Dynamics Processor

Signal Processors > Compressors

Published October 2024

Eventide’s unique, and uniquely versatile, hardware compressor has had a well‑deserved reissue, with sympathetic improvements to update it for the modern studio.

From the groundbreaking H910 Harmonizer to their current H90 stompbox, Eventide are best known for their digital gear. But back in the ’70s their catalogue included a number of analogue processors. Perhaps the most interesting of them was the Omnipressor, now reissued to celebrate its 50th anniversary.

Like the original, the reissue Omnipressor is a smart black 2U rack unit. Unlike the original, thankfully, its audio I/O is on conventional connectors rather than screw terminals: line in, line out, side‑chain in and side‑chain out are on both XLRs and quarter‑inch jacks, while stereo link in and out are on jacks only. On the front panel, the chunky white plastic buttons of the ’70s incarnation have been replaced by switches. This has freed up space for Eventide to add quite a few useful new features.

There’s now an input gain control in addition to the threshold dial, plus a wet/dry mix control, while today’s Omnipressor can independently be stereo linked and externally side‑chained, with convenient switching from the front panel. Finally, the output gain control is now continuously variable over a ±12dB range, in place of the switched 0, +10 and +20 dB choices on the older version. Happily, the all‑important meter looks very similar, and build quality on the reissue seems excellent — quite possibly better than on the originals.

Threshold Of Pain

Calling the Omnipressor a compressor is like describing a Swiss Army knife as a knife. It’s much, much more than that, but its operation is also fundamentally different from that of conventional dynamics processors, and can be confusing at first. Perhaps the best way to explain this is as follows. Most compressors start to apply gain reduction only once the signal level exceeds the threshold value. Likewise, expanders apply attenuation only to sounds below the threshold. In the Omnipressor, by contrast, dynamic changes are by default symmetrical either side of the threshold.

Consider a typical application for compression on a vocal track. On a standard compressor, we’d set the threshold level such that loud peaks exceed it and trigger gain reduction, thus reducing the average level of the vocal and its dynamic range. On the Omnipressor, though, the signal level doesn’t just get pulled down towards the threshold when it’s loud; it also gets pushed up towards the threshold when it’s quiet. The Omnipressor can act as an expander, too, in which case this equation is reversed. Signals below the threshold are made quieter, as on a standard expander, but signals above the threshold are also made louder.

In other words, the Omnipressor’s threshold doesn’t set a cutoff point below or above which nothing happens. It sets a centre point around which the dynamics of the signal are squeezed inwards or expanded outwards, according to the setting of the Function knob — which is really a ratio control. And once you get used to this concept, everything else begins to fall into place. It becomes clear why the Omnipressor can apply gain change even though the signal never crosses the threshold. And you realise why the Gain Limit and Atten Limit controls are so important to its operation.

With no input, if you set these both fully counter‑clockwise, so that each permits the full 30dB range, turn the Function knob to one of the compression positions, and switch the meter to Gain, you’ll notice that the Gain light will illuminate and the needle moves into the red. The Omnipressor has detected that the input signal is low, and is applying maximum positive gain to try to raise it towards the threshold. With a loud input that exceeds the threshold, the Atten light illuminates instead, and the needle will fall back below zero. At this point, conventional gain reduction is taking place, with a signal peak being pushed downwards towards the threshold.

Now, you might well want the Omnipressor to act just like a conventional compressor, and thanks to the Gain Limit knob, it can. If you turn this fully clockwise to the zero setting, you’re telling the Omnipressor that the maximum amount of positive gain it’s allowed to apply is zero. The net result is that signals above the threshold will be pulled down, but signals below it will be left unaffected. But equally, you could turn the Atten Limit control to zero and leave the Gain Limit knob at ‑30 — and then you would have upwards compression, where quiet signals are pushed up towards the threshold but peaks are left alone.

Ratio Metrics

The continuously variable Function control is, as was previously mentioned, a ratio setting, but it’s a ratio setting with an unusually wide range. The 12 o’clock position corresponds to a 1:1 ratio, where there is no signal‑dependent gain change. Positions to the left of this introduce increasingly dramatic expansion, while conventional compression settings occupy a surprisingly narrow range in the early afternoon, with the ratio getting exponentially greater as you go clockwise.

At about half past two on the Function control’s travel, you’ll see a symbol indicating that the ratio is infinity:1. We are used to thinking of this as hard limiting, and with the Gain Limit control set to zero, that’s what you get. But if you leave both the Gain Limit and Atten Limit controls wide open, you get what the manual describes as “infinite compression” instead. In this mode, the Omnipressor does its best to reduce the dynamic range of the input signal to nothing. Not only are peaks brickwall limited so that they cannot exceed the threshold, but quieter signals are pumped right up to the threshold. Go even further and you can enjoy one of the Omnipressor’s unique party tricks: dynamic reversal, or negative‑ratio compression, in which signals above the threshold are drastically attenuated and signals below it heavily boosted, so that what was quiet becomes loud, and vice versa.

Yes, it’s fun to mangle a drum room mic through a negative‑ratio compressor, but there’s much more to the Omnipressor than that.

Pressing Cases

Even though the basic design is 50 years old, then, there are still ways in which the Omnipressor is ahead of its time. I can’t think of another analogue compressor that offers such complete freedom to modify the dynamic characteristics of the signal. In its wilder moments, it feels as though you’re using a piece of lab equipment, or something that should be part of an experimental modular synth setup. But that’s only one facet of the Omnipressor’s design, and in fact I wonder whether its reputation as a special effects processor has overshadowed its potential as a studio workhorse. Yes, it’s fun to mangle a drum room mic through a negative‑ratio compressor, but there’s much more to the Omnipressor than that.

In place of the original’s screw terminals, the reissue Omnipressor features conventional audio I/O on jacks and XLRs.

The Omnipressor’s capabilities as a noise gate and expander are likely to be less useful now than in 1974, partly because we’re no longer recording to tape and partly because plug‑in technology is so effective at cleaning up noise or reducing unwanted spill. But its core features as a compressor remain mighty impressive. If it never became as ubiquitous as, say, the 1176 or an LA‑2A, that’s partly because it isn’t idiot‑proof. Both the attack and release times go all the way down to ‘practically instantaneous’, which will generate distortion on anything with low‑frequency content, and if you don’t put some time into understanding the two‑way action that applies positive and negative gain change either side of the threshold, it can be confusing.

Once you get to grips with the Omnipressor’s unique design, though, it’s easily mastered and reveals itself to be a very capable general‑purpose compressor. It can bring plenty of attitude to vocals, drums and bass guitars, yet with longer time constants and gentler ratios, it can also be surprisingly transparent. The side‑chain high‑pass filter is very effective, and the wet/dry mix control on the reissue makes extreme settings a lot more usable. For example, you won’t often want to use “infinite compression” in its purest form — but back the Gain Limit control off and blend in some dry signal, and you can really pin a rock vocal to the front of the mix.

Does all this justify paying around £2k for a hardware Omnipressor reissue, when you could download Eventide’s excellent plug‑in recreation for just over £100 and use it on multiple tracks at once? Sonically, the differences are not enormous. This isn’t some sort of magic mojo unit that uses transformers and valves to impart ‘analogue warmth’, whatever that is. But what playing with the review unit brought home to me was how valuable the tactile dimension of a hardware unit like this is. There’s really no substitute for getting hands‑on with physical controls, and it invariably leads me to make different decisions than I do when poking at things on screen with a mouse. Is the Omnipressor reissue a must‑have essential for the modern studio? No. But as luxury items go, it’s one you’ll get plenty of use from.