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How Enhancers Work

Exploration By Paul White
Published February 1995

Exciters and enhancers are often mentioned in SOS, but what exactly do they do to your sound, and how do the various types differ? Paul White explains.

There are often recording or mixing situations in which simple equalisation can't produce the tonal changes we're after, and it is in such situations that we might turn to enhancers or exciters to find a solution. But how can they help when traditional EQ fails?

Ordinary equalisers work by cutting or boosting a part of the audio spectrum to alter the overall spectral balance, which is why EQ can help us brighten sounds, bring up the bass or bring down the mid‑range. Most of the time, this is exactly what we want to do, but there are limitations, the main one being that an equaliser can only boost frequencies that are already there. There's often a temptation to turn up the treble control in an attempt to brighten a sound that contains absolutely no high frequencies at all, which just results in more hiss! This is often the case with miked‑up bass guitars, dull old electric pianos, and people with very smooth voices.

Another limitation is that when you add boost, it's there until you turn it off again. This may seem obvious, but if you're working on something with a lot of dynamics, such as a drum track, you might like to be able to apply some tonal boost only to the individual drum beats. One process that can achieve this effect is dynamic equalisation, where the amount of tonal boost varies according to the dynamics of the signal being processed. This allows extra bass (for example) to be added to bass guitar and bass drum sounds in a mix without making the sounds in between the beats too bottom‑heavy. Conversely, additional brightness can be achieved by adding a dynamic, high‑frequency boost to sounds such as snare drums or cymbals. Such dynamic effects are quite dramatic, because they increase the tonal contrast within the music, rather than treating the whole mix in the same way. Most exciters or enhancers combine elements of dynamic equalisation with other processes, including harmonic synthesis and phase manipulation (see the 'Psychoacoustics' and 'Just A Phase' side panels elsewhere in this article for some of the theory behind sound enhancement). Not all manufacturers use the same combination of principles, which means that each type of enhancer has its own characteristic sound. The purpose of this article is to look at some of the more popular models and to see what they actually do to the sound.

In The Beginning, There Was Aphex

Aphex was the first company to market enhancers, and they claim that their Aural Exciter was discovered quite by accident, when a stereo valve amplifier kit was wrongly assembled. One channel worked properly, but the other produced only a thin, distorted sound. To their surprise, adding the two channels together produced a result that sounded cleaner and brighter than the original. After they had spent considerable time figuring out why this was, they formed a company to exploit the discovery. The first commercial Aphex processor was shrouded in secrecy, and anyone wanting to use it on record had to hire the unit from Aphex and pay a royalty based on the length of the recording. Today, Aphex exciters may be bought and used just like any other processor.

Most of what comes out of the output of an Aphex exciter is exactly the same as what goes in at the input, but some of the input signal is diverted, via a side‑chain and a high‑pass filter, into a harmonics‑generating circuit. The high‑pass filter is necessary to remove unwanted low frequencies which, after processing, might result in a muddy or discordant sound. The filtered signal is then processed dynamically to add phase shift and to create synthesised harmonics which are musically related to the original signal. A small amount of this signal is then added into the output, which has the effect of reinforcing and emphasising transient detail without significantly increasing the signal level.

Though the Aphex principle is patented, a number of companies have produced enhancers that work by similar means and produce similar subjective results. Aside from routine track or mix processing, this type of processor is useful for restoring high‑frequency detail that has been lost after processing with a single‑ended noise reduction system, or for producing master tapes for cassette duplication, where some high end is invariably lost in the duplication process itself. The Aphex process is also effective for creating an intimate vocal sound, because the enhancement process simulates the way a sound is perceived when the source is in close proximity to the listener.

The BBE Route

BBE is one company that took a different route from Aphex in developing an enhancer; the BBE Sonic Maximizer works not by adding harmonics, but by introducing phase changes and dynamic equalisation, which just redistribute those harmonics already present. The process works by first splitting the audio signal into three frequency bands and applying different time delays to each band by means of passive and active filters. Frequencies below 150Hz are delayed by around 2.5mS, while those between 150Hz and 1200Hz are delayed by around 0.5mS. Frequencies above 1200Hz are not delayed, but are subjected to dynamic level control, which can take the form of compression or expansion, depending on the control settings and the nature of the input signal. The BBE process is also is able to influence the low‑frequency end of the spectrum by means of a Lo‑Contour control, allowing the sub‑200Hz band to be cut or boosted by ‑12dB to +10dB.

