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Author Topic: Fader as compared to main levels  (Read 2209 times)

Dan Crocker

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Fader as compared to main levels
« on: January 16, 2014, 01:43:52 am »

I am a relative newbie to all of this so please forgive the basic questions:

Let's say that I have one mic connected to a console and I set it's fader such that its level peaks at 0db (unity). If I also set my main faders to 0db, will the meters for the mains show a signal level of 0db? If, instead, I then have 10 inputs, all peaking at 0db, what will the main meters show? My understand is that the main meters will show a level that is the sum of all the inputs. But, I don't understand how the math works when the get summed. The question is related to advice I have seen to operate channel faders around 0db. If I manage to do this, don't I need to worry about getting two high a level on the main bus?
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David Morison

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Re: Fader as compared to main levels
« Reply #1 on: January 16, 2014, 07:51:11 am »

I am a relative newbie to all of this so please forgive the basic questions:

Let's say that I have one mic connected to a console and I set it's fader such that its level peaks at 0db (unity). If I also set my main faders to 0db, will the meters for the mains show a signal level of 0db? If, instead, I then have 10 inputs, all peaking at 0db, what will the main meters show? My understand is that the main meters will show a level that is the sum of all the inputs. But, I don't understand how the math works when the get summed. The question is related to advice I have seen to operate channel faders around 0db. If I manage to do this, don't I need to worry about getting two high a level on the main bus?

(This may be slightly too basic a question, and answer, for the LAB - you may find the mods move it over to the Lounge shortly).
If you sum two identical signals (ie frequency, phase and amplitude all match), then the sum will be 6dB higher than the individual signals.
If you sum completely different signals each having broadly similar level, then the resultant sum will usually be about 3dB higher, but frequency and phase could vary that a bit.
These would also apply every time you double the number of summed signals eg from 8-16 different signals, you'd expect about 3dB increase.
Many mixers include a reduction in level between the channels and the main bus (or subgroups if included) to prevent overloading the main bus as a result of the summation, so you won't necessarily see that exact amount of gain reflected on the main meters.
HTH,
David.
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Steve M Smith

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Re: Fader as compared to main levels
« Reply #2 on: January 16, 2014, 09:46:30 am »

The other thing to consider is that it is better to have your faders around the 0dB position, or about 75% of the way up rather than a lot lower.

If your running position is e.g. only a quarter way up, a small change in position will have a large change in level.  At around the 0dB position it is the other way round.  You need to move the fader more to get the same perceived change in volume so you can be more accurate with your control.

I know that isn't quite what your question was about but it is valid with regards to advice about running around 0dB.

If you find that running everything close to 0dB gives too much volume, you will need to reduce the level on your amps or system processor (or use fewer or lower powered amps!).


Steve.
« Last Edit: January 16, 2014, 01:26:33 pm by Steve M Smith »
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Tim McCulloch

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Re: Fader as compared to main levels
« Reply #3 on: January 16, 2014, 10:02:22 am »

Dan- please disabuse yourself of the notion of "unity" as it applies to fader position or the level of a signal.  There ain't no such critter, and Mackie's marketing dept should have been shot for giving this marketing effort the patina of science.
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TJ (Tom) Cornish

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Re: Fader as compared to main levels
« Reply #4 on: January 16, 2014, 10:06:06 am »

I am a relative newbie to all of this so please forgive the basic questions:

Let's say that I have one mic connected to a console and I set it's fader such that its level peaks at 0db (unity). If I also set my main faders to 0db, will the meters for the mains show a signal level of 0db? If, instead, I then have 10 inputs, all peaking at 0db, what will the main meters show? My understand is that the main meters will show a level that is the sum of all the inputs. But, I don't understand how the math works when the get summed. The question is related to advice I have seen to operate channel faders around 0db. If I manage to do this, don't I need to worry about getting two high a level on the main bus?
Fader position does not necessarily correlate to a signal level., and "0dB" or "unity gain" may or may not mean what you think they mean - as David points out, most consoles factor in summing gain so as to not overload the busses.

The two somewhat interrelated issues are gain structure, and control ergonomics.  Most mixing consoles have a target level for the channel preamp - either 0dB for many analog boards, or possibly some negative number for digital boards - often around -18dBFS.  Nuances aside, what you're trying to do is get the signal level such that it's within the operating band of the console, and that the faders are in a convenient position to mix from.  As Steve points out, fader taper is not necessarily linear, and it's generally easier to mix if your fader levels are around 0dB, because you have more linear motion to work with for small signal changes here than much lower on the fader, where 20dB may only be a fraction of an inch.

