Decibels, VU and Peak Metering, Signal to Noise Ratio, Clipping
VU Meters
Sound levels need to be monitored at all times. We get this feedback through two types
of meters
VU Meters (either analog or digital LED) measure the gradual and rather slow rise and
fall of signal levels. VU meters at the MC board are analog. VU meters in the PS are
LED. Both are VU.
Peak Meters measure sudden peaks of levels (through blinking lights of different
colors). Usually, yellow, green, red, and just one light. If you are in the red, you are
clipping or distorting sound (in both analog and digital applications). Usually found in
portable recording devices.
VU meters use decibels (DB) to measure the volume of sound (either acoustically or
electronically) by comparing one sound to another. The scale is logarithmic. In other
words, a rise or fall of 1DB is minor, but a rise or fall of 6DB is major – much, much
more than 6x a 1DB change).
The board in MC uses an arbitrary scale (0-100) as a guideline, but if you have paid
attention, below that scale are the actual DB levels associated with the arbitrary scale.
Notice the logarithmic nature of the DB scale.
The Principle of Signal to Noise: When doing either "live" on-the-air broadcasting or
recording (either analog or digital) you want to fill the tape or air with sound not noise.
The only way to do this is have your sound source (your voice, your cd, your
interviewee) fill the air or tape and thus reduce noise (getting close to the microphone is
one big solution). In other words, if your sound source is too low, the rest of what goes
on the tape or out over-the-air will be noise that gets picked up along the way. We all
know this when we hear it - fuzziness, dullness, echo, crackling, hum, etc. So, keep you
meters at the optimum levels. On the other hand, when you put too much signal over-theair or onto tape it is called…
Clipping: This means putting too much audio signal into the sound board (or a recording
device). Clipping will distort the signal, making it almost impossible to listen to. Keep
your eye on the VU meters above the board and keep them within range to avoid
clipping. Also, keep an ear on the monitor speakers and headphones - your ears are the
best guide if you are over modulating (distorting) your signal and clipping.
Level Matching: Nothing is more distracting to the listener than a radio show which
changes "volume" levels during the program. Community radio stations are notorious for
this. So, watch your meters, but most of all listen to the air, and make sure the transitions
between "live" microphones, tapes, cds, computer, promos, are all even and not jolting.
This can sometimes be difficult to achieve, but "cueing" up all sources and pre-setting
levels thoroughly before broadcast is one way to prevent problems.
We are very fortunate at KPFZ to have an Optimod (cost - $14,000). It does many
wonderful things (audio processor, our own “KPFZ sound” – unique to just us, limiting,
compression, expanding, and it is an automated board op – that raises and lowers the
levels). Because of the Optimod, you do not have to, in fact, shouldn’t “ride the board”
– adjusting levels constantly. Set a mid-range level and let the Optimod do the work. The
Optimod is extremely helpful at an all-volunteer station where programmers only come
in once-a-week and are not audio engineers. Without the Optimod, we would sound
very amateurish.
However, having an Optimod does not mean you should ignore clipping, signal-to-noise,
or level-matching altogether: extremes in those three areas still can sound bad. And once
a signal clips, not even the Optimod can do anything about it.