In a typical BBE unit, the Lo‑Contour control is used to bring up the bass, while the Definition control to brings in the high‑end enhancement. The subjective result is quite different to that produced by the Aphex unit, as new harmonics are not being added; the level of the existing ones is being modified. The result is very smooth‑sounding, but on material that is seriously lacking in top end in the first place, the process seems incapable of restoring it to the same extent that the Aphex process can. However, where the original material is of good quality, the BBE process can enhance it considerably without making it sound harsh or aggressive. On most material, the overall sense of brightness is definitely increased, and some improvement in subjective transparency is achieved. The dynamic nature of the process is also an advantage when dealing with noisy material, as little or no boost seems to be applied to low‑level signals; this helps maintain a good signal‑to‑noise ratio.

Re: Vitalizer

Yet another approach to enhancement comes from SPL of Germany, and although their process (as used in their famous Vitalizer) involves mainly equalisation, the results obtained are quite unlike those obtained using conventional equalisers. The Vitalizer works by first generating a side‑chain signal from the main signal; the frequency response of this side‑chain signal is then modified both additively and subtractively. Because of the way filters interact, the impression of an increase in both bass and brightness is created when the side‑chain signal is added back to the original, while the mid‑range is brought into sharper focus, increasing the sense of transparency. Though the SPL enhancement principle is quite complex, part of the process involves adding low‑frequency equalisation in such a way that phase cancellation occurs in the lower mid range. This has the effect of simultaneously lifting the bass and pulling back that area of the spectrum that would normally conflict with it, resulting in a very powerful but tightly‑controlled bass lift.

At the high end of the spectrum, a Harmonics control is used to pull out transient detail through a combination of EQ and (possibly accidental) harmonic synthesis. The effect is apparently created by a filter circuit employing fourth‑order filters, but this appears to generate harmonics almost as a by‑product, due to the nature of the components used.

It is possible to isolate the processed signal using a Solo button, which not only allows the user to check how much processing is actually taking place, but also provides a means of using the effect via the aux sends and returns on a mixing console. A Process Depth control determines how much of the output from the sub‑bass and mid‑high filters is added back to the original sound. This has no effect on the Harmonics control, which operates independently. A tuning control defines the area of the mid range that will be processed, and also affects the operation of the Harmonics processor, which derives its input partly from the untreated signal, and partly from the output of the mid‑high filter.

The Bass Process is interesting, in that the control has a centre‑off position and produces two distinct sound characters depending on whether it is turned right or left from centre. Advanced clockwise, the sound takes on a very tight, punchy feel, while the anti‑clockwise direction produces a much more 'rounded', full‑sounding bass, but with no apparent spill into the mid range.

SPL include a so‑called Surround Processor in their units, but this is independent of the enhancement circuitry, and simply uses the tried and tested principle of feeding phase‑inverted signal from the left channel into the right, and vice‑versa. It's the same system employed in ghetto blasters, and, if used in moderation, it can help increase the sense of stereo width. It is also fully mono‑compatible.

Like the Aphex Exciter, the SPL enhancement process lends a sense of definition and transparency to a mix, the main difference in this area being that the mid range seems better defined too. The effect is like being able to 'hear through the mix' to all its constituent parts. The Vitalizer bass enhancement is impressive by any standards, and could well be used to compensate for the 'thinner' sound of narrow‑format or budget digital tape machines. The subjective effect is like that of an exciter that works not just at the top end, but across the whole audio spectrum, increasing the sense of loudness, detail and space. Though the effect may be based on psychoacoustic trickery, the SPL process provides an easy way to add the punch and sizzle to a recording that most people associate with good pop production. Note, however, that like any equalisation process, SPL's system can bring up the level of noise that exists as a part of the programme material, and, as with the other enhancement units available, over‑processing the top end can aggravate sibilance problems and highlight any distortion already present. As ever, the key is to use the process in moderation.

Further Excitement

One of the latest companies to enter the exciter race is Dolby, who have combined their expertise in filter design with compression techniques to produce a Spectral Enhancer. Rather than adding harmonics or using simple dynamic filters, the Dolby approach relies upon treating a side‑chain signal via a bank of complex filters, which modify their characteristics according to the nature of the input signal. The filtered signal then appears to be heavily compressed before being added back into the main signal path. The system has made a strong impression on those who have used it, even though Dolby's unit is the most expensive of the current commercial enhancers. Also available is a popular range of enhancers, including the Ultrafex, Dualfex, and Bassfex, from German company Behringer.

It's worth noting that enhancers are now being included in multi‑effects units, and although all the original enhancers were analogue, attempts are being made to emulate these digitally. So far, I feel these have only been partly successful, but it's only a matter of time before these digital emulations are the equal of the original analogue processors, so if you need budget enhancement, you might be advised to wait a while. However, at the time of writing, if you need high‑quality enhancement, there's no real alternative to buying a dedicated unit.

