Need help understanding technical aspects of levels - Page 2 at DVinfo.net

Go Back   DV Info Net > The Tools of DV and HD Production > All Things Audio

All Things Audio
Everything Audio, from acquisition to postproduction.


Reply
 
Thread Tools Search this Thread
Old November 23rd, 2010, 02:53 PM   #16
Major Player
 
Join Date: Jan 2005
Location: USA
Posts: 422
Found this handy dandy page:
http://www.sengpielaudio.com/calculator-db-volt.htm
Kell Smith is offline   Reply With Quote
Old November 23rd, 2010, 04:04 PM   #17
Major Player
 
Join Date: Jan 2005
Location: USA
Posts: 422
So, then...

Referencing the earlier linked thread about the sound board being linked to my wireless, if my transmitter input levels are between -60 and -39db, and I'm sending it a +4 signal, thereby causing it to clip, then the signal needs to be attenuated on the board or through another device in-between, a minimum of 45 db? (not sure if this matters, but it's a mini plug input, also)

If a mic that required power was plugged into the camera XLR (-60db) and the phantom power button (+48) was on, it would boost the level to -12db, not that far below consumer line level?

If I'm taking a line directly into my XLR from a board, and the Sony specs say the input is +4db, set to line, it should match unless the board is hotter than that? In which case the operator would need to turn it down? Which of course I have learned the hard way to monitor it via headphones, so it would be obvious ahead of time....
Kell Smith is offline   Reply With Quote
Old November 23rd, 2010, 06:10 PM   #18
Inner Circle
 
Join Date: Mar 2005
Location: Hamilton, Ontario, Canada
Posts: 5,742
[QUOTE=Kell Smith;1591237]...
If a mic that required power was plugged into the camera XLR (-60db) and the phantom power button (+48) was on, it would boost the level to -12db, not that far below consumer line level?

...QUOTE]

Nope. Condensor mics require power to operate at all. Plug a mic requiring power into a mic level input and DO NOT turn on the +48v phantom and you'll get nothing at all, zippo, nada, because the mic won't be functioning. Turn on the +48v and the mic comes alive, sending signal to the mic input. That signal strength depends on a lot of factors but typically is in the order of -50 to -35 dBu depending on the mic. The camera has some internal amplifiers, its mic preamp and recording amps, that boost it again to drive the recording proper. When we look at the meters, the loudest signal can be recorded without overload and clipping is defined as 0dBFS. The -12dBFS is the meter reading that shows when the mic signal plus the gain iof the preamp plus the gain of the recording amp together produce a recording 12 dB below below the clipping point. That -12dB, the mic sensitivity of -60dB, and the -35dB of signal coming off the mic are different scale with different reference points selected for different purposes and can't be directly correlated to each other.
__________________
Good news, Cousins! This week's chocolate ration is 15 grams!
Steve House is offline   Reply With Quote
Old November 23rd, 2010, 07:46 PM   #19
Major Player
 
Join Date: Jan 2005
Location: USA
Posts: 422
OK.
Yeah, that makes sense now that I think about it. Mixed up the units. I should have thought that question out more thoroughly before adding it into the post.
My lav has its own battery-operated power. Should the +48 be on or off?

Last edited by Kell Smith; November 23rd, 2010 at 11:51 PM.
Kell Smith is offline   Reply With Quote
Old November 23rd, 2010, 09:47 PM   #20
Trustee
 
Join Date: Aug 2007
Location: Portland, Oregon
Posts: 1,177
Quote:
Originally Posted by Kell Smith View Post
Fit into this scale, when matched up to the Vrms, -20 should be more to the left, but instead it's to the right of the -18 level.
That sentence makes no sense without the context. What does "left" and "right" mean here?

Quote:
2. If as Richard said, the two scales are independent of each other, then these are general guidelines?
Recording levels greater than 0dBFS are as absolute as it gets. "When you're out of bits, your're out of beer"
You must develop a "feel" for where to adjust audio levels to peak when recording live dialog (for example.) To keep the signal up "out of the mud" and below clipping at 0dBFS. This is as much an art as a science.

But in our context (video production) what Claire is talking about with US vs. European standards is the "reference level" or the target you are aiming for during post-production audio mixing. This is much more in your control unless you are mixing live broadcasts, etc.
Richard Crowley is offline   Reply With Quote
Old November 23rd, 2010, 10:23 PM   #21
Regular Crew
 
Join Date: Mar 2010
Location: Edmonton, Alberta, Canada
Posts: 42
"You're probably tracking too hot. Here's why... "

Proper Audio Recording Levels | Rants, Articles | MASSIVE Mastering

Does this article get it right?
__________________
http://totalhumanbeing.com
Matthew Capowski is offline   Reply With Quote
Old November 23rd, 2010, 11:37 PM   #22
Major Player
 
Join Date: Jan 2005
Location: USA
Posts: 422
Quote:
Originally Posted by Richard Crowley View Post
That sentence makes no sense without the context. What does "left" and "right" mean here?.
Sorry about that. I had them all along a horizontal scale line, but had to edit the post to make it vertical because the post wouldn't hold the formatting. Forgot to edit that.

