FOH Masterclass with Robert Scovill – Gain Structure


In this video by Hillsong Creative, legendary audio engineer Robert Scoville gives us a masterclass on gain structure.

Robert boasts a 35-year tenure as a seasoned professional in concert sound and recording. Throughout his illustrious career, he has expertly mixed over 3500 events. His exceptional engineering and production skills have been sought after by an impressive roster of renowned music acts, encompassing the likes of Tom Petty & The Heartbreakers, Matchbox Twenty, Jackson Browne, Rush, Def Leppard, Prince, and numerous others.

FOH Masterclass with Robert Scovill – Gain Structure

Video Text:

0:00:03. –>
so uh one of the things i want to talk to you guys about though and i’m kind of on a little bit of a mission um to go out and make sure everybody kind of understands this because gain structure is such an important aspect of digital uh and to try to help you kind of get things back together on your console and make sure it’s actually operating in a very optimized way for you okay because you get out of whack really easily in digital and it’s not too forgiving you know in analog it’s a little more forgiving but in digital it is not so i want to talk to you about how to set input gain something as simple as setting input gain on a digital console because once you get that part right the rest of it just becomes so so easy but we kind of have a tendency to just skip over this first step and you know i mean in analog we can say you know there’s probably a couple of different philosophies on how to set analog input gain we can make cases for different ways to do it

0:01:01.9 –>
i know this is going to rub you the wrong way but i’m going to say it there’s one way to set input game for a digital console it’s one way anything else i can i can argue you right out of it okay so let’s talk about it you okay with that yeah okay so this has to do with and and i’ll preface this part of it by saying you know live sound engineers have some really really unique challenges we have challenges that recording engineers don’t have we have challenges that post engineers don’t have and a lot of it revolves around getting our console optimized to both kind of tracking and mixing workflows we got to do both those things at the same time right and a lot of it is built around fader position okay here’s the important part to remember faders are not linear right meaning if i if the fader is sitting up in this area and i move it this distance that represents 6 db of change right if i go down in the scale now notice the

0:02:02.1 –>
distance has decreased but the amount of change has increased and if i go down even farther this very small change represents really really big change right it’s not linear right it’s logarithmic as a matter of fact so the idea is that we want to be able to mix mix in high resolution we want to have faders sitting up in this position now i’m not here to say it has to sit at zero for my money if it sits plus 10 minus 10 you’re in high resolution okay but down here you got issues you know especially with the pa systems that are out there today i mean you can make small changes and hear small changes okay i mean it’s very analogous to mixing in a high resolution studio all right so our goal is to get our console in a set up in a way where we can mix in high resolution okay you’re buying it so far okay so let’s talk about uh input gain concepts this is where the arguments all start all right so we’re going to build a very very simple circuit here

0:03:02. –>
we’re going to have a preamp and we’re going to have a meter all right so here’s where we start the question why do we have a preamp anybody want to take a stab at it why do we need a preamp to amplify the signal why do we need to amplify the signal because the signal’s low why is the signal low being a problem because i’m telling you right now when you can answer this question you’re 99 of the way there yeah that’s part of it that’s coming but let’s let’s just stay in analog for a second why do i need a preamp well that’s true but it’s not the reason why we need a preamp to be able to take a mic level signal to line level period end of story now let’s let’s just push away all the

0:04:01.6 –>
godzilla’s the concepts of well some preamps sound better than others and all that that doesn’t deal with the why the why we need a preamp is i need to get mic level signals to line level okay why do we need to do that yeah well that’s that’s true but there’s an actual i mean just flat out reason that we need to do it because all consoles are optimized underneath the hood to work at line level not mic level okay so where’s line level how do we know where line level is anybody minus 15 zero which is it you pick one that’s true okay so the the answer to

0:05:00.6 –>
what he’s getting to there is we have a meter to tell us we have a meter to tell us where line level is all right so now that we know all of this why is it so hard to set an input gain why is there so much consternation about it why is there so many approaches about it it is simply taking a signal and moving it from mic level to line level so that it works properly and transports through the console properly right that’s all you need to care about all right now signal to noise all of that stuff that comes into play in analog for sure we can get enough signal in where we have circuit noise that is diminished with regard to it etc but i promise you that even in analog circuitry line level is the place where that is the best in digital because we’re converting that doesn’t come into play there is no signal noise at some point we just have bit depth