Aphex User Tips

The easiest way to set up a standard Aphex unit is to first turn the Mix control to full, so that any effect created is over‑emphasised. The Drive control can be advanced until the LED meter confirms a suitably high drive level, and then the Tune control can be adjusted by ear. This last control sets the frequency above which new harmonics will be generated; if it is set towards its clockwise extreme, the Exciter's action is confined to the upper reaches of the audio spectrum, meaning that only very bright sounds, such as cymbals, will be affected. Moving the control further down progressively involves more mid‑range sounds in the process. Once the filter and Drive settings are set, it is necessary to reduce the Mix setting, so that the enhancement effect is appropriately subtle; comparing the processed signal with the bypassed sound is the best way to verify this.

Note: The Drive control comes before the Tune control in the signal path so, after setting the Tune by ear, it is often necessary to readjust the Drive control for optimum results. More recent Aphex units have dispensed with the Drive control, making them even easier to use.

Of all the types of enhancer currently available, the Aphex process is probably still the most effective for producing the illusion of brightness from a source that is badly lacking in high‑frequency content. As the process only emphasises the high‑frequency end, some low‑frequency EQ may be required to maintain a proper bass/treble balance, and because of this, many modern units include some form of integral bass enhancement system to help maintain a balanced sound.

Psychoacoustics And The Theory Behind Enhancement

Various aspects of dynamic equalisation (see above) have been incorporated into enhancers to produce an effect that appears to make everything more detailed, more transparent and louder than before. The reason this works is all bound up with the psychological perception of hearing, or 'psychoacoustics', and although nobody fully understands the subject, there are tried and tested processing tricks that produce a definite and consistent result. One of the simpler psychoacoustic principles is based on the fact that our perception of the audio spectrum changes as sounds become louder. If, for example, we play a record at a very high volume, we tend to hear the high and low frequencies in a more pronounced way, whereas at lower levels, the mid‑range is more evident. Simply by using an equaliser to cut the mid range or to boost the high and low extremes, music can be made to sound louder than it really is — which is exactly how the loudness button works on a hi‑fi stereo amplifier.

The American company Aphex discovered an interesting principle, which was further developed into their Aural Exciter concept. By adding very subtle distortion to the original signal, the signal could actually be made to sound clearer and louder — but why? The answer is that whenever an audio signal is subjected to distortion, intentional or otherwise, high‑frequency harmonics are produced. Normally, these sound pretty unpleasant, as they are not always musically related to the original sound, but by using filters to confine the distortion to a specific part of the audio spectrum, it is possible to create the illusion of additional high‑frequency detail without musical dissonance. The most significant feature of exciters or harmonic enhancers is that they can be used most effectively on sounds originally lacking in high‑frequency content, because they effectively synthesise a new and musically convincing top end. Further circuitry refinements add a dynamic element to the process, with the result that more harmonic enhancement is added to percussive or transient sounds than to quieter ones. The subjective result is remarkable, producing an audible increase in detail, presence and loudness, even though the level of added distortion is minuscule. However, the user needs to beware of over‑processing a signal in this way, as excessive levels of enhancement can result in a harsh, fatiguing sound, making it imperative to use the treatment sparingly.

Dynamic equalisers, on the other hand, produce no deliberate distortion, but on well‑recorded material, the result is subjectively similar to that produced by an enhancer, as transient sounds are increased in level by the process. Because no distortion is added, the result tends to be a little less harsh when high levels of processing are needed. However, dynamic equalisers are also less effective on dull material, as they do not have the ability to synthesise missing harmonics.

Just A Phase

Both enhancers and dynamic equalisers manipulate the relative phase of various parts of the audio spectrum, and this contributes to their perceived effect. As with most audio processes, this relates to a real‑life effect; when sound travels through air, the low frequencies travel slightly slower than the high frequencies, with the outcome that distant sounds are heard with a significant phase difference between the high‑frequency and low‑frequency sounds. Nearby sounds, on the other hand, are less affected, so the sounds arrive with their phase relationships intact. If an electronic processor is able to delay the low frequencies slightly by means of deliberately introduced phase shifts, it is possible to restore the original phase relationship, making the sound source appear to be closer. This is why processed sounds seem to be very 'up front'.

Further Reading

The Aphex Type C2 Aural Exciter was reviewed in the January 1993 edition of Sound On Sound's sister publication Recording Musician, while the most recent incarnation of the BBE Sonic Maximizer, the model 462, was reviewed in the February 1994 issue of Sound On Sound. Last but not least, the SPL Vitalizer was also covered in Sound On Sound, both in its original form, in May 1992, and in its later budget stereo version, in December 1993. Behringer's Dualfex was reviewed in December '92; the Bassfex and Ultrafex were both reviewed in October '93. Back issues of all these magazines are available from: SOS Mail Order, Media House, Burrel Road, St. Ives, Cambs PE17 4LE. Telephone 0480 461244.