The question should read,
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?


Quote:
Originally Posted by Richard Crowley View Post
Recording levels greater than 0dBFS are as absolute as it gets. "When you're out of bits, your're out of beer"

You must develop a "feel" for where to adjust audio levels to peak when recording live dialog (for example.) To keep the signal up "out of the mud" and below clipping at 0dBFS. This is as much an art as a science.

But in our context (video production) what Claire is talking about with US vs. European standards is the "reference level" or the target you are aiming for during post-production audio mixing. This is much more in your control unless you are mixing live broadcasts, etc.
Oh, I wasn't referring to the 0DBFS. I know that's absolute. I was referring to the +18 and +24 dbu. Went back and edited to re-word that.
Kell Smith is offline   Reply With Quote
Old November 24th, 2010, 05:19 AM   #23
Inner Circle
 
Join Date: Mar 2005
Location: Hamilton, Ontario, Canada
Posts: 5,742
Quote:
Originally Posted by Kell Smith View Post
...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?
.
Imagine two rulers laying side by side. You can slide them back and forth so the "0" on one of them aligns alongside any number you would like on the other. One of our rulers is the meter scale on the recorder, marked in dB with "0" at the top so the only numbers on the ruler are negative. The other ruler is the the voltage of the signal sent to the recorder, marked with volts the corresponding dB in plus or minus away from 0.775v with "0" engraved in the middle. The EBU standard says we're going to take the "0" at the 0.775V mark on the voltage ruler and then line it up alongside -18 on the meter rule. The SMPTE standard says we'e going slide it down so the "0" mark lines up with -24 on the other, bringing "+4" on the voltage ruler in line with -20 on the meter ruler. "VU" is yet another "flavour" of decibels, so add a third ruler to the mix, this one in dB plus or minus dB away from 0VU with "0" also in the middle. The EBU would line up the 0VU mark against the 0dBu on the voltage ruler to bring it alongside the -18 on the meter ruler. SMPTE puts the 0VU mark alongside the +4dBu mark on the voltage ruler which in turn puts it beside the -20 mark on the meter ruler.
__________________
Good news, Cousins! This week's chocolate ration is 15 grams!
Steve House is offline   Reply With Quote
Old November 24th, 2010, 08:22 AM   #24
Trustee
 
Join Date: Aug 2007
Location: Portland, Oregon
Posts: 1,177
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?
NO value of dBFS corresponds to any particular voltage in general. Particularly when you are talking about inputs.

dBFS is a RELATIVE measure of where you are on the recording level scale. It is equally valid to say that a thimble or a shot-glass or a bucket or a 55-gallon barrel are "full to the brim" (0dBFS)

You seem to be obsessing on Decibels. And that is leaving you confused because you still don't seem to understand that dB is a RELATIVE measurement against some (several different) references for "zero". Whereas voltage is a fixed measurement. 1 volt is always 1 volt. But "0 dB" can be any of a dozen different levels or measurements. DON'T GET HUNG UP ON DECIBELS.

In the modern era, dB is a very useful measurement of gain or ratio. But it is not nearly as useful for statements of level because it is so frequent that the reference is omitted.
Richard Crowley is offline   Reply With Quote
Old February 1st, 2011, 06:59 PM   #25
Major Player
 
Join Date: Jan 2005
Location: USA
Posts: 422
Hi all,
I just realized that I forgot to get back in and post thanks for all the help in this thread. I ended up getting called away from my focus on audio levels, and planned to get back here and map out the 'ruler' example and wrap up the holes in my understanding of this issue. So now I"m revisiting it, printed out the thread to sit down and study it more in-depth, and noticed I hadn't posted back since it was still sort of "in progress."
So many thanks for all the helpful posts. I may have more questions upcoming.
Right now I've got a bit of a headache from re-reading it and getting back up to speed. =)
Kell Smith is offline   Reply With Quote
Old February 1st, 2011, 10:30 PM   #26
Trustee
 
Join Date: Jan 2011
Location: Pennsylvania
Posts: 1,359
I've been reading over this thread, and you have gotten some excellent answers.

This is difficult to grasp all at once. Many of your questions are interrelated, so the answers are also interrelated.

Some of this might be easier to understand if you knew the history of how these different scales evolved; but adding a history lesson could just make this a lot longer.

Where to begin?

This is all a matter of measurement. Let's simplify and say there are two types of measurements: absolute and relative.

