Magazine Archive

Home -> Magazines -> Issues -> Articles in this issue -> View

Behind The Aphex Aural Exciter

Marvin Caesar: The Aphex Story | Marvin Caesar

Though psychoacoustic enhancers are now made by several different companies, Aphex is where the concept of Aural Excitement was born. Paul White talks to president Marvin Caesar about the company's beginnings and his thoughts on the exciting future ahead...

I caught up with Marvin at the recent Audio Engineering Society (AES) show, held in March at Berlin's International Congress Centre. Given that most studio users know the Aphex name through their long-established range of Aural Exciters, I was keen to talk with Marvin about the roots of Aural Excitement, and about the latest in the company's range, the Type C II with Big Bottom bass enhancement.


Is it true that the Aural Exciter principle was discovered by accident when a build-it-yourself valve hi-fi amp kit went wrong?

"The story I heard is that Kurt Knoppel, the original designer, was inspired by a Heathkit stereo valve amplifier kit that he'd built incorrectly. There was one channel built correctly but the wiring harness connecting to the other channel was flipped so that when it was installed, the valves were run greatly under voltage. This put the valves in a non-linear range and there was terrible distortion at the output, but what was interesting is that, when the two channels were mixed together, the non-distorted channel actually sounded clearer. He spent many years and lot of false starts trying to figure out exactly what was going on, but he finally had a workable system in 1975 when I joined forces with him and we went out and started marketing."

Looking at it, there seems to be more to the exciter than simply adding a distorted signal to a clean one. There's an element of filtering in there, too.

"Yes, the idea is to do a frequency-dependent phase shift which is imparted by a simple high-pass filter, and then the harmonic generation is based on — without giving away any secrets — amplitude and some wavefront dependent characteristics. So it's not just amplitude dependent. This wavefront dependent characteristic is the subject of a new patent — the original model was simply amplitude dependent. One side of the waveform was compressed and the other side expanded, which is what created the harmonics. But the idea was that the distortion was musically related to the original signal and only added back in small quantities. Again, some people have over-used the Aural Exciter over the years, especially in the early days when we charged $30 a minute for it when it was used on records. People figured they had paid for it so they were going to use a lot of it. Mastering engineers cursed us because they couldn't cut the records — they were so hot with the Aural Exciter effect. But when people learned to use it effectively, there was never a problem."


How have you managed to improve the Aural Exciter over the years?

"To look at the history of the Aural Exciter, the first circuit had its own characteristic because it was a valve circuit. It introduced a lot of non-linearities which we didn't fully understand when we built our first solid-state models. Since that time, we've got closer and closer to a true emulation of that original tube circuit. We've also made further refinements to take away the non-musical non-linearities. So, the whole idea is to find a way to expand the transient nature of the input signal while not disturbing the non-transient information. In other words, if you look at a waveform and you see a transient which has an interesting harmonic structure based on the fundamental, we try to build that up based on what already exists in the input signal. Simply, we're expanding the difference between the transient information and the non-transient information. This provides much more detail and much more air without changing the spectral quality of the audio."

Your latest model is also your lowest cost model and includes a new bass enhancement system. How did the Big Bottom come about?

"The proper use of the Aural Exciter should never diminish the bass end of a piece of music — it's only when people over-use it that the high end starts to overpower the bottom end. As a matter of fact, when the Aural Exciter is used properly, it seems to enhance the bass because the harmonics of the bass sound are also enhanced. If the programme material has a muddy bass, the bass seems to tighten up and tends to be stronger.

"The Big Bottom bass enhancement circuit is something that's been knocking around in the chief engineer's head for quite some time and he built different prototypes over the years. We finally got around to building it and making a product out of it — the name was the product of excess alcohol and an over-vivid imagination! The market came to us and asked us to build a product that could do something for the low end. There are a number of other so-called enhancers that have hit the market, and to be effective, they vastly change the spectrum towards the top end. As a consequence, they had to do something about the low end. Even though they were based on simple filters, people responded to having the additional control. They didn't seem to realise that they had that same facility on their mixing board or on a graphic EQ. Following our normal design philosophy, which meant we had to do something different, we came up with the Big Bottom.

