Gain Staging in Pro Tools HD

[joswil44]

I have been having some problems with my Radial X-Amp as well as a speaker Cab that I suspect may have bad speakers.

While testing with my setup, I ran across some concerns with Gain Staging.

I am running external Preamps into a Pro Tools HD 192.

As far as Text Book Instructions, the Pro Tools 101 Training Guide says:

You want to adjust your audio source Volume until your Pro Tools Track Meter is reaching the highest peak without clipping.

Lets say your peaking anywhere between -3 and -6 avoiding 0 and above.

Starting with the signal path:

5150 dialed in, Post Gain/Volume set around 3-4 pushing nice air
SM57 mic'd near dust cap on 4x12 cab

Now hears where I get concerned.

Lets say I am using the ADL600 Mic Preamp.

I adjust the Gain on the Preamp and the Meters on the ADL600 tell me I am peaking around 0 to -6. So you would think it says I have a good signal level because the Preamp is not showing any clipping.

If it were my Grace 101 I would simply have a green light and when it gets to hot it flashes red as opposed to telling me what value of DB its at.

So on my Preamp I am just trying to get the best signal without clipping.

Now that signal is feeding into my Pro Tools 192 setup.

The Audio Track Meter in Pro Tools is seeing the guitar signal and Registering it at a peak of about -18 to -20.

Now to do what the Text Book says, you would have to turn the Preamp Gain up to bring the Pro Tools Track Meter to a peak around -3 or so.

So now the ADL600 Preamp is registering +6 and the meter is blazing and the Pro Tools Track Meter is hitting -3 to -6 Peaks.

Comparing the audio signals, the -18 sounds quieter but cleaner like it breathes more, the -3 is louder but sounds a little harsh and not as clear.

I know the 192 Manual says it is calibrated for 18 dB of Headroom.

So would that make sense that if I am putting a signal through a Preamp at 0dB it will register in Pro Tools at -18dB?

This is where I get lost.

Am I supposed to be eating up that headroom on an individual track?

If so, wont it be degrading the signal to push the Preamp Gain to the point of clipping to feed the Pro Tools Level?

I haven't been using Pro Tools very Long and my previous setup didnt seem to have such drastic signal level difference.

But I am also trying to educated myself properly so go easy on me for seeming like such a newb.

 

[bennychico11]

So would that make sense that if I am putting a signal through a Preamp at 0dB it will register in Pro Tools at -18dB?

YES!!!!!!!!

...wont it be degrading the signal to push the Preamp Gain to the point of clipping to feed the Pro Tools Level?

YES!!!!!!!!!!!


You talked yourself into the logic of it all! Congrats! Very few people get that with these kinds of questions.

However, also please note the differences in metering. Your preamp is going to be measuring in dBVU and Pro Tools will be using dBFS. There IS a difference (as you probably know, it's just it can get confusing for us when you're talking about two different units and mixing numbers around without specifying what you're measuring).

When your preamp meter is playing a steady tone (ie. sine wave) at 0dBVU, this should enter Pro Tools at around -18dBFS. When you're measuring peaks this will be different....because of the way each meter reacts to peaks. Your VU meter will respond slower than Pro Tools' meter, so even though your peaks in PT might be hitting around -10/-8dBFS....the level on the preamp may still only be around 0VU or lower.

Ignore the book. If you're running a 24 bit session, it sounds like you have healthy levels as it is already. The important thing is not to distort anywhere in the chain. As long as you have the meter bouncing around 0VU on the preamp, and the meter in PT isn't clipping...you're fine. What I like to do is pretend the PT meter is a VU meter. And what I mean by that is imagining 0 is actually lower than full scale. This prevents me from having the urge to push my levels higher.

You also should have the Signal Tools plugin in Pro Tools. Create a Master Fader and add that to it. You can even set it to VU mode and see how it's lining up with your preamp meter.

 

[joswil44]

Thanks for the answers as always.

I think the fog will start to clear now and my mixes will thank me for it.

It leads to other questions but I will try to search around first and ask if I cant find it.

Thanks again.

 

[altitude909]

This is really the difference between the analog realm and digital realm. Provided that your VU meters are calibrated correctly, the sweet spot will be around 0 dBVU. In the digital realm, the sweet spot is always around minus 18-12 dBFS . The dB scale is logarithmic so you cannot exceed the 0 in a full scale system but in a Vu system (i.e. analog) you can go as far as the tape heads can physically go without trashing the tape, so VU meters were always calibrated well below what was physically possible with the system in order order to get the best possible fidelity within the physical limits of the hardware. With digital systems, you can comfortably go to the -.2 dbFS (i.e. "11") without worrying about what happens to your gain when you work it wide open..

 

[bennychico11]

With digital systems, you can comfortably go to the -.2 dbFS (i.e. "11") without worrying about what happens to your gain when you work it wide open..

well, not TOO comfortably....if my levels were around -.2 all the time I'd be sweatin' bullets that I was about to clip!

correct me if I'm wrong, but I believe the VU meter was originally calibrated so it "looked good" with spoken voice. I believe mainly for telephone and radio purposes.

 

[Farview]

This is really the difference between the analog realm and digital realm. Provided that your VU meters are calibrated correctly, the sweet spot will be around 0 dBVU. In the digital realm, the sweet spot is always around minus 18-12 dBFS . The dB scale is logarithmic so you cannot exceed the 0 in a full scale system but in a Vu system (i.e. analog) you can go as far as the tape heads can physically go without trashing the tape, so VU meters were always calibrated well below what was physically possible with the system in order order to get the best possible fidelity within the physical limits of the hardware. With digital systems, you can comfortably go to the -.2 dbFS (i.e. "11") without worrying about what happens to your gain when you work it wide open..

Um... kind of.

Your equipment is designed to work at line level. The VU meters are calibrated so 0dbVU represents line level, because that is the target level. You can go higher than that because analog equipment has headroom built in to handle transients. You can push the levels, but once you run out of headroom, everything will start to distort. VU meters are slow and measure the average level of the signal, because that is what is important when you are setting the gain of a system.

In a digital system, 0dbFS is the upper limit. That is where you run out of headroom. Since there is an absolute ceiling with digital, it was decided that the metering scale would be a 'countdown' to the ceiling.

The db scale is logarithmic scale, which means it is a representation of a ratio. The fact that it is a logrithmic scale doesn't have anything to do with not being able to exceed "0". (if that were the case, you wouldn't be able to exceed 0dbVU either)


Different converters are calibrated differently. 0dbVU does not always = -18dbfs. My converters are calibrated to -15dbfs = 0dbVU, for example.

You can run digital signals up to almost 0dbFS without any problem occuring in the digital world, but as soon as it gets converted to analog, you run the risk of distorting the playback. (or the record path on the way in)