Symetrix 258E, User Manual, Page: 14 / 46

528E
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allows only a 1 dB increase in the output level for every 10 dB increase in the input level. Limiters
can be thought of as high ratio, high threshold compressors. They are intended to “stay out of the
way” until the level goes above threshold. However, above threshold their action is very definite.
The Threshold Concept
The threshold is the level at which a dynamic range processor’s activity begins. In operation, the
dynamic range processor’s sensing circuitry constantly “looks at” the incoming signal and com-
pares it to a reference level, which is called the threshold level. In practice that reference level is
set by the operator via the threshold control. Remember, compressors and limiters respond when
signals at the input are above threshold, while downward expanders and gates respond only when
signals at the input are lower than the defined threshold.
The VCA - Voltage Controlled Amplifier
The action of any dynamic range processor depends on some method of changing the gain based
on some external signal. Typically this takes the form of a special sort of amplifier whose gain is
controlled by a DC voltage. That part of the circuit is called a voltage controlled amplifier, or
VCA. Inside the 528E a separate buffered audio signal is sent to a group of circuits that comprise
the detector (envelope follower to you synthesists). The detector circuits turn the AC audio signal
into a DC control voltage, which is sent to the VCA under the direction of the front panel controls.
Linear vs. Downward Expanders
Expander operation is easily misunderstood unless it’s remembered that what’s being expanded is
the dynamics, or changes, of signals passing through the circuit. Expanders come in two very
different types: linear, and downward.
Linear expanders increase the dynamic range of all signals, no matter what their actual level. The
linear expander simply makes all changes greater by some ratio, which is sometimes user adjust-
able. In the real world, linear expanders aren’t too practical because clipping occurs when signals
just below maximum output level are expanded.
For instance, an unprocessed signal 3 dB below clipping that increases 2 dB won’t distort, because
it’s still 1 dB below maximum. But if that same signal is passed through an expander operating at a
1:2 ratio, the same 2 dB change at the expander’s input becomes a 4 dB change at its output.
However, that signal would be 1 dB over maximum, causing distortion. Linear expanders must be
used with care, because very few systems have enough headroom to handle the upward dynamic
range increase they produce.
The kind of processor most commonly called an expander is really a downward expander, because
it only affects signals below threshold. This gives the operator control over the expander’s activi-
ties, allowing it to be used to expand the usable dynamic range of the system without running out
of headroom.
Note: in the interests of clarity and brevity, the term expander will be defined as a downward
expander from this point forward in this manual.
How Expanders Increase Usable Dynamic Range
The lower limit restriction of a system is the noise floor, which is usually well below the 528E’s
lowest expander threshold (-50 dBu). It’s important to keep in mind that while the signal levels
may change greatly, the noise usually doesn’t change very much. The action of the expander
increases the dynamic range of all signals below threshold. This action increases the apparent
loudness of signals, while decreasing the apparent loudness of the noise.
For example, an expander operating at a ratio of 1:2 will cause an input signal that falls 10 dB
below threshold to fall 20 dB at its output. The downward action of the expander reduces the noise
floor by the same ratio applied to the signal. Since the relationship between the signal and the
noise stays the same, the noise is reduced 20 dB by the action of expander, which is responding to