Meter Matters

Jan 1, 2003 12:00 PM, By Brian Smithers

I WANT IT ALL!

So VU-style average-level metering is useful for making musical judgments about the loudness of a signal, and peak metering is a necessity for ensuring distortion-free digital recording and processing. At the dawn of the 21st century, do we really need to sacrifice one for the other?

Heck, no! Meters are now increasingly being designed (especially in software) to allow the engineer to choose which kind of metering is appropriate. Often, both types of metering can be displayed simultaneously.

Fig. 3 shows how this is implemented in Cakewalk Sonar 1.3. The drop-down menu allows the user to choose Peak metering, RMS metering, or both. RMS stands for root mean square, a method of averaging values that yields a truer representation of perceived loudness than does the good old VU meter.

In Fig. 3, the solid green bars at the lower end of the scale indicate RMS levels hovering at around -16 dBFS. The short yellow and red bars above indicate the current peak levels of -7 dBFS in the left channel and -5 dBFS in the right channel. Note that the peaks are 10 to 11 dB higher than the average (RMS) levels.

Near the top of the scale are two thin white bars indicating the highest recent peaks. This peak hold function is common in peak meters and usually retains its value for from one to three seconds. Sometimes “infinite” peak hold is available to retain peak levels until they are reset manually. Because the values in Fig. 3 are at -0.5 dBFS and -3 dBFS, we had better hope that represents the loudest point in the recording session!

If you're accustomed to peak meters, you may be surprised to find that the perceived loudness (as represented by the RMS level) is more than 15 dB lower than the peak value at the loudest part of the recording. This particular example is of a collegiate symphonic band playing a particularly dynamic and percussive piece and is representative of the sort of peak-to-average differences that one often finds in large classical ensembles.

Compare that with what would happen if we recorded a pipe organ and set levels to peak near 0 dBFS. Because the peak-to-average differences are virtually nil with an organ, the perceived loudness of the organ recording would be more than 15 dB louder than the symphonic band recording — a result that would be extremely unnatural sounding. Therefore, having both peak and VU- or RMS-style meters and knowing how to use them is clearly essential to making great recordings.

THE CASE FOR K

A great example of peak and average metering living together in harmony can be seen in mastering engineer Bob Katz's proposed K System. The K System, as detailed on Katz's Digital Domain Web site (www.digido.com), integrates metering, monitor calibration, and standardized level practices, but here we'll look at just the meters.

K System meters feature solid bars to indicate RMS average levels, with lines or dots above to indicate instantaneous peak levels (see Fig. 4). Peak-hold indicators can be set to 10-second hold or infinite hold, and an overs counter is recommended.

So far that sounds similar to the Cakewalk Sonar meters discussed earlier, but the K System meters don't count down from 0 dBFS. Instead, the 0 dB point is set 20, 14, or 12 dB below full scale, reflecting the VU meter-style practice of setting levels by perceived loudness instead of by peak. Full scale is designated +20, +14, or +12 dB. These three variations, called K-20, K-14, and K-12, respectively, reflect different amounts of headroom and are intended for different applications.

K-20 metering accommodates the demands of very dynamic music, such as symphony orchestras, audiophile recordings, and film sound. More compressed styles such as pop, rock, and R&B use the K-14 scale, and the K-12 scale is reserved for broadcast production.

Consider the ramifications of the K System for recording the symphonic band and pipe organ mentioned previously. For the band, setting levels to hover around 0 dB on the K-20 meters when the band is playing forte (full, but not maximum, volume) would leave enough headroom for the 15 to 16 dB difference between average and peak levels with a small cushion before clipping. Setting the organ's forte levels to 0 dB would appear at first glance to “waste” some dynamic range, but it would result in a level that compares naturally to the band recording.

I'D LIKE TO METER

Meter design and implementation is remarkably inconsistent. Even though the characteristics of VU and peak meters are well defined, it's hard to know whether your meters conform to those specifications precisely. The only defense is to learn how your meters respond and to compare what you see with what you hear. Find out whether your meters offer more than one type of display and how many overs set off an alarm.

If you have access to peak and average metering, use them both. If you have only one, use your ears and your good sense to extrapolate the information that your meter isn't providing. Armed with the knowledge of what your meters are telling you, you'll be able to manage your levels sensibly.

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Brian Smithers is a musician, engineer, and educator in Orlando, Florida. He teaches Audio Workstations at Full Sail Real World Education.