Aphex 323A Compellor- automatic Compressor-Leveler-Limiter

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Aphex 323A Compellor- automatic Compressor-Leveler-Limiter
The Aphex Model 323A represents the most recent iteration of the popular Compellor / Aural Exciter, a single channel dynamics processor and program enhancer. The model 323 A permits absolute control over program dynamics, a benefit made possible by the patented Compellor processor. The Aural Exciter section enhances and restores program material, resulting in increased audio presence, intelligibility, and articulation.

OPERATIONAL FEATURES A significant improvement in this version of the Compellor / Aural Exciter over its predecessor is the addition of a newly developed (patents pending) Frequency Discriminate Leveler (FDL) circuit. Exhaustive research by Aphex engineers led to the discovery that, under conditions of program leveling, the human ear perceives the onset of low frequency (bass and percussion) transients differently than transients at higher frequencies. This perception, as it turns out, is a direct function of the relative attack time of the leveler. Without FDL, there is a significantly greater chance that low frequency transients can create an audible "bass pull back" effect. In addition to potential loss of bass and/or low end "punch", mid and high frequency processing can be negatively impacted. To the listener, the effect can at times be heard as a perceived loss of bass or even a pumping effect at the mid and treble frequency ranges.

FDL eliminates this problem causing low frequency transients to generate a slower attack time on the initial transient. High frequency leveling is still controlled within the attack time determined by the onboard computer. From the listener's standpoint, the benefits are:

No more bass pullback effect Fast leveling can be used in more applications More bass punch for better music mixes Reduced audio distortion in the leveling mode Compellor Section Features Compressor - Smooth and transparent compression action due to intelligent attack and release control, the result of on-board Dynamic Verification Gate™ (DVG) and Dynamic Recovery Computer™ (DRC) analog control computers Leveler - Delivers undetectable Automatic gain Control (AGC) for maximum stability of program levels Peak Limiter - Protects system components from sudden and potentially damaging peaks. Defeatable from a front panel switch "Straight Wire" Sound Quality - Employs the proven Aphex VCA 1001 for absolute audiophile quality sonic performance Logical Metering System - One button allows you to choose between Input, Output, or Gain Reduction levels; bi-color LED's Transformerless Servo-Balanced Input/Output Circuitry - Permits compatible interface with other balanced and/or single-ended components with no comprimise in performance. RF filtered Slow / Fast Leveling Speed - front panel selectable Selectable Nominal Operating Level - your choice of -10, +4, or +8 dbm operating levels selectable via a back panel switch Aural Exciter Section Features Contains the latest refinements in Aural Exciter technology Improved performance and reduced distortion Overall System Features Inputs and outputs trimmed for the highest Common Mode Rejection ratio (CMRR) assuring ultra quiet balanced line performance Premium Quality Power Supply - precision engineered toroidal power transformer, shielded and RF filtered for silent operation Remote controllable hardwire relay bypass Designed and manufactured in the USA BENEFITS Creative Multiprocessor - A single rack space chassis containing an outstanding compressor, leveler, peak limiter and Aural Exciter Stable, reliable, accurate and extremely user friendly

About the Overall System The Aphex Model 323A represents the very latest in Compellor circuit design and technology.Aphex's Compellor is unquestionably the most sophisticated self adjusting dynamic audio processor available. Its "intelligent" on-board control electronics monitor and control long term dynamics, while simultaneously providing excellent leveling, compression and peak limiting action. Set-up and operation are simple and virtually "fool-proof". The patented Compellor circuitry automatically adjusts the processing to conform to program needs, free of the sonic side effects often associated with other gain control devices.

The Aural Exciter in the Model 323A contains all of the latest improvements for the best possible fidelity and performance, including dual mode harmonics level. The Aural Exciter can be used to restore fidelity, clarity and articulation to program material for applications in broadcasting, recording and live sound.

Nominal operating levels of -10, +4, and +8 dbm can be selected via a switch on the rear panel. Input, gain reduction, and output level are displayed on the front panel LED metering system and a single switch allows you to choose the monitoring mode.

