An explanation of psychoacoustic signal enhancers and their possible applications.
Since its introduction in 1975, the Aphex Aural Exciter has been used exclusively by top recording studios to add that extra depth and sparkle to their productions. Now, a decade later, there are other devices on the market that produce very similar effects at a price that puts them within easy reach of the home recordist. Paul White attempts to clarify this aspect of psychoacoustics and offers practical advice on the use of Aural Exciters and other harmonic enhancement systems.
Firstly the term Aural Exciter is a registered trade mark of Aphex so any other devices performing a similar function will carry a different name, though (against the wishes of Aphex) all these devices are generically referred to as 'exciters' by producers and engineers.
As the technique could probably be best described as being produced by a Psychoacoustic Harmonic Enhancement Device, I will call them all PHEDs for short to avoid upsetting anyone. Current PHEDs on the market are:
Aphex Aural Exciter Model B
Much mystery surrounds these PHEDs, though they are electronically quite straightforward devices, so the first step is to explain exactly what they do and how they do it. The word 'psychoacoustic' is often bandied about when talking about PHEDs, but all that it adds up to is that a PHED can give the clarity and brightness to programme material that simply cannot be achieved using conventional EQ circuitry. This doesn't sound too exciting on paper, but in practice the effect can be very dramatic and can even be used to salvage poor recordings as well as to enhance good ones.
If you have a recording that has lost some of its high frequency content, either due to close mixing or due to extensive track bouncing, it's not enough just to turn up the treble; after all, you can't boost what isn't there, you'll only increase the tape noise.
The PHED on the other hand actually synthesises these missing harmonics by processing part of the upper midrange frequency spectrum of the programme material and then adding a small proportion of this processed signal back into the main signal, and because some of these new harmonics may never have been present in the original sound, the result can sound even larger than life. Figure 1 shows a much simplified block diagram of the system.
The processing is done in a sidechain circuit, and after a variable gain stage to set the drive level, the signal is passed through a variable high pass filter so that the operator can select just how much of the frequency spectrum is to be processed.
This portion of the input signal is then passed to the harmonic generator section where musically related overtones are produced. Without giving too much away, the harmonics are produced by disturbing the symmetry of the waveform in a carefully calculated way so that the required new harmonics are generated without giving the impression of distortion, though technically speaking, distortion is what's being added. Also, time constants within the circuitry lend emphasis to the attack of percussive events so that, for example, the impact of a drum stick on ride cymbal can be lifted out of a mix. There are also other side effects caused by the filter circuitry which fortuitously add to the overall effect, but it must be remembered that these harmonics are added to the main signal in quite small proportions. This is enough however to fool our brains into perceiving extra loudness, clarity and stereo separation which is why this process is sometimes referred to as being 'psychoacoustic.'
An 'Exciter', and I use the term advisedly, is simple to use and may be patched into individual tracks or complete mixes as required. It is necessary to set up a reasonable signal level in the sidechain in order to optimise the working of the harmonic generator and this is done on the Aphex B using the 'drive' control.
Metering is very basic as nothing sophisticated is really necessary. The Aphex system uses a tricolour LED, which shows green, yellow and red (yellow being optimum), whilst other devices sport anything from a single 'peak' LED to a full bargraph display.
The filter usually precedes metering so it must be set up before finalising the drive settings. As previously intimated, the filter is a high pass type, usually with a 12dB/octave response, and may be switched, continuously variable or fixed, again depending on the type of processor.
"If it's got the name Harmonic, Psychoacoustic or Enhancer in its title, you'll know what it is."
With the filter frequency set to its highest position, only the very high sounds such as cymbals or toppy synths will be affected, whilst lowering the frequency will also affect voice, drums and even bass guitars.
Having decided on the 'drive' and 'filter' settings, the required amount of excitement is achieved by using the effect control, but be careful not to add too much; the effect is addictive and after a day in the studio, you could end up with a mix like a shower of razorblades, so take care!
This could be an endless list as I tend to use the effect on anything from Glockenspeil to bass drums but it is probably true to say that the applications can be split into two groups - enhancement and salvage.
