528E
12
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 defi ned threshold.
The VCA - Voltage Controlled Amplifi er
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 amplifi er whose gain is
controlled by a DC voltage. That part of the circuit is called a voltage controlled amplifi er, 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 dif-
ferent 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 ad just -
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. How-
ever, 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 ac tiv i-
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 defi ned as a downward ex-
pander from this point forward in this manual.
How Expanders Increase Usable Dynamic Range
The lower limit restriction of a system is the noise fl oor, 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
fl oor 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 a 10
dB drop in the signal with its 1:2 ratio.
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