Aphex Systems 720

Aphex Systems 720

The Aphex Systems Model 720 / 722 Dominator II is a stereo MultiBand peak limiter designed to fit a wide range of applications. Through the use of MultiBand techniques along with new proprietary circuits, the audibility of limiting action has been greatly reduced, especially when compared to conventional limiters. This means that greater limiting depth is possible, resulting in higher loudness with maintained audio quality. At virtually any limiting depth, the Dominator II is free of "hole punching", "dullness", and most other effects normally associated with limiters. As a peak overshoot protection limiter, the Dominator II is undetectable in line while it absolutely prevents peak levels from exceeding a user settable output level. In addition, the desired limiting effects of greater audio density and increased "punch are readily available with the Dominator II.The Model 722, for broadcast use, has the Pre and De-emphasis circuitry, while the Model 720 Does not.

FEATURES

¡         Peak Ceiling Trimmable in 0.2dB Steps Over a 34dB Range 

¡         3 Bands of Limiting with Switchable Crossover Frequencies 

¡         Patented Automatic Limit Threshold (ALT) Circuitry

¡         Freedom from Pumping

¡         Freedom from Spectral Gain Intermodulation

¡         Adjustable Density (Relative Crest Height)

¡         Calibrated Detented Potentiometers

¡         104dB Dynamic Range

¡         LF and HF EQ Provides Shaping Equalization Below Peak Ceiling

¡         Relay Bypass, Remote Controllable

¡         Servo-Balanced Transformerless Inputs and Outputs

APPLICATIONS

¡         Sound Contracting -- protection of amplifiers and speakers from overload; increased loudness; maximized use of available power.

¡         Recording -- preventing sudden peak overload of mixer or recorder; tightening tracks; special effects, etc.

¡         Mixing -- used as a program limiter, the Dominator II will keep a track "rock steady" for "layering" into or on top of a mix.

¡         Digital Sampling -- obtaining good full scale samples free from peak overload, i.e. no more missed samples.

¡         Digital Recording -- insuring clean recording by stopping clipping of peaks and overshoots. Maximizes bit usage for less distortion.

¡         Satellite Uplink -- Modulation control to prevent splattering on high frequency audio, gives reduced distortion, better signal-to- noise.

¡         Broadcasting -- AM and FM modulation control for increased loudness; cleaner sound; use in production for greater consistency of tapes, punchier voice-overs.

¡         Location Film Shoots -- anti-crash for dialog and sound effects recording.

¡         Post Production -- Soundtrack peak control; managing difficult dialog; controlling transient sound effects.

¡         Optical Recording and Transfer -- prevents "valve clash", gives higher average level with low distortion and better signal-to- noise performance.

¡         Analog Disk Mastering -- peak control for high allowable average cutting levels; less limiter degradation to the program; brighter, punchier sound.

¡         C/D Mastering -- peak and density control for more accurate digitizing, cleaner sound requiring less error correction on playback; no limiter induced sound degradation.

¡         STL & Phone Line Driver -- maximize signal-to-noise without overload distortion.

¡         Video and Audio Tape Duplication -- "Hotter" transfers without saturation.

Multiband vs Wideband Processing

A very significant problem with wideband processing is "spectral gain intermodulation" which occurs when one part of the spectrum controls the level of another part. A typical situation is a vocalist being "sucked down" every time the kick drum hits.

Since most energy is contained in the lower frequencies, they tend to control the level of the entire spectrum. When the lower frequencies are above the limit threshold the higher frequencies are attenuated thus causing the output to be dull.

MultiBand processing solves these problems by splitting the audio into two or more frequency bands and processing each band separately. However, more bands often result in many more parameters to control including a method of summing the bands together again. While giving the user flexibility, it also requires different settings for almost every different source.

The Dominator II uses program dependent, intelligent circuits to reduce the number of controls. The user, therefore, has flexibility to shape the sound while quickly and easily achieving the goal of consistent, effective limiting.

ALT (Automatic Limit Threshold)

A MultiBand processor splits the audio into separate bands, limits each band individually and then sums the bands together again. Even though each band's peak output is predictable, summing the bands together produces an unpredictable peak output.

One conventional approach to making the summed output predictable is to use a wideband limiter after the summing. This, however, introduces all the drawbacks of wideband limiting discussed above.

Another approach is to use a clipper on the summed output. This causes too much clipping distortion if the summed output is too high. In order to avoid this distortion the limiters' thresholds are set very far below the clipper threshold. The drawback is a loss of loudness and, due to the lower thresholds, much greater amount of processing.

The Dominator II uses a patented method to produce a predictable peak output while maintaining maximum loudness without audible distortion- the Automatic Limit Threshold (ALT). The outputs of the three bands are summed and sent to the ALT detector circuit. If the sum exceeds a reference value, the ALT reduces the thresholds of the individual limiters. When the summed output falls below the reference value the limit thresholds return to their original setting.

The ALT circuit has a self-adjusting finite attack time. The amount of time it takes to lower the thresholds of the limiters is the length of time the limiters' overshoot may be in the clipper. The reference value of the ALT in relation to the clipper determines the depth of clipping.

