Meters, Levels and Loudness, Oh My!
Recently, a colleague at a Central American television station wrote asking about levels. Here is his question:
In the on-going process of trying to professionalize the operations here at the station, I am tackling the issue of QC on our audio levels. We frequently transmit distorted, saturated audio, but the reasons for it are harder to nail down than I wish they were.
One issue I’m encountering is the difference in meters between tape machines. My Betacam SP recorder players have the typical LED display where the high end on the read out is at about 5 over zero. I’ve always understood that tone should register at zero, and that the program should read a little below zero with only the highest, occasional peaks registering over zero, or briefly into the red.
But I’m finding that the programs we get on digital tapes digi- beta or dv-cam) read differently. The machines themselves have zero at the very top of the read out, and tone plays at 20 below zero. The program plays with peaks approaching 10 below zero.
Is the standard different now with these digital media?
My Reply
Audio levels and metering have become chaotic since we first started moving to digital production in the 1980s. I don’t think it is much of an exaggeration to say that few people understand it thoroughly. Although I probably understand more than some, I’ve been grappling with these issues for 20+ years and had the (mis)fortune of sending programming to a very picky television network every week during their own conversion to digital. Here is what I’ve learned over the years.
There are few standards, in the old sense of the word, and what few there are, are often ignored, at least in North America. However, in recent years I believe the situation has gotten better as some common practices have gotten more… well, common. Also, a few manufacturers and individuals have tried to impose order where none has existed. More on this later.
Often overlooked is that two issues are inextricably linked: reference levels and meter ballistics. My colleague’s problems appear to stem from both issues. I’m going start with some basics, for the sake of completeness. If you know some of this already, you can skip ahead.
We often think of the old style, mechanical VU meters as providing a standard reference level, which they did, if properly set up. If they adhered to the ASA spec (which not all did) they also had standardized ballistics (with specs for things like overshoot and settling time) aa well as the famous standardized scale (two, really). Later the BBC and DIN, then EBU and Nordic came along with their own standards which, again had their own (differing) standards for reference level, ballistics and scale. We often referred to VU meters as “averaging” and the European meters as “peak.” But, in fact, neither term was entirely accurate. I’ll spare you the reasons why.
Ballistics of the old, standard VU meters were often referred to as “syllabic” because they were developed for speech and their integration time was similar to that of the human ear. In other words, what we saw on the VU meter was a visual representation of how our ears perceived the loudness. This meant that transient sounds did not register accurately on VU meters because the were too fast for the rise time of the meter. As engineers, we learned to print to (analog) tape at different levels than were shown on the VU meters, depending on the type of signal we were recording. You could record a close-miced tambourine so that it peaked at 0 VU, but you only did it once if you didn’t want to have to sell shoes for living.
The European-style “peak” meters were designed to help eliminate this problem. Their rise times were much faster, though not instantaneous, since they were still mechanical. They did give you a much better indication of what levels you were actually printing to tape (or sending to air). In order to make the meters readable, the fallback time was slower than VU meters. As a result of these ballistics, they did not do a good job of giving you an accurate estimation of how consistent your levels would sound to the human ear.
Enter digital. Now we really need to know what levels we were printing because attempting to go beyond 0 dBFS is disastrous.
About dB
As you may know a decibel (dB) is just a ratio, or more accurately a logarithm. So, to say a tone is recorded at 0 dB (or -3 dB or +6 dB) is meaningless. A dB figure must be in reference to something. With digital devices, we often refer to decibels re “full scale” or “dBFS.” Full scale is when the digital word describing the level is all ones and no zeros. For practical purposes, you can’t go higher that that. Therefore, a level relative to full scale will always be at or below 0, such as -20 dBFS. In digital systems 0 dBFS is the only level that really has any measurable, distinguishing characteristics, and so it is really the only reference that makes any sense in an absolute sense.
Where Does That Leave Us?
Forget about the meters on your VTRs being any kind of a standard at all. The best you can hope for is they are accurately calibrated to some actual level as recorded on tape in the machine: 0 dBFS for digital machines, or for analog machines, a known reference fluxivity. The only way to know this for sure is to get a standardized test tape for your format. (I know analog ones exist; I don’t know about DigiBeta.) Or you could do like some people (including me, at times, I’m embarrassed to say): don’t think about it and hope for the best. This is not really the best method.
When playing back your mix (or doing it live to tape) set your zero reference level from your recorded mix or your console to your chosen house standard, as shown on the record machine already aligned electronics and meters. Then don’t look at those meters again. You’ll only confuse yourself.
So, what should your house standard be? In the case of digital machines, -20 dBFS is a widely accepted “standard” nowadays. Another accepted “standard” is that we limit our peaks to about -8 dBFS or -6 dBFS, giving us +12 to +14 dB of headroom for our peaks, above our -20 dBFS reference. Doesn’t this “waste” some digital headroom? The answer is, it depends on what you mean by “waste.” In the real world, we cannot assume that every machine that plays back your tape will do it accurately. If you have 0 dBFS peaks, some machines may actually distort due to miscalibrated converters. Even more important, you do not know how much headroom a downstream piece of equipment will have at playback, but in pro gear +12 dB is pretty easily achieved.
This accepted practice — reference at -20 dBFS and maximum peaks at -8 dBFS — is what major networks seem to want nowadays and so has become pretty much a de facto standard. This is also how I deliver material I regularly mix for radio broadcast. (Audio CDs are different.)
The real magic comes in how you meter levels in mixing and how what you see on the meters affects what you do in the mix. Having at least one set of reference quality meters is a requirement for professional work. It is not optional. Sometimes you get lucky and such meters will come with your console, but not always, especially nowadays. The thing is, with digital you need to meter the peaks for the benefit of the recording medium. But you also need a syllabic metering system for the sake of the listener, including the guy that mixes. So it’s best to have a metering system that shows both peak and average simultaneously. If you only have one or the other, you can get levels that are too low and/or inconsistent or too hot and distorted. How you calibrate these meters is integral to how you set up the level structure of your audio system. Good, professional meters will have clear reference position that works (with a sine wave) for both the peak and average metering. They will also tell you how to calibrate the meters relative to your house levels.
Does This Make Your Head Hurt?
OK, here’s what I do. Although this is for radio, I’ve found radio practice to be even more chaotic than TV, so I’ve adopted TV practices. I use two metering systems: a pair of analog Dorrough 40-A and the Inspector XL software plugin from Roger Nichols Digital, which incorporates Bob Katz’ K-system.
When using the Inspector XL plugin, I use the -20 K-scale. This has reference at -20 dBFS marked “0″ with positive numbers above it and negative numbers below it. With this scale I can easily see where my levels, both peak and average, are at any time and what the highest levels were (peak & average) within the last n seconds (n is adjustable).
I also use the pair of Dorrough 40-A by calibrating them so that their reference mark is equivalent to -20 dBFS (as indicated by Inspector XL) coming out of the DAW’s analog hardware. I use both these metering systems because I have them. You could use either one by itself, or any of a few others on the market, with good results.
If everything is calibrated properly and you are using a good bus limiter (lightly!), at around -6 dBFS or -8 dBFS, then the person doing the mixing should be able to focus mainly on the average, with an occasional check of the peaks. That’s easy enough with either system I’ve mentioned. The advantage is that he or she can mix instinctively, by ear, getting consistent levels to the listener while staying out of digital danger.
Oh, and don’t forget to set your monitors to a moderate and consistent volume level.
If you have any question, email me.
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