Those of us who tour without production endure the interesting daily experience I refer to as "PA dú jour" – basically we get whatever equipment has been provided by the promoter at a given venue.
Sometimes it can be top-of-the-line gear, and sometimes it can be (to put it politely) challenging. Fortunately, audio gear has significantly improved over the years and as digital desks get cheaper they replace their analog counterparts, typically resulting in audible improvements.
Just like engineers, mixing consoles and their meters have personalities. The power supply for the original Mackie 8•Bus didn't have a ton of headroom. If you PFL'd each channel "properly" (i.e., raised the trim control until your meter showed 0 dB as suggested in the manual), no doubt you'd get a clean signal per channel. But when you started mixing, say, 24 or 36 channels of audio into the L/R mix, the desk sorta ran out of poop, even before the meters went red. It wasn't a case where the audio obviously sounded distorted, but the mix sort of collapsed and lost clarity in a subtle way, maybe even starting to sound a bit compressed. To stop that from happening, you need to set the PFLs a bit lower and feed the L/R bus level a bit lower than you might like to see. Similarly, I remember the first time I found the "sweet spot" on a Yamaha PM4000. If I recall correctly, I was doing a sound check and commented to the systems engineer that the system was very loud, even though the VU meters were showing a level well under 0 VU. His reply was something to the effect of "yeah, but don't run your VU meters for the L/R mix at 0 VU. This desk sounds better when you mix with an average around -7 to -5." Lo and behold, when I hit that mix bus the way he suggested, the desk sounded great.
VU, Peak (PPM), Full Scale
On a daily basis, an engineer might encounter any or all of three different audio meters: VU, Peak (or "PPM") and Full Scale. A VU meter employs a mechanical pointer that swings across the face of a scale where 0 is approximately three-quarters of the way to the right. VU meters respond to audio much like your ear, displaying average signal level as opposed to peak or instantaneous signal level. A VU meter is a very slow device requiring almost 1/3-second to respond.
LED ladder meters like the ones we find on a lot of analog desks usually indicate peak or instantaneous. It's important to know that a VU meter may lie to you when it sees a transient: most snare drums or high hats are too fast for the meter's response characteristics, or ballistics. The reason many VU meters also have a ‘peak' LED is that the LED responds faster to transients and shows you things that the VU meter is too slow to catch.
It's also important to know that in the analog world, 0 VU means "here is your average operating level." Your ability to push a mix beyond 0 VU is a function of how well you know the desk. Some consoles will let you push a mix way beyond 0 VU and still sound good – often indicating that the desk was designed with high voltage rails yielding a lot of headroom. Other consoles do not have a ton of headroom, and you may need to mix lower than the 0 VU indication to get optimum sound quality.
As the name suggests, Peak or Peak Program Meters (PPMs) show instantaneous signal level. These are usually multicolored LED ladders and are great for checking input level for sources like tambourine, snare drum and high hat, because they are fast enough to respond to transients. They also provide accurate indication of the signal level leaving your mixing console and hitting the next piece of gear. If your desk uses PPMs for the main L/R bus, you have to take them with a grain of salt. Don't be alarmed if you see occasional red on a PPM, because this is an indication of very brief signal peaks – and most audio gear (power amps included) are capable of easily handling brief peaks. If, however, your PPM is consistently showing red, you may have a problem.
PPM and VU meters are a stark contrast to the digital world. When you see a meter on a digital desk, "0" probably means 0 dBFS, which in English means "zero decibels below Full Scale." When you hit Full Scale (red), the bucket is typically full, and adding more audio causes distortion – though this varies to slightly from manufacturer to manufacturer. Some will show 0 in red on a dBFS meter when one audio sample has clipped, while others show 0 dBFS when three samples have clipped. Still others may allow this to go up to nine consecutive clipped samples. On the Mackie TT24, the first red LED on the L/R bus meter illuminates when your signal level reaches -2 dBFS, warning you that the bucket is about to spill.
A Mixing "Target"
The net result is that the meters on a digital desk provide a mixing ‘target' different from that of an analog desk. It's always smart to leave room as you build your mix. For example, if you tend to start your mixes with the drum microphone faders and bring the kick into the L/R bus at 0 VU, you're screwed (and you've just started). Sure, 0 VU is a great level for a kick drum, but what happens when you add the rest of the drums, bass, instruments and vocals into that mix bus? You'll be at plus-twentysomethin'. When I was a young audio pup, someone (probably Tony Ungaro, one of the best engineers I've ever worked with) showed me that – if you start a ‘pop' mix (i.e. anything other than jazz or classical) with the kick drum somewhere between -10 and -7 VU, by the time you add in the rest of the band and vocals, the level of the L/R bus will be "right" (somewhere between -3 and +3). This was invaluable information. On a PPM, you can probably start with a kick at around -3 dB, depending upon the peak output level of the console.
There is no such thing as +3 dBFS, because once your mix bus has reached Full (Scale), your mix is distorted and level doesn't matter anymore. At this point, there are no more bits available in the digital system to describe the volume levels you attempting to achieve, so it ain't gettin' any louder.
Complicating matters further is the fact there is no clearly defined standard between analog and digital signal levels. If you are mixing on a digital desk with analog outboard (or vice versa), you need to be careful. Some manufacturers define -14 dBFS = 0 VU, while others use -18 dBFS = 0 VU, or even -20 dBFS = 0 VU. This reference number gives you a clue as to how much headroom is available; when 0 VU is defined as -20 dBFS, there is more headroom available than there is when -14 dBFS = 0 VU. You need to know that information so you can anticipate how the digital desk's output (once converted to analog) will hit the analog outboard, or – in the case of an analog desk with a digital processor – how the analog desk's output will hit the input of digital outboard.
Of course we'll probably need to wait another 15 years before the audio organizations have a worldwide summit and declare a true standard dBFS level for 0 VU. In the meantime, read the manuals and pay attention.
