it. Otherwise, you don't turn it on. This is quite straightforward.

Quote:
, or for that matter the mic att switch, or more importantly, why.
You use the Attenuator when the incoming signal is too "hot". If it is distorting the mic preamp, and/or if you can't turn it down enough using the record level.

Quote:
I have the general concepts down: line to line, mic to mic, generally how to get sound and keep it below 0db, when in doubt ask the sound guy etc. But I want to be able to look at the Greek in the specs and know what it's saying. I want to understand it in technical terms. And I don't.
You will generally be dealing with either line level or mic level signals. And as a sub-species, consumer line level and pro line level. These are the major considerations, and most everything else is a second-order effect at best. If you keep track of the basics, you can pick up details about the rest along the way.
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Old November 21st, 2010, 08:04 PM   #12
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Thank you guys
I am going to have to spend some time going over your posts.

One quick question in the meantime. I'm going through an old project which had an audio problem and I suspect it was from the +48v switch being turned on by mistake. I'm trying to remember now what the setup was. But what would be the result if that switch was incorrectly turned on
1) in the case of an on-camera mic that didn't require it,
2) in the case of a feed being taken out of the board (and probably not attenuated), or
3) hooked to a wireless Lav which already had its own battery-operated power supply?
Thanks

Edited
The track in question is so low in level that it is unrecoverable. You can barely hear anything on it at all when it's turned up, although there is some very faint sound.
The other track was a lav that overdrove and I'm trying to figure out why. It may have been placement or settings.
The other camera operator was able to record a decent track but I suspect it came from the on-camera mic. There was a lav hooked to that camera that overdrove, also. It was a very loud performance, but I have recorded them before with no problems, same mics, similar placements. I suspect that the +48v switches were flipped on all tracks but the on-camera mic that got good sound.

Last edited by Kell Smith; November 21st, 2010 at 08:47 PM.
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Old November 22nd, 2010, 04:28 PM   #13
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Quote:
Originally Posted by Kell Smith View Post
Thank you guys
I am going to have to spend some time going over your posts.

One quick question in the meantime. I'm going through an old project which had an audio problem and I suspect it was from the +48v switch being turned on by mistake. I'm trying to remember now what the setup was. But what would be the result if that switch was incorrectly turned on
1) in the case of an on-camera mic that didn't require it,
2) in the case of a feed being taken out of the board (and probably not attenuated), or
3) hooked to a wireless Lav which already had its own battery-operated power supply?
Thanks
Almost all microphones that don't require phantom power will just ignore it. But there are a few that will be affected, either temporarily (while the P48 is on), and SOME permanently (like some old ribbon mics, etc.) It is a good rule to NEVER turn on P48 unless you know you have a mic that needs it, and at no other time.

Outputs from mixer boards and wireless receivers may not tolerate the P48 very well. Some will produce distorted sound if you try to feed P48 back into their outputs. Some might even be damaged. Yet another good reason to never turn on P48 unless you know for sure that you really need it.

Quote:
The track in question is so low in level that it is unrecoverable. You can barely hear anything on it at all when it's turned up, although there is some very faint sound.
The other track was a lav that overdrove and I'm trying to figure out why. It may have been placement or settings.
The other camera operator was able to record a decent track but I suspect it came from the on-camera mic. There was a lav hooked to that camera that overdrove, also. It was a very loud performance, but I have recorded them before with no problems, same mics, similar placements. I suspect that the +48v switches were flipped on all tracks but the on-camera mic that got good sound.
There is only a miniscule chance that P48 had aything to do with low recording levels. You are on the wrong track. Look elsewhere.
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Old November 22nd, 2010, 04:36 PM   #14
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Audio Operating Levels

An old article of mine reproduced in the Avid Training stuff of recent past. It might be useful in your quest:

EBU or SMPTE? VU or PPM? Dbu or DBFS? An array of standards and specifications with which the new entrant into broadcast audio confronts. Broadcasters have standardised on one or the other as a matter of history, and of good engineering practice as defined by specifying bodies.

But how many professionals understand the difference and the consequences of operating level? How does it relate to digital consoles, DBFS scales, and to the programme content?

EBU R68 as used throughout most of Europe specifies a test tone level at -18db DBFS which corresponds to a voltage output of 0.775V RMS. Most recognise this as the 0dbu reference, that is, the resulting voltage across a 600 ohm resistance in order for 1mW of power to be dissipated. A throw-back of the old days in the UK to the General Post Office (GPO) and BBC specification relating to balanced line feeds terminated into 600 ohms. What goes in, was what was supposed to come out - but it's okay leaving me!

SMPTE RP155 used throughout most of North America specifies a test tone level of -20db DBFS which corresponds to a voltage output of 1.23V RMS (+4db greater) recognised by users in North America as the 0VU reference. Therefore, it can be seen that things start to hot up very quickly at the business end of the scale if levels and standards are ignored. Allowing content to reach 0DBFS using EBU operating level corresponds to an output of 6.16V RMS (+18dbu), but using the SMPTE operating level 0DBFS corresponds to a colossal 12.28V RMS (+24dbu) - a +6db difference, in practical terms a voltage difference twice that of the EBU operating level. Consequences too for the audio console as output headroom becomes all the more important. Therefore console specifications of more the +24dbu output headroom should be available, typically +26dbu, or even better, +28dbu.