0:06:00.3 –>
that is so poor that the signal turns into noise okay so let’s get back to our original discussion here so we have our mic level signal coming in we need to get it to line level because the console is built to work at line level in our situation that has to do with zero db okay not dbfs this is dbvu okay zero db all right so let’s talk about this for a minute what do all these different scales mean all right so what that means what it means is i take a mic level signal in and i turn up the preamp until it is sitting at zero line level all right i want it operating somewhere around zero what does that actually mean in terms of the electronics part of it what that means is if i took a direct output from that channel and went out of the console with it it would be plus 4 dbu analog all right in venueland it’s called dbr meaning we have an analog scale right and 0 db there actually represents

0:07:02.1 –>
-20 dbfs because it’s digital right but we’ve given you it at an analog scale but here’s the important part to remember when that cons or that signal is operating around zero the analog portion of a console would put out plus 4 dbu venue would put out plus 4 dbu and pro tools would put out plus 4 dbu if that signal is sitting at those levels okay all right so zero is you know it’s the same it’s just different ways of showing the damn the the same signal okay everybody with me okay just different ways to look at the same thing all right so let’s go back to our oh no here we go so uh yeah sorry here’s the other piece of it that we have to pay attention to in digital right in in analog it’s noise right that’s the enemy in noise or in analog is noise right as we if we didn’t have enough if that mic mic level signal was too low

0:08:01.5 –>
noise actual signal noise would compete with it right that’s not the case in digital what we have in digital is a bit depth issue right meaning how many bits are being used on the signal and here’s where we’re gonna we’re gonna run into probably some conflict with recording engineers etc but i’m gonna try to explain the difference between these two things in live and recording in in our world we work in 24 bit depth preamps right 24 bit what that translates to over the scale is 6 db per bit okay so if i have the signal operating at -6 i’ve used 23 bits of conversion right minus 12 22 bits right on down the line right so you just keep subtracting so what that means is if i can get a signal rmsy is hanging at zero line level i’m actually converting at around 21 bit that’s plenty that’s plenty of bit resolution especially for live sound all right now

0:09:01.9 –>
here’s the other thing to take into account here if my main signal is sitting here and it’s converting at 21 bit in live sound signal let’s say it’s a vocal mic right and we have vocal signal what’s all the signal that’s sitting down here if this is the vocal what’s down here yeah room background noise amplifiers drums etc well what do you think those signals are converting at yeah very low bit rate right they’re not going to be very pretty that’s why it’s very important in digital even though it’s nearly impossible as you’re going to hear on some of the tracks i’m going to play you today to keep that signal low and out of the way and the the worst part about it is if we don’t get that signal coming in properly to line level if we run it too low then not only do we get low bit resolution of the main signal but even worse resolution of the background noise right so it’s uber important to get your

0:10:02 –>
levels right on the console yeah recording so that you’re just short of zero to maximize resolution yeah you have to just be careful with it uh in this and here’s the difference between like the studio mentality and the live sound mentality in the studio mentality let’s go back to that situation again where i was just talking about let’s say we’re recording a vocal right what’s all this stuff sitting down here in the recording yeah that’s your million dollar room right so we we don’t want that to be recorded low bit rate at it as well we want to get all of it recorded hotter now obviously there’s some place where we can’t get more level you know this this ratio is not going to change unless we change it meaning i compress the signal or limit the signal pre-conversion pre-conversion

0:11:01.3 –>
right so if i if i reduce that pre-conversion and then turn the whole thing up i now have a much higher bit rate conversion of my my background noise my room right so that’s the recording mentality the problem is when we do that in live sound we first of all we don’t have the ability unless we really really put together some specialized uh signal paths we don’t have the ability to compress or limit pre-conversion okay so that’s the first thing if we do it post conversion we’re not gaining anything in bit depth and we’re actually taking all of this not so well recorded background noise and moving it closer to our signal what is this in there this is echo this is pa system this is symbols this is you know that’s where all of these heavy heavy compression techniques for live microphones don’t serve you very well in live sound okay you’ve got to be much more judicious with it okay we don’t have that luxury of isolation

0:12:00.5 –>
that we have in the studio okay you’re with me so far okay so let’s get back to it here so the idea here again is just to get maximized operating system here now why is this more important in live sound or why is this really important in live sound because it plays directly into fader levels as well and not only that as you’re going to see here in a second it’s also what our record level it’s our record level as well directly off the pre and the conversion is also our recording level so here’s the challenge that we have in live sound this is it in a nutshell let’s uh let’s make this let’s make this the bass drum for a second okay if i’m sitting at the console and i gain up that bass drum and get it optimized right around line level and it’s not loud enough in the room what’s that telling me

