38
sive signal processing is required. However, in some situations such as live performances, the principal goal
is simply to protect the system from brief overloads. In those cases a short release time is more appropriate
to ensure that the limiter only intervenes when it is needed and the level returns to normal as soon as
possible.Long release times are better suited when the limiter should remain "inaudible", for instance in
broadcasting, club applications, or when a signal is transferred to (analog) tape.
4.1.4 Expanders/noise gates
The dynamic range of signals will often be restricted by noise. Outdoor recordings generally include a high
level of background noise such as traffic, wind, etc. Synthesizers, effects devices, guitar pickups, amplifiers
etc. usually produce a high level of noise, hum or other ambient background hiss, which can restrict the
dynamic range of the program material. Normally these noises are inaudible if the level of the desired signal
lies significantly above the level of the noise. This perception by the ear is based on the "masking" effect:
noise will be masked and thus becomes inaudible as soon as considerably louder sound signals in the same
frequency band are added. Nevertheless, the further the level that the desired signal decreases, the more the
noise floor becomes a disturbing factor.
An expander can be used to enhance the dynamic range of signals, thus providing the inverse function of a
compressor. These devices attenuate signals when their amplitudes drop, thereby fading out the background
noise. Gain controlling amplifiers such as expanders can thus effectively extend the dynamic range of a
signal.
The most common audio application for this system is in complementary noise reduction systems (coding
decoding).
The advantages of expanders for the removal or background noise and crosstalk between individual tracks in
multitrack recording was recognized at an early stage.
The noise gate is the simplest form of an expander: The signal passes through the unit unchanged above a
user-definable threshold. If the signal drops below this threshold, it is simply "cut off" completely. Needless to
say, this method is not suitable to most applications, as the transition would be too abrupt and would be
perceived as unnatural by the listener. A device of this type can be improved by the option of user-definable
control times.
4.2 Digital audio processing
In order to convert an analog signal such as music into a series of digital words, a so-called "Analog to Digital
Converter" or ADC is used. These converters measure the signal at constant time intervals and output the
current signal amplitude as a numerical value. This form of measuring the signal regularly over a period of
time is known as sampling, the conversion of the amplitude into a numerical value, quantizing. The two actions
together are referred to as digitizing.
In order to carry out the opposite - the conversion of a digitized signal into its original analog form - a "Digital
to Analog Converter" or DAC is used.
In both cases the frequency at which the device operates is called the sampling rate. The sampling rate
determines the effective audio frequency range. The sampling rate must always be more than twice the value
of the highest frequency to be reproduced. Therefore, the CD sampling rate of 44.1kHz is slightly higher than
twice the highest audible frequency of 20 kHz.
The accuracy at which quantization takes place is primarily dependent on the quality of the ADCs and DACs
being used. The resolution, or size of digital word used (expressed in bits), determines the theoretical Signal/
Noise ratio (S/N ratio) the audio system is capable of providing.
The number of bits may be compared to the number of decimal places used in a calculation - the greater the
number of places, the more accurate the end result. Theoretically, each extra bit of resolution should result in
the S/N ratio increasing by 6 dB. Unfortunately, there are a considerable number of other factors to be taken
into account which hinder the achievement of these theoretical values.
4. TECHNICAL BACKGROUND