About the Compellor Process The Compellor's simple audio path is composed of a servobalanced input stage, the world-renowned Aphex 1001 VCA, and a new electronically servo-balanced output stage which can be used balanced or single-ended. The nominal operating level of the Model 320A Compellor (O VU on the meter) is rear panel selectable between -10dBV, +4dBu and +8dBu to match virtually any system.

There are three main detector circuits for Compression, Leveling, and Peak Limiting:

Compression is accomplished with a variable ratio, attack and release. The ratio changes depending on program content from 1.1:1 to 3:1. The attack and release times are derived from and vary with program material. The "soft knee" threshold helps to prevent the "choked" sound character often associated with deep compression. Additional program dependent characteristics are imparted by other sections of the Compellor's on-board computers, the Dynamic Verification Gate (DVG), and the Dynamic Recovery Computer (DRC).

The DVG monitors short term and long term average levels, compares them and impedes gain changes when program dynamics might be sacrificed for arbitrary gain reduction. The DVG also prevents gain release during short term program pauses which otherwise might cause audible "pumping" or "breathing" effects. Vocal program material is especially benefited by this feature, allowing voices to sound natural, even under heavy compression. The DRC allows very rapid recovery from gain reduction under certain complex program waveforms. Signals that are high in peak amplitude, but low in relative power, can cause an increase in compression release rate. Hence, undesired gain reduction is inhibited, preventing loss of transient waveforms, holes, etc. The sonic benefit of the DRC is substantial, contributing toward the natural, open sound of the Compellor, even when the signal is highly compressed.

Leveling is performed in a manner related to the way the ear perceives loudness over long time intervals. The circuit effectively maintains output level within 1dB for a 20dB input level change. This action is slow enough to have a minimal impact on program transients or short term dynamics. The addition of FDL further improves the ability of the Leveler circuit to operate smoothly and undetected by introducing a measure of control over the different dynamic characteristics of low frequency vs. high frequency program material. By allowing the Leveler to discriminate between high and low frequency dynamics, the attack time applied to low frequency program is proportionately slower than at higher frequencies. The result is the virtual elimination of "bass pull back" or "pumping" at mid and high frequencies, a condition that is sometimes caused when bass or low pitched percussion dynamics impact the attack characteristics of mid and high frequency program.

When leveling and compression are used together, the Leveler maintains the gain platform so that compression is consistent over the varying levels of program material, providing smooth sounding dynamic compression.

The Peak Limiter provides further dynamic control, capable of holding an absolute ceiling of 12 dB above the nominal O VU level. It may be bypassed using a switch located on the front panel. It is recommended that the limiter be bypassed when using the Compellor with a precision multiband limiter such as the Aphex Dominator.

The Silence Gate detects significant gaps in the program material and freezes the processing, thus preventing noise "swell" or "build-up", a condition commonly audible in other automatic gain control devices. The Silence Gate immediately releases when the program resumes.


1. METER SELECT Selects meter display to read program input or output level (in peak and VU) or Gain Reduction

2. DRIVE controls the amount of gain reduction; clockwise = more.

3. PROCESS BALANCE varies the ratio of compression to leveling without changing total gain reduction

4. OUTPUT provides 20 dB of gain control to set the desired output level after gain reduction settings are made.

5. LIMITER ON/OFF switches the peak limiter in or out of the process.

6. SILENCE GATE THRESHOLD sets the level at which the 323's gain reduction "freezes" preventing noise buildup during quiet or silent passages.

7. AURAL EXCITER IN/OUT switches the Aural Exciter in or out of the process. Does not affect gain reduction.

8. TUNE Controls the point at which Aural Excitement enhancement begins from 700 Hz to 7kHz, by tuning a filter in the process loop.

9. PEAKING varies the shape of the highpass filter (similar to bandwidth or "Q") to allow accenting a wide band (flatter sound) or narrow band (peakier sound) of the program material.

10. HARMONIC LEVEL SWITCH selects the amount of harmonics generated by the Aural Exciter; Normal for general program material and High for additional enhancement.

11. MIX controls the amount of enhancement mixed back into the program, from none to maximum

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