Rupert Neve 543 - 500 Series Mono Compressor-Limiter - Manual (Page 4)

4
THE NEED FOR DYNAMIC CONTROL OF SOUND LEVELS
The dynamic range of sounds we hear around us in normal life greatly exceeds the capability of our
best recording and processing equipment - but even if this were not so, the scale of dynamic range
must be accommodated to the venue in which it is to be reproduced. For example, actual volume
levels of the dance hall would be deafening in a students bedroom. In the same way, late night
listening in a quiet living room demands careful adjustment of dynamic range. In the constantly
changing background noise of a car, drama dialog does not work without constant attention to the
level control. In the field of communications, it is often necessary to ensure that the best possible
signal-to-noise ratio is obtained, in the interest of intelligibility, within the limited performance of,
say, a reporter’s recording device.
Digital recorders are unforgiving when overloaded. Overload can be avoided with careful use of
high ratio compression - on the verge of limiting - with careful choice of time constants. A
recording that still sounds “loud” can be produced without non-musical harmonic distortion.
A compressor-limiter is one of the most powerful, yet subjective items in the sound engineer’s
armory. Compression should never be obvious to the listener and this needs intuitive and effective
controls on the part of the designer together with considerable skill on the part of the sound
engineer.
A NOTE ON DISTORTION
The human hearing system is a remarkably complex mechanism and we seem to be learning more
details about its workings all the time. For example, Oohashi demonstrated that arbitrarily filtering out
ultrasonic information that is generally considered above our hearing range had a measurable effect on
listener’s electroencephalo-grams. Kunchur describes several demonstrations that have shown that our
hearing is capable of approximately twice the timing resolution than a limit of 20 kHz might imply
(F=1/T or T=1/F). His peer reviewed papers demonstrated that we can hear timing resolution at
approximately with 5 microsecond resolution (20 kHz implies a 9 microsecond temporal resolution,
while a CD at 44.1k sample rate has a best-case temporal resolution of 23 microseconds).
It is also well understood that we can perceive steady tones even when buried under 20 to 30 dB of
noise. And we know that most gain stages exhibit rising distortion at higher frequencies, including
more IM distortion. One common IM test is to mix 19 kHz and 20 kHz sine waves, send them through
a device and then measure how much 1 kHz is generated (20-19=1). All this hints at the importance
of maintaining a sufficient bandwidth with minimal phase shift, while at the same time minimizing
high frequency artifacts and distortions. All of the above and our experience listening and designing
suggest that there are many subtle aspects to hearing that are beyond the realm of simple traditional
measurement characterizations.
The way in which an analog amplifier handles very small signals is as important as the way it behaves
at high levels. For low distortion, an analog amplifier must have a linear transfer characteristic, in other
words, the output signal must be an exact replica of the input signal, differing only in magnitude. The
magnitude can be controlled by a gain control or fader (consisting of a high quality variable resistor
that, by definition, has a linear transfer characteristic.) A dynamics controller - i.e. a compressor,
limiter or expander - is a gain control that can adjust gain of the amplifier very rapidly in response to
the fluctuating audio signal, ideally without introducing significant distortion, i.e. it must have a linear
transfer characteristic. But, by definition, rapidly changing gain means that a signal “starting out” to be
linear and, therefore without distortion, gets changed on the way to produce a different amplitude.