Eventide Omnipressor 2830AU Compressor and Expander

Originally invented by Eventide’s founder, Richard Factor (and later redesigned for production by Jon Paul), the 1976 Omnipressor made a host of effects possible. It featured variable control of all aspects of dynamic modification.

 

It also introduced the notion of the side-chain and foretold techniques that today we take for granted like lookahead processing. One of its first customers was a young Brian May, lead guitarist of Queen. With the Omnipressor in his rig, his sound was never the same.

One of the novel features of the Omnipressor is its ability to make signals sound backward. This is a consequence of the Dynamic Reversal feature, which enables loud sounds to come out more softly than soft sounds. Speech waveforms, for instance, generally consist of loud peaks followed by trailing-off envelopes.

 

By making these envelopes louder than the peaks, the illusion that the sound is coming out backward is generated. Likewise, drum sounds consist of peaks roughly coincident with mechanical impact, followed by a decay envelope. In general. any material with wide dynamic range can be reversed, and by experimenting with forward and reversal playback, you can achieve an incredible array of effects.

Fully counterclockwise, the Omnipressor gain varies sharply from full attenuation to maximum gain as a threshold level is exceeded. As the control is turned clockwise from the center divider, the gain begins decreasing with increasing input level.

 

• Dynamic Reversal: An input level of +10 dB results in an output of -10 dB, and vice versa.
• Gate: As the signal decreases below +10 dB, the device gain rapidly goes to minimum.
• Expansion: A 40 dB input range results in a 60 dB output range, increasing dynamic range.
• Control Centered: Input level equals output level.
• Limiting: Gain is unity until input is 0 dB. Above 0 dB, a 30 dB change in input produces a 6 dB output change.
• Infinite Compression: Output level remains unchanged regardless of input level.

The analog VU meter is calibrated over a 60 dB range in a linear/logarithmic scale. When the three-way switch is in the Input position, the meter reads the input signal level. When in the Gain position, the meter reads the relative gain of the device, giving an indication of the gain control function. In the Output position, the meter reads the output level of the Omnipressor.

• Line switch controls the relay-bypass function
• Input and Output level controls
• Mix control for wet/dry adjustment and parallel compression
• Side-chain enables or disabled side-chain audio path
• Input Threshold controls the operating point of the Omnipressor
• Bass cut for reducing low-end leakage into your signals
• Control Attack Time and Release Time
• Three-way switch for Meter Function: Input, Gain, or Output monitoring
• Main Function knob scrolls through basic modes of operation (compression, limiting, reversal, etc.)
• Atten Limit adjusts the limits of maximum attenuation, with a max reduction of 30 dB
• Gain Limit commands the maximum gain of the Omnipressor
• Link activates or deactivates the unit to unit linking function

The unique ability of the Omnipressor to separate the gain control from the level detector enables one to build what is most conveniently termed a predictive compressor. Such a unit should go a long way towards eliminating the unavoidable imperfections standard units have.

 

Connect an Omnipressor and an Eventide Digital Delay Line, or similar delay device. Enable the side-chain input. What you have just fabricated is a compressor that can read the future, or, in more common parlance, one which has a negative attack time. 

The Omnipressor makes a good compression/expansion noise reduction unit for enhancing the transmission capability of some media such as tape, digital equipment, low grade phone lines, etc. If the Omnipressor is set up as a compressor on the input end (feeding the tape machine or phone line) and as an expander on the output end, then the input dynamic range is compressed during transmission and a medium with, say, 40 dB dynamic range can appear to have a much wider range.

 

Since identical circuitry with identical time constants is used to produce compression and expansion, perfect dynamic tracking is obtained. If compression and expansion ratios are set properly, the system should be transparent to the listener.