My workflow has evolved to the following:
 - I set my preamps such that the signal level is approximately -6dB on the input meter (my board is an A&H GLD).  I find that if I normalize to 0dB, I have less headroom before clipping than I'd like, and since with modern gear there really isn't an issue with noise floor, normalizing to -6dB gives me extra room for dynamic signals with no loss of quality.
- I run my channel faders in the vicinity of -5dB, for a similar reason - more room on the fader to adjust for dynamic signals.
- If with these starting points I've got faders way off my -5dB baseline, I may adjust channel gain, keeping in mind that any adjustments I make after setting up monitors will need to be reversed in all aux sends to keep the relative monitor mix the same.
- I adjust final volume with either the main fader of the console, or amp gain to compensate for running the desk a little cool.

During the show, unless there is a drastic problem, I leave channel gains alone and don't worry about my fader positions too much.
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Dan Crocker

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Re: Fader as compared to main levels
« Reply #5 on: January 16, 2014, 01:14:58 pm »

Thanks for all the replies. I'll respond here as opposed to each one individually.

David: This was very helpful. You actually addressed one of the things I was confused about. I was worried about overdriving the main bus. I thought that if I operated my channel faders around 0, I'd end up needing a huge amount of headroom on the main bus (3db for every doubling of channels -> 12db for 16 channels). It sounds like maybe I don't need to worry about this if the console automatically attenuates the combined signal from the channels before sending it to the main. In case it helps, the console is the Roland M200i.
One thing that puzzles me is that our previous console (Presonus 16.4.2) seems to behave differently wrt the main fader meter. I didn't spend enough time to correlate the numbers, but Roland's main level seemed to be quite a bit lower than the level for the single channel I was playing with. Meanwhile, the Presonus main level was very similar to the channel's level.

Steve: Actually, this is precisely what started all this thinking. With our previous console, we've been operating channels on the low side and I wanted to change this. I do know that this is where I want to be running. With our new Roland, I want to re-balance things better.

Tim: By unity gain, I guess I mean 0db...no amplification and no attenuation of the signal. I expected these terms to be interchangeable. Marketing BS aside, aren't they the same thing? Don't get me wrong, I'm not saying I like that term or need to use it :).

TJ: Could you clarify your first comment about 0db? What I think it means is that the amplitude of whatever signal comes into the fader section of the circuit is not modified. Of course, there can be all kinds of things before the  fader in question (channel, subgroup, or main) that can modify signal amplitude. Was this your point or was it something else. And yes, I do know that none of this necessarily implies any specific "loudness".
Also, your workflow example is very helpful. A few follow-up questions:
-Once you have your channel faders running around -5db, do you care about what the main fader is set to? I assume you would ideally want it around -5db as well for the same reason you want the channel faders this way. If it ends up being too low (turning it up more would make the mix too loud), it seems like you'd want to to turn down the power amp, right?
-As of right now, the way our church amp is set up, the main fader needs to be turned way down to get the right output volume (or the channel faders end up being low). The M200i has an attenuation setting on the mains. Would adding attenuation with this control be the same as turning down the power amp? Or, is there some downside to doing this?
-For subgroups, I'm assuming I'd want these faders to be set around -5 or 0, right? If so, I'd be inclined to start with all my subgroup faders set to -5db and, more or less, forget about them while adjusting the mix with the channel faders. Then I would use the subgroup faders during mass to play with overall levels for the groups. Does this sound reasonable?

Thanks,
Dan


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TJ (Tom) Cornish

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Re: Fader as compared to main levels
« Reply #6 on: January 16, 2014, 01:24:03 pm »

Thanks for all the replies. I'll respond here as opposed to each one individually.

David: This was very helpful. You actually addressed one of the things I was confused about. I was worried about overdriving the main bus. I thought that if I operated my channel faders around 0, I'd end up needing a huge amount of headroom on the main bus (3db for every doubling of channels -> 12db for 16 channels). It sounds like maybe I don't need to worry about this if the console automatically attenuates the combined signal from the channels before sending it to the main. In case it helps, the console is the Roland M200i.
One thing that puzzles me is that our previous console (Presonus 16.4.2) seems to behave differently wrt the main fader meter. I didn't spend enough time to correlate the numbers, but Roland's main level seemed to be quite a bit lower than the level for the single channel I was playing with. Meanwhile, the Presonus main level was very similar to the channel's level.

