36
whole audio spectrum. If these currents are highly amplified, the result will be perceived as noise. Since all
frequencies are equally affected, we term this white noise.
It is fairly obvious that electronics cannot function without components. A certain level of background noise is
inevitable despite the deployment of special low-noise components.
This effect is similar when replaying a tape. The non-directional magnetic particles passing the replay head
can also cause uncontrolled currents and voltages. The resulting sound of the various frequencies is heard as
noise. Even the best possible tape biasing can "only" provide signal-to-noise ratios of about 70 dB, which is
not acceptable today since the demands of listeners have increased. Due to the laws of physics, improving
the design of the magnetic carrier is impossible using conventional means.
4.1.2 Audio dynamics
A remarkable feature of the human ear is that it can detect the most wide-ranging amplitude changes - from the
slightest whisper to the deafening roar of a jet plane. The attempt to record and reproduce this broad spectrum
of amplitudes with the aid of amplifiers, cassette recorders, LPs or even digital recording media (CD, DAT, etc.)
soon runs into the physical limitations of electronic and acoustic playback options.
The usable dynamic range of electroacoustic equipment is limited as much at the low end as at the high end.
The thermal noise of the electrons in the components results in an audible basic noise floor and thus repre-
sents the bottom limit of the transmission range. The upper limit is determined by the levels of the internal
operating voltages; if they are exceeded, audible signal distortion is the result. Although in theory, the usable
dynamic range sits between these two limits, it is considerably smaller in practice, since a certain reserve
must be maintained to avoid distortion of the audio signal if sudden level peaks occur. Technically speaking,
we refer to this reserve as "headroom" - usually this is about 10 - 20 dB. A reduction of the operating level
would allow for greater headroom, i.e. the risk of signal distortion due to level peaks would be reduced.
However, at the same time, the basic noise floor of the program material would be increased considerably.
Ear
Microphone Amplifier
Power Amplifier
T
ape Recorder
Radio
Cassette
Recorder
P/dB
140
120
100
80
60
40
Fig. 4.1: The dynamic range capabilities of various devices
4. TECHNICAL BACKGROUND