Drum kits, (even electronic ones) sound better for a touch of excitement, especially close miked acoustic kits as these can sound a bit on the lifeless side and the cymbals always seem to lose some of their splash during recording.
Vocals not only sound clearer but their breathy quality is also emphasised giving an 'intimate' feel and the Buggles used this trick to liven up the girls vocals on 'Video Killed the Radio Star' some years back. The effect also appears to move sounds to the front of the mix without actually altering their level and, if the whole mix is processed, the speakers seem to take a step towards you when you punch in the effect button.
One obvious candidate for the salvage treatment must be the old Fender Rhodes electric piano, early models of which were difficult to record due to their somewhat muddy sound. EQ doesn't help here as there is no top to boost but the use of an exciter can make a world of difference.
Reverb or delay devices can be given a degree of sparkle that they couldn't otherwise achieve but no matter what you choose to process, the technique always seems to improve the separation and 'upfrontness' of sounds within a mix, a fact that the makers of advertising jingles exploit to the full.
To the home recordist, the feeling of having a dull, lifeless mix where all the cymbals have disappeared must be a familiar one and without the physical separation that can be achieved in a professional studio, it can happen all too easily. It is here that an 'exciter' becomes not only desirable but practically invaluable as all these missing or muddy sounds then start to come back to life. Of course you can't polish a turd as they say in the profession, but this way you can put a fair gloss on one.
It's always disappointing when a good sounding master cassette tape is copied on to tape with the inevitable loss of quality that always ensues. However, by patching an exciter into the chain, the signal can be livened up before it goes onto cassette to compensate for the deficiencies in the copying system. Anyone who runs a cassette duplicating plant without the facility of some sort of PHED would be well advised to try one out so that he can appreciate the difference that using one can make.
"After all, why should big studios have all the excitement?"
Apart from the Aphex Model B which retails at around £400, offering two independent channels of excitement, there is also the EXR projector which retails at a similar price, but uses fixed filters and operates in a somewhat different way.
Vesta fire produce the 'Driving Enhancer', previously called the 'Driving Exciter' which offers the same facilities as the Aphex B but is based on slightly different circuitry resulting in a slight, but not serious difference in perceived sound quality. At around £250, it's worth checking out.
Bandive announced a much simplified 'PHED' type of device at the Frankfurt Music Messe this year which was packaged to fall into line with their other Accessit products and, though no absolute price has been announced, it is expected to sell for around £60 excluding the power supply.
Offering true stereo operation and ganged controls, the only compromise seems to be the fixed filter frequency which is not a problem at this low price.
Rumour has it that even Paul Williams is in the process of cooking up something similar for his Tantek rack system, the main problem being in finding a name that doesn't upset Aphex. If its got the name Harmonic, Psychoacoustic or Enhancer in its title, you'll know what it is. After all, why should big studios have all the excitement?
It seems crazy to me that the Aphex Exciter system has been in existence for over ten years and the technology is only now becoming available to semi-professional and amateur recording enthusiasts.
It's difficult to classify the excitement principle as an 'effect' as it doesn't really change the character of your sound, it just makes it sound as though it's been recorded using more sophisticated equipment and/or techniques than may actually be the case.
One of the cheaper units on the market may well be useful to give that extra edge to a plate reverb and it would be left patched in permanently for this purpose.
The Americans seem to have gone for 'excitement' in a big way and Aphex sell units for such diverse uses as improving factory PA systems (where they are used to improve speech intelligibility), and political speeches where their psychoacoustic properties reputedly work wonders. Also, in the highly competitive world of radio and TV advertising, the technique of excitation is widely used to make commercials stand out from the competition as the perceived loudness of the soundtrack may be significantly higher than the level actually being transmitted.
Finally, if any of you ever risk losing your studio tan by going out to play live, you could do a lot worse than to put your music through a PHED to increase the subjective level and clarity of the lyrics without provoking feedback. You owe it to yourself to at least try one out.
Gear in this article:
Feature by Paul White
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