dbx 128 - COMPRESSOR;LIMITER AND EXPANDER, A SUMMARY; Compressor and Peak Limiter Functions; Expander and Peak Unlimiter Functions; Internal Circuit Operation

COMPRESSOR/LIMITER AND EXPANDER,,
A SUMMARY
Compressor
A variable gain amplifier whose gain decreases as its input
level increases. A compressor uses low compression ratios
such as 2.0 to 4.0 (selected by the EXPANSION/COMPRES-
SION control). In the LINEAR mode, the 128 decreases gain
above the threshold point (the threshold point is set by the
LEVEL contro!) and increases gain below the threshold point
(see figure 19C). In the ABOVE THRESHOLD mode, the
128 decreases gain above the threshold point, and does not
act on levels below the threshold point (see figure 19D).
Peak Limiter
A compressor with a high compression ratio, such as 10.0
or greater. The high ratio maintains essentially constant out-
put level despite any increase in input level above the thres-
hold. While the 128 can be used in the LINEAR mode with
high compression ratios, it would normally be used in the
ABOVE THRESHOLD mode with high compression ratios,
making it a peak limiter (acting to limit only musical peaks).
Expander
A variable gain amplifier whose gain increases as its input
level increases. An expander normally uses low expansion
ratios such as 1.2 to 1.5 (selected by the EXPANSION/COM-
PRESSION control). In the LINEAR mode, the 128 increases
gain above the threshold point (the threshold point is set by
the LEVEL control) and decreases gain below the threshold
point (see Figure 19A). Inthe ABOVE THRESHOLD mode,
the 128 increases gain above the threshold point and does not
act on levels below the threshold point (see Figure 19B).
Peak Unlimiter
A peak unlimiter is an expander with a high expansion
ratio such as 1.4 to 1.6 which is used to counteract the
effects of peak limiting described above. Although the 128 33
works best with moderate expansion ratios while in the
LINEAR mode, HIGH EXPANSION RATIOS WOULD
NORMALLY BE USED TO PROVIDE PEAK UNLIMITING
IN THE ABOVE THRESHOLD MODE.
How the 128’s Compression/Expansion Circuits Work
An expander or compressor is made by combining two
devices. One device changes the level of the audio (by chang-
ing its gain), and the other senses the audio level to control
the gain change. The 128 uses VCA’s (voltage controlled
amplifiers) to change the level. The VCA gain is linearly pro-
portional, in dB, to the controlling voltage. The 128’s level
sensing devices respond to the RMS (root-mean-square) value
of the sum of the stereo input signals, converting that RMS
value to a D.C. (direct current) control voltage, which changes
in proportion to the dB change of input signal. Depending on
the setting of the COMPRESSION/EXPANSION control
(i.e. compress or expand) the polarity of the control voltage
is reversed (to plus or minus), and the fraction of a volt per
dB of level change is varied. The control voltage is fed to the
VCA where it modifies the audio level appearing at the 128’s
outputs. ,
RMS level sensing is used, rather than peak or simple
average sensing, for several reasons. First, the human ear and
brain perceive loudness in a way that more closely follows
the RMS value than any other value. Thus, when the 128
expands or compresses, it does so on the same basis the
human ear uses to determine loudness. In contrast, a peak
sensing system would respond to short-term transients that
do not contribute to loudness and can lead to audible dis-
tortion. RMS level sensing results in very natural changes in
level.
A particularly interesting feature of linear decibel com-
pression and expansion is that it works evenly over the entire