A few years ago, the common wisdom was to "record as hot as you can." By HOT, I don't mean temperature wise. The wisdom at that time was to record your signal (from your guitar, keyboard, mic, etc.) so that the on-screen digital VU meters on your ProTools, Logic, DP, or Sonar is just hitting 0 dB, but not going over. Everybody knows that as soon as the red light turns on, you'll get clipping and digital clipping sounds nasty... like a fingernail scratching on a blackboard. ... <nasty sound>
The above argument sounds LOGICAL. There was even talk "you must use all of the bits as much as possible, so 0dB must be 1111111111111111 and that's good since we used up all the bits, nothing got wasted."
But something got forgotten along the way by the people who advised you to record close to 0dB in your DAW (Digital Audio Workstation... just a fancy name for a fast computer optimized for audio recording)
And that "something" is before your signal gets digitized as a WAV or AIFF waveform existing as a DIGITAL file inside your computer, it has to come from the ANALOG source/equipment. That ANALOG equipment is your mic preamp, or analog mixing board. Basically, every audio you'll record into your computer (with the exception of those generated by software synthesizers running as software plugins) will have to pass through either a microphone preamp, or line input preamp.
And here comes the $64K question.... what or where should I set the volume/gain/trim knob on my preamp/mixer when I'm recording into my DAW? Should I adjust the gain/volume knob on my preamp so that the VU meter lights on my DAW software just hover before 0dB but not CLIP? Before we answer that question, let's take a short side trip.
Now, you'd probably notice that your mic preamp or Mackie/Behringer mixer has it's own VU meter. It's in the form of LED lights, and you have "green lights" for okay, and "yellow lights" for getting close and "red lights" for overload. If you're some guy with nice gear, you'd have an analog VU meter with a needle that sways and dances to the beat.
And this is where a lot of people go wrong. It's probably misunderstood, or they just don't know it. Here it is....
The VU meter on your analog piece of gear (preamp/mixer/etc) is NOT the same as the VU meter on your software DAW. Oh yeah, they may look the same, both have dB markings, but they're not the same.
ANALOG VU METER is not equal to DIGITAL VU METER.
Let me tell you the reason why.... in analog gear, we have what you call "headroom." Headroom is that cushion or buffer or comfort zone you have between analog 0dB and the point where your audio waveform starts clipping (in your analog gear). The point where analog signal gets clipped in your analog gear is in the (+) dB range... expensive, professional gear can have +30dB headroom, while others are typically +24dB or +22dB. With the advent of cheap mixers for the home recording crowd, it's more in the +22dB or lower range.
In digital, your available "headroom" is everything before the 0dB. If you exceed the headroom (go over 0dB), then you get digital clipping (think of it as your waveform hitting a ceiling and no matter how loud you turn it up, it won't get up and beyond the ceiling.)
And of course, we do not want digital clipping because we don't want distortion and we don't want to lose anything in our recorded signal.
So how do we reconcile this two facts?
Answer: You'd want to calibrate your analog gear so that by the time it's hitting it's maximum headroom, your digital gear is just hitting the 0dB digital mark.
Here are my steps:
1. You'll need a signal generator or a test tone oscillator and an AC voltmeter. Our test signal will be a 1Khz simple sine wave. Some big board mixers will actually have a built-in test tone oscillator (1Khz, 5Khz). The cheaper mixers or the smaller mixers would not have such a feature. If yours doesn't have a built-in oscillator, purchase one at an electronics supply store. Any inexpensive ones will do. Just make sure it outputs sine wave 1Khz (and not square waves or triangular waves).
I used an AC RMS voltmeter, and adjusted the output of my signal generator so it measures 0.774 volts. Why 0.774 volts? Because 0dBu Analog corresponds to 0.774596669 Vrms. And since we want to read 0dB on our analog mixer, we need to feed it a voltage of 0.774 volts rms
. <---- This is the key to calibrating your input levels, and gain settings to read 0dB.
2. Connect the output of your signal generator or oscillator to the input of your mixer or mic preamp. Adjust the trim knob or gain knob of your preamp/mixer so it reads 0dB on your VU meter lights. Keep your faders at Unity level (marked as "U"). See photo below.
Notice the VU meter (yellow LED lights) lighting up to the 0dB mark? Your faders should be set to Unity (marked as "U") on your mixer.
And that's it. We're done. Our preamp/mixer is now calibrated for use.
And some of you may be scratching their heads and saying "wait a minute? that's it? I don't believe you... it must be more complicated than that! "
Okay, here's what I did next.... I unplugged the signal generator from my mixer and connected it to the inputs of my MOTU 828mkii. Then I started my DAW software (in this case MOTU Digital Performer). And I viewed the input signal level.
At this point, some of you may be wondering "How come it's not showing 0dB on your DAW software?"
Don't forget what we said earlier... DIGITAL VU meter is not the same to ANALOG VU meter (and vice versa).
So how much dB level do we have? Let's find out! I used the TRIM plugin of Digital Performer. You can see the screen shot below.
Did you see that? 0dB ANALOG corresponds to -24dB DIGITAL.
What does this mean? This means, I have +24dB of analog headroom before I reach digital clipping. Okay, what does that REALLY mean? It means, I can really crank up my guitar, keyboard, mic, and without adjusting the gain knob on the mixer (just the fader), I can be confident that I won't get analog and/or digital clipping when I start recording.
And just for kicks... I pulled up the technical specification sheet of the Mackie 1402VLZ mixer. Guess what I've found? It's maximum level is +22dBu. The way I've currently calibrated my system, my Mackie mixer will be hitting it's headroom of +22dBu before I get digital clipping. In fact, I even have an extra +2dBu cushion before I hit digital clipping.
And that's how you calibrate your gear to get the the correct recording level.... Have fun!
I made a quick diagram so you can see the relationship between 0dB Analog and 0dB Digital and how the Analog headroom corresponds to the Digital headroom.