Symetrix 528 - The VCA - Voltaire Controlled Amplifier; Linear Vs. Downward Expanders; How Expanders Increase Usable Dynamic Ranae; Sidechain Processina

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1.11 The VCA - Voltage Controlled Amplifier
The action of a dynamic range processor is determined by one of the amplifier circuits
inside the unit whose gain is controlled by a DC voltage. That part of the circuit is
called a voltage controlled amplifier, or VCA. Inside the 528 a separate buffered audio
sillQal is sent to a group of circuits that comprise the detector. The detector circuits
tum the AC audio sillQal into a DC control voltage, which is sent to the VCA under the
direction of the front panel controls.
1.12 Linear vs. Downward Expanders
Expander operation is easily misunderstood unless it's remembered that what's being
expanded is the dynamics, or changes, of sillQals 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 alt changes greater by some ratio, which is
sometimes user adjustable . Linear expanders can cause distortion, because clipping
occurs when sillQals just below maximum output level are expanded.
For instance, an unprocessed sillQal 3dB below clipping that goes up 2dB won't distort,
because it's still ldB below maximum. But if that same sillQal is passed through an
expander operating at a 1:2 ratio, the same 2dB change at the expander's input would
become a 4dB change at its output. However, that sillQal would be ldB over maximum,
causi~ 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 sillQals below threshold. This gives the operator control over the
expander's activities, allowing it to be used to expand the usable dynamic range of the
system without running out of headroom.
1.13 How Expanders Increase Usable Dynamic Range
The lower limit restriction of a system is the noise floor, which is usually well below
the 528's lowest expander threshold (-60dBm) . It's important to keep in mind that while
the sillQal levels may change greatly, the noise usually doesn't change very much. The
action of the expander increases the dynamics of all signals below threshold that do
change. This action increases the apparent loudness of those changing signals, while
decreasing the apparent loudness of the noise .
For example, an expander operating at a ratio of 1:2 wilt cause a signal that falls lOdB
at its input to fall 20dB at its output. The downward action of the expander reduces th e
noise floor by the same ratio applied to the signal. Since the relationship between the
sillQal and the noise stays the same, the noise is reduced 20dB by the action of
expander, which is responding to a lOdB drop in the signal with its 1 :2 ratio.
1.14 Sidechain Processing
The sidechain is a patch point in the control circuit of a dynamic range processor, which
provides access to the part of the circuitry that tells the VCA what to do. The 528's
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