4.6 Glossary
•
Compression ratio: Compressors limit the audio signal amplitude whenever it surpasses a
certain preset threshold level. The characteristic curve’s slope equals 1 below this threshold,
meaning that the output amplitude is the same as the input amplitude. At the threshold point the
curve has a knee, beyond which the slope decreases with increasing compression ratio. Here,
the output grows slower than the input (see fig. 15). The compression ratio is the slope of the
curve above the compressor’s threshold. Stand alone compressor units feature an adjustable
compresion ratio, as opposed to a limiter, which usually shows a fixed compression ratio of 1:
∞
(i.e. a slope of 0: the output amplitude is constant over varying input levels).
Fig. 15 Illustration to help explain the term compression ratio.
•
Decibel (dB): In technical acoustics all kinds of signal amplitudes, power levels and sound
pressure levels are usually expressed in decibels. Since a decibel is a logarithmic measure, it
can only express the ratio of two signals, and no absolute values.
By definition, the ratio of two power levels is:
L
dB
P
P
p
=
⋅
10
0
log(
)
With the help of a known impedance, this can be expressed in voltages as follows:
L
dB
U
U
u
=
⋅
20
0
log(
)
The reference voltage U
0
in audio electronics is by convention 0.775V.
The reference power level P
0
is usually assumed to be 1 mW, which equals 0.775V into 600
Ω
.
•
Phase shift: The superposition of two identical waveforms of the same frequency and
amplitude that cross zero at the same time (i.e. they are in phase) results in the same wave with
its amplitude doubled (see fig. 16a). Likewise, if they are 180° out of phase, the wav e is
extinguished (see fig. 16b). The time delay between the two waves is called their phase shift.
The effect of the amplitude amplification/attenuation depends strongly on the amount of phase
shift. It is not necessary for the two signals to have the same amplitude, it is just that then the
effect is at its extreme. But even lesser effects due to different amplitudes (at the same
frequency) can be very noticable in audio reproduction.
