gain staging - interface to monitors

[lomky]

I posted this over at GS, just thought I would also post here for more info on the subject:

So, I need to wrap my head around something that has me a little perplexed. I am going out of my Audiofire 12 into channels 15 and 16 of my A&H mixwizard, I then have the output from the A-B out to my Dynaudio BM5A's. Now when setting the output from my mixwizard I need to engage the pad and have my gain as low as it can go. The levels do match the levels on the front panel of my Audiofire, well sort of, 0db on the Audiofire matches to about +9 db or so, which in my mind should be about right.

I know there is some fundamental issues that are at play here. Just want to get this straight as it bugs me a bit.

I have since switched from channel 15 and 16 from the Audiofire to ST2 IN.

I still find that the meters peak at +16 when I set the ST2 knob at 0.

 

[SouthSIDE Glen]

Two questions I need to ask you, lomky:

First, do you know what the calibration level is on your A/D//D/A interface? If you take the maximum output specification in dBu, subtract 4, and stick a minus sign on the front of it, that should tell you roughly what 0dBVU on the analog side converts to in dBFS on the digital side. That should help give you a good gain structure guideline bridge from which you can help judge levels in that area of your signal path.

Second, you need to specify when discussing this just which kind of dBs you're referring to at any given point. When you say "0db on the Audiofire matches to about +9 db or so", while that may make sense in your mind, I honestly have no idea what you're actually referring to for each of those 'dB" values. Offhand I can't think of a standard type of dB conversion where an equation of 0 and +9 makes a whole lot of sense, unless I'm thinking of the wrong dB scales. So my second question is if you could clarify the "0db on the Audiofire matches to about +9 db or so" statemet a bit more?

G.

 

[lomky]

Two questions I need to ask you, lomky:

First, do you know what the calibration level is on your A/D//D/A interface? If you take the maximum output specification in dBu, subtract 4, and stick a minus sign on the front of it, that should tell you roughly what 0dBVU on the analog side converts to in dBFS on the digital side. That should help give you a good gain structure guideline bridge from which you can help judge levels in that area of your signal path.

Second, you need to specify when discussing this just which kind of dBs you're referring to at any given point. When you say "0db on the Audiofire matches to about +9 db or so", while that may make sense in your mind, I honestly have no idea what you're actually referring to for each of those 'dB" values. Offhand I can't think of a standard type of dB conversion where an equation of 0 and +9 makes a whole lot of sense, unless I'm thinking of the wrong dB scales. So my second question is if you could clarify the "0db on the Audiofire matches to about +9 db or so" statemet a bit more?

G.

Hi Glen, thanks for chiming in on this! It was you that I had in mind when I was trying to think through this. I'm at my day job, so let me try an get some answers without having the gear in front of me:

#1. The interface is set on +4 in the control panel, this doesn't really answer your question, I know. So how do I determine the "maximum output specification"? The manufacturer specs state: "Maximum Output Level: +17.5dBu" is this what you are refering to?

#2. I mis-spoke my numbers here, I'm refering to your "Metering and Gain Structure" window as I type this, I hope I understand it correctly. If my audiofire is peaking at 0 dBFS(when listening to highly limited music) then the meters on my Mixwizard (which go up to +16 dBu PPM, at least I think the meters on the Mixwizard are PPM) peak between +9 and +16.

You have made me look closer at the Audiofire, and the meters on the front panel go up to -6 dB (I'm assuming this would be -6 dBFS) so this would equal +16 dBu.

I'm wondering if my question is more of a proper "Hook up" thing rather than a "Gain Staging" thing.

 

[SouthSIDE Glen]

Hi Glen, thanks for chiming in on this! It was you that I had in mind when I was trying to think through this.

Oh, boy, no pressure on me then! (j/k )

#1. The interface is set on +4 in the control panel, this doesn't really answer your question, I know. So how do I determine the "maximum output specification"? The manufacturer specs state: "Maximum Output Level: +17.5dBu" is this what you are refering to?

Those are two good pieces of info. The fist tells us that your interface is set to a "pro" line level of +4dBu. All this really means (in a kind of oversimplified kind of way) is that your interface does indeed consider +4dBu as being calibrated to 0dBVU. This means that the second piece of info can indeed be applied to the formula I gave in the first post.

The MOL of +17.5dBu is on the low end of the spectrum but it's certainly workable. This means that because your interface considers +4dBu to be 0dBVU analog, that there is about 13.5dB of "headroom" between 0VU analog and the maximum output level of your interface. Since that MOL is going to also mean maximum digital level, which is 0dBFS on the digital side, that means that there is about 13.5dB between 0VU analog and 0dBFS digital. or to put it another way, 0VU analog converts to -13.5dBFS digital.*

This is all the A/D on the way in, but the D/A on the way out (as in your monitor path) should work the same, except in the reverse direction.

I've not used the Echo Audiofires, so I can only go by what I see on Echo's product page for the Audiofire 12. I see what you mea by the meters. It's funny in a strange way that they are labeled "Analog In" and "Analog Out", but seem to be reading a digital scale, and further that scale only goes to -6, with no 0 or clip indicators. However, I do note that page says that it comes with software that lets you control calibration and metering in some way. Maybe there's something in there or in it's online help that may help make sense of what those meters are actually showing. When you get home you might want to check that out mad make sure there's nothing unusual set in there.

#2. I mis-spoke my numbers here, I'm refering to your "Metering and Gain Structure" window as I type this, I hope I understand it correctly. If my audiofire is peaking at 0 dBFS(when listening to highly limited music) then the meters on my Mixwizard (which go up to +16 dBu PPM, at least I think the meters on the Mixwizard are PPM) peak between +9 and +16.

Well, based upon what we calculated earlier, that sounds just about right - maybe. If 0 dBFS does indeed equate to 17.5dBu then you should be showing peaks at +16 or greater. My quiestion though is whether that is showing dBu or dBVU. If it's showing dBVU, then +13.5 would be right in between 9 and 16 and would be equal to 0dBFS, and all would make sense. But check that, and make sure just which dB type that's actually displaying (I'll bet it's peak VU.)

Also, all this applies as long as the output controls on your computer software are set for unity gain. Bump those in either direction and all bets are off.

HTH, at least until you get home .

G.

 

[lomky]

Excellent info! So I'm home now and able to check things out in earnest. Yes there is a "Console" for the Audiofire. It basically shows output levels for each channel, and main Analog Outputs. Levels on the main outputs go from -60 dBFS (at least I think it is dBFS, still wrapping my head around this) to 0.0 dBFS. Levels on the main output are set at 0.0 dBFS. The levels on the external unit basically stay slammed on -6 dBVU (I suspect) When the Console floats between -3 to 0.0....

As I type this I am getting a bit confused. Why would the external meters be showing dBVU? Shouldn't they be dBFS? So if the meters on the External box are peaking at 0 dBFS the meters on my board are peaking at +16 dBu PPM which they are when everything is set at unity.

However, if I am hitting the Stereo input of my board at +16 dBu am I not overloading my board? Should I simply turn the ST2 gain down?

In fact, if I turn the output on the Audiofire Console down to -16 dBFS the meters on my Mixwizard board now show peaking at 0 dBu PPM. This makes sense to me, I think