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Author Topic: the dB Scale  (Read 11361 times)

Christy L Manoppo (okky)

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the dB Scale
« on: November 21, 2010, 10:48:49 pm »

one of my contacts in tweeter sends a pic like this..

http://img28.imageshack.us/img28/4681/confusedi.jpg

imho... I guessed that his gain structure isn't properly done right.

So we discussed about this, he said that the -20 on the M7CL is on a digital scale. -20 on digital is equals 0 on analog...

but, afaik, there's no such thing as "+" or amplification on the digital realm, only analog can do that. (cmiiw)

so that raised a questions..

what's the real truth?

I also noticed that in most digital boards, the fader uses analog scale, but the meters are in dBFS. -20-0. This confuses me little bit.


nb. Running that board so "low" makes minute adjustment kinda hard, and in my experience, running that "low" also introduces some unpleasant sonics, as it kinda "restrains" the sound, not "open". (cmiiw)

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Christy L Manoppo
Coordinator for AVL Dept,
Bethany Indonesian Church of GOD,
Philadelphia, PA

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Lee Buckalew

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Re: the dB Scale
« Reply #1 on: November 22, 2010, 07:27:58 am »

Chirsty,
First, the fader is at -25 not -20 (but not necessarily -25dBfs).  
Secondly, 0dBfs is zero dB Full Scale.  A full scale digital signal (0dBfs) can get no higher, it is all 1's with no clipping occurring.  It is at it's peak useage and increasing the level means you are going into digital clipping.  
If I am correct and this is an M7CL-32 then your friends picture shows him at about -35 dB on the fader.  IF the gain structure is actually set so that the scale next to his fader represents dBfs this would be -35dBfs buttThat would all be dependent on the gain structure through the board and is most likely not the case since you can easily vary the level without moving the fader.  Just change your mix, increase or decrease head amp levels, etc.  That scale is not dBfs.

To answer your other question.
What - dBfs level references to "zero" in analogue.  This varies with musical style, there has been no set standard.  Typically it is somewhere between -13 and - 18 dBfs.  Bob Katz has developed multiple scales for better metering of different musical styles since he was tired of there not being any standardization.

Hope this helps,
It's early here.

His,
Lee Buckalew
Pro Sound Advice, Inc.
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Brad Weber

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Re: the dB Scale
« Reply #2 on: November 22, 2010, 07:54:51 am »

Your friend is correct in a way, but there also seems to be some confusion.  Signal levels for digital signals are usually referenced in dBFS (Full Scale) with 0dBFS being the peak level possible.  Thus on digital mixing consoles 0dBFS typically relates to the maximum analog input and/or output levels for the console, so whatever the maximum analog levels are for the console will be 0dBFS in the digital realm.  In the case of the M7CL, the maximum input level is +30dBu, which represents 0dBFS in the mixer and a maximum analog output level of +24dBu (there is a 6dB loss included in the D/A conversion of the M7CL), while the nominal levels are -20dB relative to those maximum levels.  So your friend is right that what would be the nominal "0" level on analog dBu meters would be -20dBFS on the digital meters of the M7CL.

However, the level markings on the faders have nothing to do with absolute signal levels and are instead a reference to relative levels.  -20 on a fader does not represent a -20dBu level or -20dBFS level or any specific level, it simply means -20dB or 20dB of attenuation relative to the fader being at the 0 setting.  The faders alter the signal level but they do not set the level.

So while they are right that on the dBFS digital signal level meters of the M7CL you typically want to run about -20dBFS, that has nothing to do with setting the fader at -20.  The fader should be set to provide appropriate signal levels and not necessarily at the markings denoting the desired signal level.

And yes, you can have amplification in the digital realm.  A digital audio signal is just bits and bytes and can be mathematically manipulated like any other data.  The caveat is that the result cannot exceed the maximum value that can be represented, the result of doing that is digital clipping and that is to be avoided.
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Matthias Heitzer

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Re: the dB Scale
« Reply #3 on: November 22, 2010, 09:33:08 am »

Even in the analoge world, a fader can't boost the signal, it's just a variable resitor. +6 or (+10) printed on the surface next to the slot just means that the signal is amplified by that amount before the fader.

So even if all your sliders are near the "0" mark, the signals get dampened, and thats a good thing, otherwise you are very likely to overdrive the summing amps.

Often, the best resolution seems to be above zero, too.

In my opinion, the whole discussion about gainstructure is useless.