Steve: Actually, this is precisely what started all this thinking. With our previous console, we've been operating channels on the low side and I wanted to change this. I do know that this is where I want to be running. With our new Roland, I want to re-balance things better.

Tim: By unity gain, I guess I mean 0db...no amplification and no attenuation of the signal. I expected these terms to be interchangeable. Marketing BS aside, aren't they the same thing? Don't get me wrong, I'm not saying I like that term or need to use it :).

TJ: Could you clarify your first comment about 0db? What I think it means is that the amplitude of whatever signal comes into the fader section of the circuit is not modified. Of course, there can be all kinds of things before the  fader in question (channel, subgroup, or main) that can modify signal amplitude. Was this your point or was it something else. And yes, I do know that none of this necessarily implies any specific "loudness".
Also, your workflow example is very helpful. A few follow-up questions:
-Once you have your channel faders running around -5db, do you care about what the main fader is set to? I assume you would ideally want it around -5db as well for the same reason you want the channel faders this way. If it ends up being too low (turning it up more would make the mix too loud), it seems like you'd want to to turn down the power amp, right?
-As of right now, the way our church amp is set up, the main fader needs to be turned way down to get the right output volume (or the channel faders end up being low). The M200i has an attenuation setting on the mains. Would adding attenuation with this control be the same as turning down the power amp? Or, is there some downside to doing this?
-For subgroups, I'm assuming I'd want these faders to be set around -5 or 0, right? If so, I'd be inclined to start with all my subgroup faders set to -5db and, more or less, forget about them while adjusting the mix with the channel faders. Then I would use the subgroup faders during mass to play with overall levels for the groups. Does this sound reasonable?

Thanks,
Dan
Dan - kudos for trying to understand all this.

My point about "unity" being a simplification was to take into consideration the various level shifts that happen behind the scenes in the console that you don't need to know or care about, as reasonable boards are going to handle summing without issue.

Modern gear is so good - north of 100dB signal to noise ratio, that you can only really get in trouble through severe clipping - even adding and subtracting 10dB or more at various stages really doesn't make much difference in final quality - unlike 20 years ago.  Everything else comes down to workflow style.

Answering several of your questions at once, I only care about fader position for faders I actually use - I tend to only occasionally touch group faders, and even less often the master fader, so those are less critical.

It is easy to potentially end up with too much system gain, leading to your current situation of having to either run the channel faders pretty low, or the main fader pretty low.  The best solution is probably turning down your amp gain (depending on the type of system - if you have a professionally-tuned DSP, it's probably better to do it on the input level setting inside the DSP rather than change settings on amps which could have side effects to your system tuning), but the attenuation setting on your master section would also work well.

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dick rees

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Re: Fader as compared to main levels
« Reply #7 on: January 16, 2014, 01:25:41 pm »

"Unity gain", if it means anything, is the desired optimum level of signal above noise floor THROUGHOUT A SYSTEM.  As such, it proceeds from your pre-amp settings through your channel faders to the summing amps and so on out to the amplifiers and speakers.  So there may or may not be a "unity gain" setting at each point in the chain, but it definitely has nothing to do with numbers, "o'clocks" or physical positioning of any knobs or sliders, simply denoting that the signal is not overly boosted or attenuated at any given stage. 

That's where the term "gain staging" enters in, a better and more comprehensive term which includes the concept of "unity gain".

At least that's the way I think of it...
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Steve M Smith

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Re: Fader as compared to main levels
« Reply #8 on: January 16, 2014, 01:30:36 pm »

Steve: Actually, this is precisely what started all this thinking. With our previous console, we've been operating channels on the low side and I wanted to change this.

I would much prefer to run the main left and right faders low and keep the channel faders up around 0dB as you are more likely to be adjusting channels than the main output level.


Steve.
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Dan Crocker

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Re: Fader as compared to main levels
« Reply #9 on: January 16, 2014, 01:34:35 pm »

Thanks.
So, I heretofore will no longer be using this term. It sounds relatively meaningless :)
Dan

"Unity gain", if it means anything, is the desired optimum level of signal above noise floor THROUGHOUT A SYSTEM.  As such, it proceeds from your pre-amp settings through your channel faders to the summing amps and so on out to the amplifiers and speakers.  So there may or may not be a "unity gain" setting at each point in the chain, but it definitely has nothing to do with numbers, "o'clocks" or physical positioning of any knobs or sliders, simply denoting that the signal is not overly boosted or attenuated at any given stage. 

That's where the term "gain staging" enters in, a better and more comprehensive term which includes the concept of "unity gain".

At least that's the way I think of it...
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