With colossal voltage levels appearing at the output possible, specifications relating to maximum (operating) output levels would generally be held at +12dbu (3.09V RMS). This corresponds to -6db DBFS EBU. This maximum output level applies to many of the European broadcasters. In North America, maximum (operating) output levels would be from +3dbu to +7dbu (1.10V RMS to 1.74V RMS). Therefore, employing good practice and the correct engineering standards whatever the operating levels will guarantee programme content is held within the dynamic constraints of the transmission path.

One further element to be considered however is the ballistics, dynamics, and integration time of the signal measuring instrument itself. The simultaneous use of the Volume Unit instrument (VU) and the Peak Programme Meter (PPM) with typical programme content will show marked differences in results. One may seem to underdrive, while the other might seem to overdrive. Therefore an appreciation of exactly what each instrument measures and how it responds mechanically should be realised.

The VU meter has a typical integration time of 300ms, simply, its ability to 'see' the signal and mechanically register its value takes about one-third of a second. For this reason the VU meter is considered to be relatively slow. However, the instrument detects averages not peaks - the Volume Unit so to speak. For signal paths terminating at an audio recorder using ferric oxide tape, the VU meter was considered by some a bonus, enabling maximum signal to tape in order to minimise the effects of tape noise and to maximise signal to noise ratios - tape saturation and print-through allowing (0VU at 320nw/m, DIN +4dbu). However, this does not help a transmission path where the output may be a radio transmitter. Often, a limiter device is employed to brick-wall the peak signals to the transmitter location.

The PPM on the other hand has a very fast integration time, typically a rise time of about 5ms, and a decay time of 1.5 to 2 seconds. As the name Peak Programme Meter implies, it detects peaks of the programme content. For transmission paths and transmitters sensitive to peak clipping, and at worst transient shutdown, it provided accurate monitoring and safety.

Measuring Average or Peak values has advantages and drawbacks, but providing good practice and recognised standards are employed consistently, and more important, understood, then all should be well. This being the case, the audio engineer is able to concentrate on the content and not whether their output is melting someone's transmitter, video tape or audio tape. This being true, then they can confidently say: "It's okay leaving me!"
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Old November 23rd, 2010, 01:44 PM   #15
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Ok. So if I'm understanding this correctly, combining the posts from Claire, Richard, and Steve, putting it all together (and using a few online converters to combine the terms), would this be correct?
I had a nice horizontal scale line here, but the post wouldn't save the formatting. So here it is in vertical format.


(In order)

Mic Level
-60 db

Consumer line level
-7.78 dBu
-10 dBv
0.316 Vrms

Reference Level
0 Dbu
-2.2 dBv
.775 Vrms

Another reference level
+2.2 dBu
0 dBv
1Vrms

US Max Operating Level
+3-+7 dBu
0.78-4.78 dBv
1.10-1.74 Vrms

Pro Line Level
+4 dBu
1.78 dBv
1.23 Vrms

0 DBFS Europe
+18 dBu
15.78 dBv
6.15 Vrms

0 DBVS U.S.
+24 dBu
21.78 dBv
12.28 Vrms


If that's right, then this part is still unclear:
1 (Claire quote): EBU R68 as used throughout most of Europe specifies a test tone level at -18db DBFS which corresponds to a voltage output of 0.775V RMS. Most recognise this as the 0dbu reference, that is, the resulting voltage across a 600 ohm resistance in order for 1mW of power to be dissipated. A throw-back of the old days in the UK to the General Post Office (GPO) and BBC specification relating to balanced line feeds terminated into 600 ohms. What goes in, was what was supposed to come out - but it's okay leaving me!

SMPTE RP155 used throughout most of North America specifies a test tone level of -20db DBFS which corresponds to a voltage output of 1.23V RMS (+4db greater) recognised by users in North America as the 0VU reference. Therefore, it can be seen that things start to hot up very quickly at the business end of the scale if levels and standards are ignored. Allowing content to reach 0DBFS using EBU operating level corresponds to an output of 6.16V RMS (+18dbu), but using the SMPTE operating level 0DBFS corresponds to a colossal 12.28V RMS (+24dbu) - a +6db difference, in practical terms a voltage difference twice that of the EBU operating level. Consequences too for the audio console as output headroom becomes all the more important. Therefore console specifications of more the +24dbu output headroom should be available, typically +26dbu, or even better, +28dbu. (end quote)

I was wondering why -20 DBFS, corresponding to 1.23 Vrms or line level +4dbu, is higher than the -18dbfs level, corresponding to .775Vrms? Shouldn't -18 be the higher level?

2. If as Richard said, the two scales are independent of each other, then the dbu levels are general guidelines?

Last edited by Kell Smith; November 23rd, 2010 at 11:47 PM.
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