Both parameters are set by the Density control. When the Density control is set higher, the ALT reference gets closer to clipping, and the attack time is slower, producing more clipping. The opposite occurs when Density is set lower. The "0 RCH" position for the Density control emulates the standard parameters of the original Studio Dominator Model 700, and is recommended for general use.

It should be noted that there is only one ALT circuit controlling both channels equally. This provides global stereo balance and imaging by assuring that both channels always limit at the same threshold. This does cause an interaction if the Dominator II is used as two independent channels. Therefore, we do not recommend such a practice.

Model 722 Pre- and De-emphasis

Pre-emphasis is an equalization curve expressed as a time value based on the ratio of a resistor and capacitor. The higher the value, the greater the equalization. It has been employed as a noise reduction technique for broadcast and transmission links. There are primarily two world standards- 50 and 75 microseconds.

The Dominator II Model 722 has separately switchable pre and de-emphasis curves. When pre-emphasis is switched in, either 50 or 75 microseconds, the equalization curve is added after the input stage and before the limiters. When de-emphasis is switched in, the complementary de-emphasis curve is inserted after the final clipper and before the output stage.

When both pre and de-emphasis are switched in, the frequency response of the output is flat. When the input is below threshold and as the input increases above threshold the output takes the shape of the de-emphasis curve.

When both pre and de-emphasis are switched out, the Model 722 works exactly like the Model 720.

Using the Aphex Model 720 / 722

¡         INPUT Adjusts input level ±15dB. Center detent is unity gain.

¡         PROCESS IN / OUT Switches the audio through the unit or through bypass relays.

¡         LF EQ Adjust input level to the low frequency band ±5dB. Center detent is "flat" response below limiting.

¡         LF XOVER Switches the crossover point between low and mid bands from 100Hz to 210 Hz.

¡         HF EQ Adjusts input level to the high frequency band ±5dB. Center detent is "flat" response below limiting.

¡         HF XOVER Switches the crossover point between mid and high bands from 1.7 kHz to 3.4kHz.

¡         RELEASE TIME Adjusts release time of the limiters between 150 msec (fully Clockwise) and 7 seconds (fully Counter-clockwise)

¡         STEREO COUPLING Forces the limiting in each band to be equal to the limiting in the same band of the other channel if the band in the other channel has a greater amount of limiting.

¡         DENSITY Adjusts the RELATIVE CREST HEIGHT (RCH) of the output. The higher the RCH (Clockwise) the louder the output. The lower the RCH (Counter clockwise) the lower the average level output.

¡         FINE Adjusts the peak ceiling by ±1dB in .2dB steps. 

¡         RANGE When switched to the "-10" position it adds a 10dB boost to the input and a 10dB cut in the output. The PEAK CEILING is therefore 10dB lower than the settings shown on the "COARSE" and "FINE" controls.

¡         METER Displays the limiting (from gain reduction and clipping) in the channel with the greatest amount of limiting.

¡         POWER Switches AC mains on and off. In the "Off" position the audio signal is hard-wired bypassed through bypass relays.

SPECIFICATIONS - Aphex Model 720 / 722 Precision MultiBand Peak Limiter

AUDIO    

¡         Frequency Response ±0.5 dB 10Hz to 50 kHz

¡         Nominal Gain 0dB +14dB / -21dB 0dB +8dB / -21dB

¡         Signal/Noise (Ref +4dBm) -84dB -94dB

¡         THD (Ref 1kHz, no limiting) <0.006% (+12dBu in) <0.006% (+2dBu in)

¡         SMPTE IMD <0.006% <0.006%

¡         Max Input (MIL) +26dBu +16dBu

¡         Max Output Level (MOL)* +24dBm Peak +14dBm Peak

     *(MOL is limited by the peak ceiling setting. The output stage is capable

     of+25dBu into 600 ohms)

¡         Crosstalk -60dB up to 10kHz

¡         Dynamic Range 104 dB  

INPUT and OUTPUTS  

¡         Input Circuits Servo Balanced Transformerless

¡         Output Circuits  Servo Balanced Transformerless

¡         Input Connectors 3-pin XLR Female

¡         Output Connectors 3-pin XLR Male

¡         Input Impedance 19.5k ohms unterminated; 600 ohms by rear panel switch

¡         (terminator lifts in bypass mode) 

¡         Output Impedance 65 Ohms

¡         Input CMRR Better than 50dB 20Hz to 20kHz

¡         Input RF Rejection Better than 40dB at 800kHz; better than 60dB above 2 MHz

CONTROLS and ADJUSTMENTS

¡         Input Gain +14dB / -21dB +8dB / -21dB

¡         LF EQ ±5dB

¡         LF Crossover 100Hz / 210Hz

¡         HF EQ ±5dB

¡         HF Crossover 1.7kHz / 3.4kHz

¡         Release Time 150msec to 7 seconds

¡         Density -5 to +5 RCH

¡         Output Ceiling -9 to +24dB (peak) (=peak value of sinewave) 

OTHER SPECIFICATIONS

¡         Power Requirements 120 VAC 50-60 Hz   30 Watts  (100, 220, 240 options)

¡         Power Fuse 100/120 VAC = .375A SloBlo; 220/240 VAC = .25A SloBlo

¡         Dimensions 19"W x 1.75"H x 9.5" D  (482.6mm x 44.5mm x 241.2mm)

¡         Net Weight 9 lbs (4.08 kg)