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Gain Staging
By Jay Walsh on September 5 2008 09:51 PM | Permalink | Author Info
How stuff is supposed to work together part 1

In the days of old, when the earth was still cooling, it was a simpler time.  A time when most audio devices had meters and there were standard operating levels.  Things were less confusing.  Here is how it worked:

1.You start with a mic. 

2. You plug the mic into the preamp. 

3. You set the preamp so the VU meter hovers around 0dbVU. 

4. You plug the preamp into a compressor and set it to do what you wanted, then you set the makeup gain so that the VU meter hovers around 0dbVU. 

5. Then you plugged the compressor into the tape deck which would naturally have the meters hovering around 0dbVU.

6. At mix time, a 0dbVU signal would come out of the tapedeck and into the console.

7. All the tracks would be mixed together and the mix would hover around 0dbVU on the output meters of the console

8. That mix would be recorded to a 2-track deck and the meters would be hovering around 0dbVU


You might have noticed a theme.  You see, every piece of equipment has a sweet spot.  A place where the self noise and distortion are the lowest.  Where the circuitry is the most linear.  Most designers put that point at line level.

>So, the object of the game, once you got the signal up to line level with the mic preamp, was to keep it there no matter how many things you ran the signal through.  All the way up until mix time.

Now, there are two common line levels: +4dbu and -10dbV.  In the old days, the professional stuff was +4dbV and your home stereo stuff was -10dbV.  Now, it is common to have a mix of things in your recording rig.  This can lead to gain staging problems.

Some of the confusion comes in with the VU meter.  0dbVU will be line level on the piece of equipment that it is on.  If the piece has a +4dbu line level, 0dbVU will be +4dbu.  If the piece has a -10dbV line level, 0dbVU will be -10dbV.

The difference between the two line levels is about 11db.  I know it seems like it should be 14db, but we are working with two different db scales (dbV and dbu) that have different voltage references.

It's always best to pick a standard and just use that, but sometimes that's not practical. 

When running a -10 piece of gear into a +4 piece of gear, you have the choice of turning up the output of the -10 piece or turning up the input of the +4 piece.  (or a little of both)  Turn the output of the -10 piece up and you risk clipping the output.  Turn up the input of the +4 piece and you raise the noise floor.  You will need to experiment and listen carefully to determine which to do in your circumstance.

Running a +4 piece into a -10 piece is the same problem.  You can just turn down the output of the +4 piece, but the noise floor will be higher because the signal to noise spec on the +4 piece is referenced to the +4 line level.  Again, this may or may not be a problem.  It really depends on the specific equipment involved.

The real danger with running a +4dbu signal into a -10dbV input is that the +4dbu unit will have a ton more headroom the the -10dbV unit.  Since VU meters only measure the average signal level, your signal could have transient peaks of +15dbu or more.  That is about 26db hotter than the -10dbV units line level!  You can imagine that it won't sound very good.

That is really all there is to it.  No matter what your signal path is, this is how it is designed to work.  Remember, 0dbVU is just a target.  A few db either way doesn't make much of a difference.  Also, there may be artistic reasons for running the levels higher or lower.  The point of this article is to illustrate how things were meant to work and be used.  If your preamp does something special when it's begging for mercy, by all means abuse it until it sounds great.

Next time, we will find out what all this means in a modern world. Have a nice day.











 
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