Voltage is an absolute measurement; as someone else said, one volt is one volt is one volt. (And a rose is a rose is a rose.) If all signal levels were expressed in volts, one part of your question would be simpler to answer.

Someone mentioned that you're obsessing about dB. Let's try to clear that up simply. A dB, or decibel, is one tenth (deci-) of a Bel. A Bel is named for Alexander Graham Bell (we capitalize Bel to honor Bell). Someone has already said that a measurement in dB is actually the relative ratio between some reference and some quantity being measured. The "thimbleful, cupful, bucket full, barrel full" analogy was excellent! If your reference is a cup, than a cupful is 0dB(cup). In that case, half a cup would be -6dB(cup), one fourth of a cup would be -12dB(cup). etc. I don't mean to plagiarize here, but I want to reinforce some excellent explanation that has already been given.

At this point, I think it will be very helpful to look at a VU meter with a conventional "A" scale: the dB scale on top, and a percentage scale underneath. Note that there is nothing on this meter that says Volts, Watts, Amps, cups, or thimbles. It's all about ratios. As you see, a standard "A" scale VU meter does not use fractions to express ratios, rather is uses percentage. Just stare at that for a minute and try to absorb it. The only thing a decibel represents is a percentage, or a ratio... just relative relationship to some reference. 0dB is always 100%. -2dB is always actually 79.43%, which is darned close to 80%. 50% is always actually -6.02dB... -6 is close enough. Those relationships, percent to dB, never change when measuring voltages.

In the old days, that meter would be connected to a broadcast telephone line, to measure the audio level on the line. A broadcast line was 600 ohms, and 0 dB was defined as 1 milliwatt (0.001 watts) of power. With a little math (which you don't need to know) we find that it takes 0.775Vrms to generate 1 milliwatt of power into a 600 ohm load. So actually, the if meter reading was 0dB, the meter was connected to 0.775Vrms. (This is actually oversimplified, but please just accept it for now, or we will become tangled in math.)

But..... that meter could be connected to the output of a 500 watt power amplifier! In that case, you would need a resistive attenuator (or "pad") between the amplifier and the meter terminals, because, of course, the meter is quite sensitive (it can respond to 1/1000 watt). But, with the correct attenuator, you could make the meter read 0dB with 500 watts, or with 200 watts, or with 50 watts, or any other power level, coming out of the amplifier. 0dB is only a *relative* number!

In fact, we could let 0dB = 200 pounds, (the reference weight at the beginning of my diet). Next month, I weigh myself again, and find I weigh 160 pounds. 160 (measured weight) / 200 (reference weight) is 80%. Look at the scale on that meter. I've lost 2dB... see it? What if I weigh myself next month and I weigh 250 pounds? 250/200 = 125%. You don't see that number on the meter scale, that's "in the red" and if I had gained 50 pounds in a month, you can bet I'd be in the red, too. But actually, a ratio of 125% corresponds to +2dB. And, by the way, 1/80% = 125%. When you invert a ratio, you just change the sign of the dB.

Fifty years ago, life was easier. Almost all US professional audio equipment had evolved around telephone company practices. 0dBm was a common reference so you didn't need to convert that to volts, milliwatts, calculate it at different impedances, etc. Today, it's much more complicated, because different references are used at different times. So you need to be very aware of the reference level! Several other people have already explained parts of that for you, and eventually you'll see the light!

OK, I hope this hasn't gone too far afield. I sensed that you were getting confused by trying to cover so many different things at the same time, so I thought I'd simplify and talk just about decibels. If you're puzzled, keep looking at that meter scale while you re-read this. It's really easy, once you see the light! If you understand dB now, you've made some important progress!
Greg Miller is offline   Reply With Quote
Old February 8th, 2011, 08:35 PM   #27
Major Player
 
Join Date: Jan 2005
Location: USA
Posts: 422
Thank you everybody,
This is making a lot more sense now.
Awesome responses from everyone. A lot of very helpful information!
Kell Smith is offline   Reply
Reply

DV Info Net refers all where-to-buy and where-to-rent questions exclusively to these trusted full line dealers and rental houses...

Professional Video
(800) 833-4801
Portland, OR

B&H Photo Video
(866) 521-7381
New York, NY

Z.G.C.
(973) 335-4460
Mountain Lakes, NJ

Abel Cine Tech
(888) 700-4416
N.Y. NY & L.A. CA

Precision Camera
(800) 677-1023
Austin, TX

DV Info Net also encourages you to support local businesses and buy from an authorized dealer in your neighborhood.
  You are here: DV Info Net > The Tools of DV and HD Production > All Things Audio

Thread Tools Search this Thread
Search this Thread:

Advanced Search

 



Google
 

All times are GMT -6. The time now is 09:37 PM.


DV Info Net -- Real Names, Real People, Real Info!
1998-2017 The Digital Video Information Network