Sure, there are some big mistakes that can be made, but modern audioelectronic has a much better S/N ratios than most real world environments. Just think about one of the biggest struggles for churchsound folks: the stage volume. Or a mid sized audience with some people coughing and others rustling with their jackets. Or how about HVAC?

Especially in a church situation, i want some extra gain available at the faders.

Most modern faders have a smooth and constant resolution down to -30dB, so if you set them to -10 in the first place, You have 40dB of flexibility without leaving the comfortzone.



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Christy L Manoppo (okky)

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Re: the dB Scale
« Reply #4 on: November 22, 2010, 10:42:28 pm »

Thanks for the help for clearing this. This helps a lot. Again many thanks..
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Christy L Manoppo
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Brad Weber

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Re: the dB Scale
« Reply #5 on: November 23, 2010, 09:03:59 am »

Matthias Heitzer wrote on Mon, 22 November 2010 09:33

Even in the analoge world, a fader can't boost the signal, it's just a variable resitor. +6 or (+10) printed on the surface next to the slot just means that the signal is amplified by that amount before the fader.

This is not always true.  If the fader is a varistor directly affecting the audio signal as noted then it is valid, however if the fader is actually varying a control voltage to a VCA that provides the actual level change in an analog mixer or a DCA as occurs in probably every digital mixer then you could indeed have amplification.

The important thing seems to be understanding that the markings on the fader are simply relative, the fader at '+10' should result in a 10dB greater signal level than if it is at '0' while being at '-10' would make the signal 10dB less in level.

Matthias Heitzer wrote on Mon, 22 November 2010 09:33

In my opinion, the whole discussion about gainstructure is useless.

Gain structure is a critical issue, especially since you have not limited the comment to just the mixer.  Even within a mixer, it makes sense to employ good gain structure.  While your example of ambient noise may apply to many church environments, it does not necessarily apply to recording, broadcast, streaming, etc. that may also be outputs from the console.  And you also have to consider the impact of excessively high or low output signals on the system downstream of the mixer.  I believe that good mixer gain structure considers both the effect on the electronic signal and the usability for the operator.
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Matthias Heitzer

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Re: the dB Scale
« Reply #6 on: November 23, 2010, 11:08:29 am »

Quote:

 
Quote:

Matthias Heitzer wrote on Mon, 22 November 2010 09:33

Even in the analoge world, a fader can't boost the signal, it's just a variable resitor. +6 or (+10) printed on the surface next to the slot just means that the signal is amplified by that amount before the fader.



This is not always true. If the fader is a varistor directly affecting the audio signal as noted then it is valid, however if the fader is actually varying a control voltage to a VCA that provides the actual level change in an analog mixer or a DCA as occurs in probably every digital mixer then you could indeed have amplification.


If the cicuit is able to send positive and negative voltages to the VCA.

Quote:


The important thing seems to be understanding that the markings on the fader are simply relative, the fader at '+10' should result in a 10dB greater signal level than if it is at '0' while being at '-10' would make the signal 10dB less in level.



Yep, that's what it is about

Quote:

Quote:

Matthias Heitzer wrote on Mon, 22 November 2010 09:33

In my opinion, the whole discussion about gainstructure is useless.



Gain structure is a critical issue, especially since you have not limited the comment to just the mixer. Even within a mixer, it makes sense to employ good gain structure. While your example of ambient noise may apply to many church environments, it does not necessarily apply to recording, broadcast, streaming, etc. that may also be outputs from the console. And you also have to consider the impact of excessively high or low output signals on the system downstream of the mixer. I believe that good mixer gain structure considers both the effect on the electronic signal and the usability for the operator.


Yes, I should have narrowed my statement. Gainsturcture itself is important, but the whole "system" which inludes the room and even the human ear has to be considered.

I had to watch a bunch of soundmen failing because of executing the textbook aproach.

