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Aphex Type C, BBE Sonic Maximizer, SPL Vitalizer

Article from Recording Musician, September 1992

Three psychoacoustic enhancers, all designed to make your music sound bigger, clearer and more impressive.

Paul White studio-tests three psychoacoustic enhancers, all working on different principles, and finds that they all produce quite different subjective effects.

Equalisers (or tone controls) are invaluable in shaping the sounds that we record, but there are occasions on which simple equalisation is not enough. 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, but there are limitations, the first being that an equaliser can only boost frequencies that are already there. This might sound obvious, but I've seen people trying to turn up the treble control in an attempt to brighten a sound that contains absolutely no high end at all, and then they wonder why all they hear is an increase in hiss! This is often the case with miked-up bass guitars, where the speakers are incapable of producing much above 4kHz, so trying to add boost at 10kHz in the hope of achieving a brighter sound is clearly pointless.

Another limitation is that when you add boost, it's there all the time. Again, this seems 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 during the drum beats. One process that can achieve this effect is dynamic equalisation — the amount of tonal boost varies according to the dynamics of the signal being processed allowing, for example, extra bass to be added to bass guitar and bass drum sounds in a mix without making the sounds in between the beats sound too bottom heavy. Conversely, additional brightness could 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.

The three processors under review all work on different basic principles, but all are designed to exploit some psychoacoustic principle in order to make the processed sound more attractive and more attention grabbing. They may be used to treat a whole stereo mix or just selected sounds, the latter approach being more effective in underlining the tonal contrast within a piece of music. Additionally, all these processes are useful for treating production master tapes for use in cassette duplication, where the processing can help to compensate for the loss of clarity inherent in mass cassette duplication.

All three devices should be used in series with the signal being processed (using an insert point rather than the aux send/return system), though the SPL Vitalizer has a switch option that does allow use in aux send systems if required.

Aphex Aural Exciter Type C

The original Aphex process 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. Fortunately for the private studio owner, you can now pick up a genuine Aphex unit for around £200. Housed in a stylish 1U rack case, the Aural Exciter Type C contains two independent channels of processing designed to work at the -10dBV, unbalanced semi-pro operating level, making it compatible with most home recording mixers. For those working on professional equipment conforming to the +4dBu operating level, the unit has sufficient headroom to run at this level with no problems. To make connecting up simple, both phono and jack inputs and outputs are fitted, the phonos having priority so that if both jacks and phonos are plugged in at the same time, the jack inputs are disabled.

The control layout is quite simple, consisting of Drive, Tune and Mix controls, plus a bypass button for each channel. A tri-colour LED acts as a simple meter for setting up. Most of what we hear at the output is exactly the same as what we send in at the input, but some of the input is diverted via a so-called side-chain into the harmonics generator circuit. This side chain signal is routed through a high-pass filter to remove unwanted low frequencies and then processed dynamically to add phase shift, synthesised harmonics and equalisation. Some of this signal is then added into the output, though the typical amount is a scant few percent of the main signal.

I found the best way to set up the unit was to first turn the Mix control to full so that any effect created is over-emphasised. The Drive control is advanced until the LED flickers mainly between green and yellow, with the occasional red flash on signal peaks, and then the Tune control can be adjusted. The tune control sets a frequency above which new harmonics will be generated, so if it is set towards its clockwise extreme, its action is confined to the upper reaches of the audio spectrum and so only very bright sounds, such as cymbals, will be affected significantly. Moving it downwards starts to affect all bright sounds, whereas going below the half way mark starts to process some mid-range sounds as well, which can produce a harsh result. However, when processing a single sound such as a snare drum, using a low tune setting can be useful in creating an aggressive, cutting sound.

The Drive control comes before the Tune control in the signal path so, after setting the Tune by ear, it is often necessary to reset the Drive control for optimum results. Once the filter and drive settings are OK, it is necessary to reduce the Mix setting so that the enhancement effect is suitably subtle; comparing the processed signal with the bypassed sound is the best way to verify this.

In Use

After a few minutes' practice, it becomes easy to set up the unit and the correct setting for the Tune control is largely intuitive. The effect tends to be addictive, in that the unprocessed sound appears unattractively dull by comparison, but you have to take great care not to use too much of the process or the mix will sound harsh and abrasive. Of all the units tested, the Aphex is the most effective for producing the illusion of brightness from a source that is badly lacking in high-frequency content, but as the process only emphasises the high-frequency end, some low-frequency EQ may be required to maintain a proper bass/treble balance. Anyone hearing an Aphex unit for the first time cannot fail to be impressed by the sense of clarity and detail it creates.

Apart from general 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. The Aphex process has a very distinctive 'sizzle' to it, which is useful for creating an intimate 'in-your-face' type of sound, especially on vocals.