0:13:00.5 –>
we need a bigger pa system we need a bigger pa system right there’s nothing i’m going to be able to do between here and there to magically make that work there are no plugins to magically make it work and give me more headroom etc when you turn it on and there’s not enough of it there’s not enough pa okay now compare that to the studio do we ever say there’s not enough control room monitors no this is a dynamic that we have to tend with in live sound right now here’s the other problem that we have to tend with that is again not in recording what do we do let’s say let’s say it’s well we started out with the bass drum let’s let’s stick on the bass drum for a second let’s say i turn up the bass drum in the pa i’ve got my fader set at zero high resolution and of course i’ve got my master setting at zero but it’s now way too loud for the music i’m going to be presenting this is the bigger dilemma

0:14:01.1 –>
now what do i do where do i turn it down well so now if i turn it down at the amps i’m going to penalize the level of all inputs the potential of all inputs just because the bass drum was too loud where do i turn it down how do i make it right for the music i’m about to present what kind of subgroup yeah you’re on you’re right on the money there we have a we have an excellent student in the audience today thank you for coming so let’s play the game though i’ll back up even though you’re exactly right let’s play the game because what i see people do more often than not is one of two things more often than not they will turn this down they say well just turn down the preamp what’s the ramification of that lower bit rate lower resolution right

0:15:00.6 –>
okay so there was a in analog you can kind of get away with that at times but in digital you really can’t get away with it right so we’ve got to come up with a way to turn this down the other part of it that we don’t want to do for live remember we’re setting we’re adjusting this to make it right in the room well we’ve now also destroyed the record level now it’s not recording at the right level either now the guys are going to get it in post go dude what’s up with your levels well it had to be right for the room right so the other piece of it like i said is the digital recorder is tapped right off that the other thing is once it’s optimized here it’s perfectly set up for effects processing on the channel meaning gates compressors etc when this is optimized for xero you can literally go to the gate and punch it in without doing anything on the settings if the threshold is set right at zero and the gate will probably start operating correctly right if you don’t have that if i turn down that kick drum there to make it

0:16:02. –>
work in the room let’s say i turn it down 20 db maybe that’s what it’s got to be for the room now i’ve got to find an additional 20 db of threshold in my noise gate to make it work i as all this calculation has got to take place in my head now to kind of go well it’s down 28 you might not even be able to get enough threshold to get it to work right again line level is where the console is optimized to work right so the other place that comes into play is if i turn it down i’ve stolen gain potential from aux levels if i’m mixing monitors there now i’ve taken away gain from the aux level just just to get it to sit right in the room okay and here’s the bummer about it and this is probably the thing i see people do the most they just apply a vca subgroup to this right think well i’ll just use the vca to do it and we know what a vca does right as it moves down oops i think i got ahead of myself there as it moves down

0:17:01.6 –>
it is essentially moving the fader down right now that can work to our advantage don’t get me wrong but in this situation to use it to create an offset or to use it to set the level in the room honestly it’s the wrong choice because you’ve now stolen any that that same amount of gain away from that aux bus you don’t really have it optimized in the aux bus now right so if you had something sitting outside here maybe it’s not monitor mix in this situation maybe it’s an effects processor or a compressor maybe it’s bus driven compression now you’re in the same problem right you’ve stolen a gain away from there and you’re gonna have to make it up in threshold out there all of a sudden the the tail starts chasing the dog a little bit you know whereas if we make this an audio sub group and i’m going to give you a piece of language here to help you kind of understand this a little better because subgroup is really not the best term to use in this situation it really should be sub master right it’s a sub master meaning if this is the sub master for my

0:18:02 –>
drums this is going to represent the overall level of the drums in the room it’s a master fader for the drum kit i may create a master fader for the guitars a master fader for the keyboards etc right it’s not a vca it’s an audio subgroup all right so i can leave this where it’s supposed to be optimized here i still have a good record level i have optimized signal for dynamics processing i still have the full game potential for any aux buses pre or post fader because i’ve used an audio subgroup there to set the right level not a vca which would still gain potential away post fader right and the other thing keep in mind if you’re doing bus style compression if you’ve got compression being driven off of an aux bus and you have this set up as a vca every time you move the vca you’ve now changed the threshold to the compressor you’re going to be chasing your tail for a long time to get that right

0:19:02.5 –>
okay is this making sense to everybody right so how do we set gain rms level to line level on the console

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