My biggest problem with those gainstructure discussions is that people often stop thinking for themselves and repeat the mantra of unitygain (or whatever gainstructure-dogma they believe in)

In- and output levels are within a narrow range by now. (this luxury becomes obvious if you hook up vintage equipment)
But still, different manufacturers design different gain stages for their consoles, and this internal gainstucure has to be followed by the user.
I've met desks with max. input levels higher than +20dBu, but summing amps that distort if a few channels driven at +10dBu are routed to the same subgroup.
Or an Aux send that provides +30dB of gain (15 at the channel and 15 at the master) While another manufacturer gives their console a max. gain of +10dB, or just a "passive" circuit for a post-fade aux. (with OPamps of course, but without additional gain).
Judging by pictures won't lead to a good result.(Well, i can't think of a situation where it is usefull to max the gain knob and press the activate the PAD.... oh wait, perhaps this button is also switching XLR and TRS inputs)
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Brian Ehlers

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Re: the dB Scale
« Reply #7 on: November 23, 2010, 01:11:08 pm »

I still think you're dismissing gain structure more than you should.  (Please, I'm trying to discuss, not argue.)  While the noise floor of good equipment is generally low enough to not worry about more than a little (for live sound anyway), it's worth keeping the signal level of any given stage out of the mud if for no other reason than to make your meters useful.

More significant is the issue of headroon/clipping.  I would argue that understanding your gain structure is not only important between pieces of gear but also within each piece of gear -- especially the mixing board.  Understanding the maximum voltage of each channel strip, mix bus, and output driver helps to ensure that an optimal setting for one gain stage doesn't cause problems in a latter gain stage or bus.

It's also not true that "in- and output levels are within a narrow range by now."  An easy example is a pro CD player which outputs +24 dBu for CD data at 0 dBFS.  That's WAY above the "standard" pro interface level of +4 dBu.  Another example would be a compressor/limiter without an input gain knob.  Due to limitations in its available threshold settings for compression or limiting (or gate for that matter), you may want the board to send it a much higher or lower signal than you otherwise would.  And the ripple effect for that decision will be different depending on whether the compressor/limiter is on a channel insert or aux or group bus.
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Matthias Heitzer

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Re: the dB Scale
« Reply #8 on: November 23, 2010, 04:47:38 pm »

Since the question of the original poster has been cleared, i think it's ok if we continue.


Quote:

in- and output levels are within a narrow range by now


I think i expressed myself unclear, i tried to use this sentence as an introduction to the lines that follow, beginning with the word "But", explaining that the differences regarding the internal gainstructures of differnt consoles alone renders a judgement based on a picture useless.

You are right, there is a huge difference between the output voltage of a dynmaic micophone and a cdplayer or console output, but today's desks are flexible enough to manage all those situations. You don't need a bunch of different preamps and impedance transformers anymore. If your desk is near the sources, there would not even be the need for a lot of DIs, thanks to high Z inputs.
As the felt distance between the continents shrinked with the spread of air traffic, the output levels of microphones, instruments and devices converge if the gain knob you're twisting controls a preamp with a 80+ dB range. That's my subjective feeling, i suppose.

Quote:

 I would argue that understanding your gain structure is not only important between pieces of gear but also within each piece of gear -- especially the mixing board.
Understanding the maximum voltage of each channel strip, mix bus, and output driver helps to ensure that an optimal setting for one gain stage doesn't cause problems in a latter gain stage or bus.


Imo, the user does not have a lot of choices to "set" a gainstructure, he has to comply to the restrictions and the dictates the combined devices make. Matching input and output levels is very important, no doubt, but it seems to me that the broad discussion about an abstract term called "gain structure" only distracts from hearing. Too often i saw people with their eyes glued on meters showing everything was fine while the signal distorted at some unseen point in the signal chain.
(Actualy not that often in absolute numbers, but in situations where it should not have happend). And too often i witnessed people arguing  whether the amps should be opened completly or just until the point where amp and desk clip at the same time. And often both were wrong...
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Andy Peters

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Re: the dB Scale
« Reply #9 on: November 25, 2010, 03:40:57 pm »

Matthias Heitzer wrote on Mon, 22 November 2010 07:33

Even in the analoge world, a fader can't boost the signal, it's just a variable resitor. +6 or (+10) printed on the surface next to the slot just means that the signal is amplified by that amount before the fader.


Not true; pretty much all modern consoles use the fader as a variable voltage divider in front of a gain-of-10 dB op-amp. When the fader is at unity, it's actually attenuating by 10 dB, which is made up by the following op-amp.

That op-amp does more than provide that make-up gain. It also buffers the fader from the following stages (post-fader aux sends, pan, etc).

Now there are some consoles where the group and possibly master faders top out at unity (some Soundcraft and Soundtracs, for example) so the gain structure is somewhat different but the point still stands that the fader circuit is a variable voltage divider followed by a buffer amplifier.

-a
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Re: the dB Scale
« Reply #9 on: November 25, 2010, 03:40:57 pm »


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