Further Information
Aphex Aural Exciter Type C £305.50 including VAT.

Stirling Audio Systems, (Contact Details).

BBE Sonic Maximizer

Also a 1U, 2-channel unit, the BBE Sonic Maximizer works not by adding harmonics, but by introducing phase changes and dynamic equalisation. Like the Aphex unit, the inputs and outputs may be on jacks or phonos and there is sufficient headroom to accommodate either -10dBV or +4dBu signals. A further socket takes an optional bypass footswitch.

The BBE 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. Consequently, the device does not add new harmonics, but rather attempts to redistribute what already exists.

The control layout is fairly simple — a 'Low Contour' knob and one labelled 'Definition'. Four LEDs show whether the HF band is being compressed or expanded, while also monitoring any peak overload. The red (-) LED indicates compression, the yellow LED no compression or expansion and the green (+) LED expansion. The red Clip LED should not come on under normal circumstances, as it shows that the input signal has exceeded the maximum +16dBu level of the circuitry.

Both channels are affected by the same bypass button, while a second button, labelled 'Auto or Manual', affects the way in which processing is applied. Normally the unit will be used in Auto mode, which means that the compression or expansion of the HF band is controlled by the dynamics of the mid-band signal. In Manual mode, the high band is treated with a fixed degree of expansion. The Definition control sets the dynamic nature of the effect: in its counterclockwise position, there is no dynamic effect but the relative time delays between the three bands still apply. As the control is advanced, the level of the high band is increased, producing a brighter sound. Unlike the Aphex Type C, the BBE Sonic Maximizer is able to influence the low-frequency end of the spectrum too, by means of the Lo-Contour control. This allows the sub 200Hz band to be cut or boosted by -12dB to +10dB.

In Use

Using the Maximizer is simplicity itself: set the unit to Auto, turn up the Lo-Contour to bring up the bass and turn up the Definition control to bring up the high end. The subjective result is quite different to that produced by the Aphex unit, no doubt due to the fact that no new harmonics are being added, but rather the level of the existing ones is being modified. Effects such as this are always hard to describe, but the result seems to be somewhere in between what you'd expect with an exciter and an equaliser. The top end is certainly smooth, while the Lo-Contour control provides a valuable low EQ function which seems able to add punch without clouding the bass end too much.

The effect seems rather less 'focused' than that achieved using harmonic synthesis, though the overall sense of brightness is most definitely increased and some improvement in transparency is achieved. The dynamic nature of the effects is a positive advantage when dealing with noisy material, as no boost seems to be applied to low-level signals (at least in Auto mode) which helps maintain a good signal-to-noise ratio. The difference between the Auto and Manual setting is quite subtle on some material, the manual setting being more constant rather than varying with the signal dynamics.

Further Information
BBE Sonic Maximizer £351.33 including VAT.

Stirling Audio Systems, (Contact Details).

SPL SX2 Vitalizer

Designed and built in Germany, the SPL Vitalizer again occupies a conventional 1U rack case and is configured as two independent channels. Two versions of the unit are available, offering either jack or XLR connection, both being electronically balanced, with a switchable, rear-panel input attenuator allowing connection to systems operating between -10dBV and +8dBu.

The main controls and switches are located on the front panel, each channel having its own bypass button; there is LED status illumination on all switches. A separate section is labelled Surround Processor, and this controls a simple stereo width expansion system that functions independently of the rest of the processing.

The Vitalizer works by generating a processed side-chain signal which is then added to the original. This modifies the frequency response of the signal both additively and subtractively, and because of the way the filters interact, the effect gives the impression of being dynamically related to the input signal. The result is a perceived increase in both bass and brightness while the mid-range is brought into sharper focus, increasing the sense of transparency.

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, the Harmonics control is used to pull out transient detail through a combination of dynamic EQ and harmonic synthesis. It is possible to isolate the processed signal using the Process Solo button, which not only allows the user to check how much processing is actually taking place, but which also provides a means of using the effect via the aux sends and returns on a console — something the other two units can't do.

The first control to note is a dual-concentric pot, the outer part of which sets the output level of the unit, unity gain corresponding to the fully clockwise position of this control. The inner section controls the Process Depth and 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 or Stereo Width controls, which operate independently. A peak signal LED is fitted to assist in setting up; the best effects seem to be achieved when the input signal level just causes the LED to flash on signal peaks.

Bypass is self explanatory, while Process Solo isolates the processed part of the signal for checking or aux send/return use. The Mid-High tuning control defines the area of the mid range that will be processed and also affects the operation of the Harmonics control; the harmonics processor derives its input partly from the untreated signal and partly from the output of the Mid-High filter.

"The three processors under review all work on different basic principles, but all are designed to exploit some psychoacoustic principle in order to make the processed sound more attractive and more attention grabbing."

