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Rocktron Hush IICX

Single-ended Noise Reduction System By Paul White
Published February 1996

Rocktron Hush IICX

As recording equipment becomes more advanced, I become more depressed about the amount of noise being generated at source, due to noisy synth modules, guitars picking up buzz from computer monitors, ground loops, and the cumulative hiss from countless ICs in my mixers and processors. Even in the most carefully‑designed system, an irritatingly high level of noise, compared to the noise floor of, say, a DAT recorder, is eventually funnelled down to the final stereo mix, and I feel there really is a case for pressing synth manufacturers to quote noise figures.

As you've no doubt read umpteen times, a great many noise problems come about because the rules of gain structure have been ignored, or because unused sound sources or processor outputs are still patched into the mixer rather than being muted. However, even if you're meticulous about such things, source noise can still pose a real problem. One partial solution is to use a single‑ended noise reduction unit (see 'The Sound Of Silence' box for a description of how these units typically work).

The Hush Process

Rocktron's Hush system has been around for a number of years, and although patented, it is broadly similar to the system used by Symetrix and Drawmer in their own DNR (Dynamic Noise Reduction) processors, in that it combines a downward expander and a dynamic filter. The IICX is a two‑channel, 1U, rackmount processor powered via an external adaptor, with both inputs and outputs on unbalanced, quarter‑inch jacks. A high input impedance of 470kΩ means that electric guitars can be plugged directly into the unit if required.

The two channels may be linked by means of the Master button, for stereo operation, or they may be used independently — each channel has separate controls for expander threshold and filter sensitivity, as well as switchable +4dB/‑10dB operation. Channel bypass buttons are fitted, but it isn't possible to bypass the expander and filter separately, something I like to do while setting up if possible. Neither does the expander have a variable ratio or release time, though a button does provide a choice of preset Fast or Slow release times.

So that you can see what's going on while using the IICX, both the expander and filter are furnished with simple four‑LED meters. The expander meter shows how much gain reduction is being applied to low‑level signals, while the filter's meter shows the cutoff frequency. This will normally vary, as the incoming signal varies in level and frequency content; the optimum setting is usually with filter fully open during all louder passages, only closing during obvious pauses or at the end of a piece. The expander should be set to apply little or no gain reduction during playback, unless a total pause is encountered.

A Class‑A VCA is used in the expander circuitry, in order to maintain the highest possible signal quality. Bearing in mind that very often the whole mix will be going through this unit, that's an important consideration.

In Use

As expected, setting up the Hush system involves a compromise between acceptable noise and acceptable top‑end loss. If the Hush can be applied to specifically noisy individual tracks, such as electric guitars or a noisy synth module, it's possible to bring about a worthwhile reduction in the amount of subjective noise without noticeably affecting the sound of the overall mix. When using the process over a complete mix, however, you have to be doubly careful not to compromise the sound quality, which usually means leaving in some audible noise and preserving the tone of the track, rather than taking out all the noise and dulling the track. I found that unless the filter was allowed to close quite a long way, the level of hiss was only reduced very slightly, and if the filter was allowed to close right down, the sound being processed tended to lose all its top end.

Setting up the Hush IICX is actually very easy, and in cases where the noise contamination is not serious, significant improvements can be achieved.

Setting up the Hush IICX is actually very easy, and in cases where the noise contamination is not serious, significant improvements can be achieved. However, if the material is very noisy, it's often impossible to eliminate audible noise pumping and/or top‑end loss, and very often the only real benefit comes during complete pauses or at the start and finish of a mix, where the first note emerges from total silence, and the last note fades away into total silence.


I don't think the Hush IICX works quite as effectively as the more expensive Symetrix and Drawmer units, but considering its relatively low price, it represents very good value nevertheless. Setting up requires some care — you need to use your ears to choose the best compromise — but is not difficult, and the unit undoubtedly works best when fed with individual signals, when it may be used instead of a conventional gate, to keep pauses silent and remove hiss from behind low‑level signals.

The Hush IICX is also useful on complete mixes, but only if a modest amount of noise reduction is required while the track is actually playing. It will take care of starts and ends of tracks with no problems, but when music is present, you shouldn't expect to achieve more than a few dB of hiss reduction before the side effects start to show.

Ultimately, all single‑ended noise reduction units of this type are only effective when used with reasonably well‑recorded material, and even then, they can only help to reduce hiss — they have no effect on clicks, crackle, or hum, except during pauses. There's little benefit in using them to try to salvage hideously noisy mixes — for that you need a sophisticated DSP system, which will cost considerably more money! Providing you appreciate and accept these limitations, the Hush IICX is something of a bargain.

The Sound Of Silence: How Single‑Ended Noise Reduction Works

Most analogue, single‑ended noise reduction units (not to be confused with encode/decode tape noise reduction devices), work by combining the action of a low‑level expander with a dynamic filter. The expander acts like a 'soft' gate by significantly reducing the level of very low‑level signals (and noise) during pauses, while the filter 'rolls off' the high‑frequency content of the signal to a greater or lesser extent, depending on the mid‑ and high‑frequency content of the material being treated. The dynamic filter idea is actually quite clever, because when the signal is loud enough and bright enough to mask any background noise, the filter opens fully, so as to have no adverse effects on the signal. However, when the mid‑ and high‑frequency content of the signal falls, the filter cutoff frequency is reduced accordingly; in the case of the Hush system reviewed here, the filter frequency can drop as low as 700Hz during quiet passages.

Natural sounds are usually brightest when they are at their loudest, so the filter tends to open fully at the start of a sound, then starts to close down as the sound decays. This way, the sound isn't made to sound unnaturally dull, because the important attack gets through unchanged. Even so, inappropriate settings of the controls will cause dulling, and long reverb tails can sometimes suffer, no matter how carefully you juggle the settings. Because of this latter consideration, many engineers will apply DNR (Dynamic Noise Reduction) to the dry portion of a mix, but leave the reverb returns unprocessed.


  • Cost‑effective.
  • Easy to use.
  • Works well within the limitations of a dynamic filter/expander DNR system.


  • Irritating external PSU.
  • No separate expander and filter bypass switches.


A useful tool for polishing up already well‑recorded mixes, for cleaning up cassettes, or for reducing the level of hiss on individual tracks. Should only really be used after you've taken all other possible precautions to minimise hiss.