.METERING


Truth in Metering

1999 marks the 60th anniversary of the VU Meter standard, yet many people still don't know how to read a VU! Despite all its defects, the VU meter has survived because it works. The VU meter, with its 300 millisecond averaging time constant, is closer to the loudness-sensing of the human ear, while sample accurate peak-reading meters tell you nothing except whether the capabilities of the digital medium are being exceeded. Two different programs, both reaching 0 dBFS on the peak meter, can sound 10 dB (or more) apart in loudness! This makes an averaging meter an essential supplement to the mastering engineer's ears. Some meters have dual scales, displaying both average and peak. While mixing or mastering, use the average meter and glance at the peak meter. For popular music mastering, here's a conservative calibration setting that will help to produce masters in a similar ballpark to the best-sounding CDs ever made: With sine averaging meter to read ZERO. If the averaging meter reaches 0 on typical musical peaks, and occasionally +3 or +4 on extreme sustained peaks, you're probably right in the ballpark. Every decibel of increased average level means that considerably more than 1 dB additional compression has been applied; which might or might not be the perfect thing for your kind of music. Listen and decide.


The Ear Is The Final Judge

Wide dynamic range material, such as classical music, folk music, some jazz and other styles are often mastered without any dynamics processing at all. In such cases, you may find the averaging meter reading well below 0. This is probably not a problem as long as the music sounds proper to the ears. Some mastering engineers working with wide-range music recalibrate their averaging meters to -20 dBFS = 0 VU, or else recognize that the averaging meter may read well below 0 VU with such music. Also realize that meters are generally not frequency-sensitive, but the human ear judges loudness by frequency distribution as well as level. Thus, two different programs reaching 0 VU (average) may have different loudness.


Quasi-Peak Meters and Judgment of Quality

The ear is the final arbiter of quality, but meters can help. The VU helps demonstrate if average levels are too hot, but as I've described, it requires interpretation. An objective measure of quality is to measure transient loss - to see if audible peaks are reduced. The ear has a certain "rise time"; we probably can't hear the difference between a 10 millisecond transient and a 10 micro second transient.

The digital Peak Program Meter is too fast; it measures inaudible (short duration) peaks as well as audible ones. A popular meter for detecting audible peaks is a quasi-peak meter, or analog PPM, defined by an EBU standard. It's usually made with analog circuitry, but can also be constructed with digital circuits. This meter's 10 millisecond integration time is much slower than the 22 microseconds of the sample-accurate digital PPM. Short overloads, or short bursts of limiting can be inaudible, as long as the level on the quasi-peak meter does not drop.
Peaks shorter than about 10 ms can usually be limited without audible penalty. Wide range program material with a true peak to average ratio of 18 to 20 dB can be transparently reduced to about 14 dB. That's one of the reasons 30 IPS analog tape is desirable, as it performs this job very well. The Finalizer can also do this job, with the aid of a quasi-peak meter to verify the audible peak level is not coming down, and/or the VU meter to see if a 14 dB peak/average level is obtained. A rule of thumb is that short duration transients of unprocessed digital sources can be transparently reduced by 4 to 6 dB;
however, this cannot be done with analog tape sources, which already have removed the short duration transients. Any further transient reduction (e.g., compression/limiting) will not be transparent but may still be esthetically acceptable or even desirable.


Over Counters and Increased Level

0 dBFS (FS=full scale) is the highest level that can be encoded. Most mastering engineers have discovered that you can often hit 0 dBFS on a digital PPM without hearing any distortion. In fact, a single peak to 0 dBFS is not defined as an over level. Over levels are measured with over counters. Conventional wisdom says that if three samples in a row reach 0 dBFS, then an overload must have occurred somewhere between the first and third sample. In an A/D converter, even if the source analog voltage exceeds 0 dBFS, the end result is a straight line at 0 dBFS. However, the ear forgives certain overloads. Note that a 3 to 6 sample over will often be inaudible with drums or percussion, but the ears may hear distortion with only a 1-sample over with piano material. The original Sony digital meter established the standard of 3 contiguous samples equals an over, but has a dip switch to indicate 1-sample overs. Some engineers conservatively use the 1-sample standard, but I've had no problems with a set of good ears and a 3-sample over counter. You can often raise gain by 2 or more dB without having to limit or compress, when you trust the over counter and your ears, instead of a digital PPM.


Quelle: Bob Katz