The Mid-High (high-pass) filter range is from 1kHz to 22kHz, so the higher the setting, the less of the spectrum is treated. Settings of between 3 and 5kHz seem to produce the best results. A preset pot allows the Q at the cutoff point to be increased, if desired, for special effects, though it requires a screwdriver to reset it.

The Harmonics control is independent of the Process Depth control and has the effect of enhancing high-frequency detail in much the same way as a traditional exciter. The effect is apparently created by a dynamic filter circuit employing fourth-order filters, but this appears to generate harmonics almost as a by-product. The Bass Process 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 counter-clockwise direction produces a much more 'rounded', full-sounding bass, but with no apparent spill into the mid range. The Deep button changes the filter response of the Bass Process circuit, providing an even deeper bass effect.

The Surround Processor uses the tried and tested principle of feeding phase-inverted signal from the left channel into the right, and vice-versa, to provide a stereo image that appears to be wider than the speaker placement. It's the same system employed in ghetto blasters and is mono-compatible.

In Use

The controls need to be set up in a logical sequence, and you really need to read the manual first, but otherwise it's pretty straightforward. By setting the Sub Bass control to off and the Harmonics control fully anti-clockwise, the effect of the Mid-High tune control can be heard in isolation once the Process Depth control is turned up. Next, the Bass Process control can be used to bring up the low end. Finally, the Harmonics control can be brought into play to enhance the high end. The Surround Processor is, as stated, an entirely separate effect and is best turned right down unless it is needed. It is not affected by the Bypass button, which means it can be used on its own if required.

Like the Aphex Exciter, the SPL Vitalizer lends a sense of definition and transparency to a mix, the main difference in this area being that the mid range seems much better defined. The effect is like being able to 'hear through the mix' to all its constituent parts, while the bass enhancement is impressive by any standards and could well be used to compensate for the 'thinner' sound of narrow-format tape recorders. Using the stereo width expander, the stereo image is certainly widened but at the expense of localisation of the individual sound sources — as the sound gets wider, it also gets more vague. Nevertheless, the effect is useful for spreading reverb or stereo keyboard sounds.

In terms of audio quality, the Vitalizer is quiet enough for all serious audio applications, but like any equalisation process, it can bring up the level of noise that exists as a part of the programme material. And, as with the other units reviewed, over-processing the top end can aggravate sibilance problems and highlight any distortion already present.

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 Vitalizer represents an easy way to add the punch and sizzle to a recording that most people associate with good pop production. I feel this device has something genuinely different to offer, and though it is the highest priced of the three units by a significant margin, feel it is the most versatile.

Further Information
SPL Vitalizer £757.88; unbalanced jack version £699.13. Prices include VAT.

The Home Service, (Contact Details).

Aphex Type C Aural Exciter

Easy to use.
Can synthesize missing HF detail.
Dramatic increase in perceived clarity.

Only treats the HF end of the spectrum. Over-processing can produce harsh or sibilant results.
Tends to emphasise any noise present in the original material.

SPL SX2 Vitalizer

Enhances the whole audio spectrum to accentuate detail, punch and transparency.
Low-frequency enhancement does not interfere with mid-range clarity.
Is effective on inherently dull programme material, though less so than the Aphex Type C.
Built-in stereo width enhancer.
May be used in an effects send/return loop.
Fully balanced operation with jack or XLR versions.

Control operation is not obvious until you read the manual.
More expensive than the other two processors. There is no single control to regulate the overall effect — the harmonics section and stereo width expander work independently.
Over-use can cause harshness.

BBE Sonic Maximizer

Very easy to use.
No tendency to sound harsh unless grossly over-used.
Useful low equaliser.
Improves overall brightness and clarity.
Does not significantly increase programme noise.

Unable to compensate for harmonics missing in the original.
Seems less able to create a sense of 'transparency' than either the Aphex or the SPL units.

Psycho acoustics

Various aspects of dynamic equalisation have been incorporated into signal processors to provide 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 though 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 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 midrange 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, it 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 these so-called exciters or harmonic enhancers is that they can be used effectively on sounds originally lacking in high-frequency content because they, in effect, 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, over-processing a signal in this way can result in a harsh, fatiguing sound, making it imperative to use the treatment in moderation.

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. Because no distortion is added, the result tends to be a little less harsh when high levels of processing are needed, but on the other side of the coin, they are less effective on dull material, as they do not have the ability to synthesise the missing harmonics.

Both types of psychoacoustic processor also manipulate the relative phase of various parts of the audio spectrum and this also contributes to the 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 should be possible to restore the original phase relationship, making the sound appear to be closer. This is exactly what happens — which is why processed sounds seem to be very 'up front'.

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Publisher: Recording Musician - SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

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Recording Musician - Sep 1992

Donated & scanned by: Mike Gorman

Review by Paul White

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