"The proper use of the Aural Exciter should never diminish the bass end of a piece of music — it's only when people over-use it that the high end starts to overpower the bottom end."

"Big Bottom is a low-pass filter followed by a high ratio compressor. The output of the compressor, which has a fast attack and fast release, is mixed back with the original source with a certain phase relationship. The effect is dramatic. You can get to a point where the bass is nicely enhanced but doesn't sound effected or muddy. That seems to be the point where there's no increase in peak output; when you go beyond that point, that's when you start getting the bad effects in terms of muddiness."

On the other hand, deliberately over-using the Big Bottom effect could be useful in creating powerful basslines for dance music, for example.

"Give anybody a tool and they'll find a way to abuse it, but the advantage of this enhancement system is that the lower the quality of the playback source, the more dramatic the improvement. Similarly, the way the Aural Exciter section enhances transient details improves sound separation and gives an impression of greater stereo spread. These fine details can be lost in digital recording, especially if the recording is made at a low level. People say, 'I have digital so I don't need an exciter now.' They're wrong — that's exactly the time they need an exciter."

With the future being digital, do you foresee a digital Aural Exciter in the near future?

"We're not satisfied that what we do can be reduced into the digital domain, and certainly not yet in real time. Listening to gain control processors, they all seem to introduce a buzz or a haze across the top end. Of course, we're talking about devices that are supposed to be aggressive, but from the engineers I've spoken to, you don't seem to be able to back out of that buzz. I'm sure that as time goes on and we get faster and faster processing or parallel processing, you may able to approach what's going on in a Compellor or perhaps a Dominator, but when the analogue version is more cost effective and works better, and can be used at the point prior to where the signal enters the digital domain, what's the incentive to go digital? We could build an analogue product with a converter built in, but there are so many good converters on the market that it wouldn't make sense to get into building A to D converters.

"We have been asked to do digitally-controlled analogue boxes to implement gain control because the engineers out there aren't satisfied with currently available digital systems. One of the areas where digital scores is in its resettability, so one of the things that we've done is to start to use digital control of analogue circuitry. The Digicoder is a stereo encoder for FM multiplex and we're using that concept quite effectively to produce a 'set and forget' device. All other stereo devices need to be tweaked to maintain their separation, but the Digicoder is absolutely repeatable and stable."

"Give anybody a tool and they'll find a way to abuse it, but the advantage of this enhancement system is that the lower the quality of the playback source, the more dramatic the improvement."


How much further do you feel you can take the analogue Aural Exciter concept? Do you think you might ever build a valve exciter for the audiophiles?

"Anything is possible, but that's not on the drawing board — we have other plans. The application for the Aural Exciter has grown and there are a number of companies that are licencing the Aural Exciter. One interesting thing is that it seems to work very well for the hearing impaired. In the US last year, there was an act passed called the Persons with Disabilities act which means that, in every public place, you have to accommodate people with handicaps and people with hearing impairments have a right to have a personal PA system. Gentner Engineering in the US is manufacturing an assisted listening device. But people who have a hearing impairment need more detail and more clarity, especially in a high ambient noise field. So Gentner licensed the Aural Exciter, and by transmitting audio treated with the Aural Exciter, people report greater clarity and intelligibility. We have another company, Mackenzie, who use it for the message on hold on telephones. You'd think the effect would be all gobbled up by the limited bandwidth of the telephone system, but in practice, the result is quite dramatic. There's also interest in using it for in-car system where the music has to compete with the road noise. The ambient noise level in a car is around 80dBA so you have a very small window between losing the sound in the road noise and being in pain from excessive level. The Aural Exciter tends to cut through the noise and the Big Bottom gives a tremendous amount of bass information without the need for a sub-woofer — you're going to see the Aural Exciter and Big Bottom in systems coming from car manufacturers in the not-too-distant future."


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 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.

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 are 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 devices is that they can be used 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 miniscule. However, overprocessing a signal in this way can result in a harsh, fatiguing, sound, making it imperative to use the treatment in moderation, as Marvin Caesar points out.

More with this artist

Previous Article in this issue

Retro Rocker

Next article in this issue

Hard & Fast!

Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Sound On Sound - Aug 1993

Interview by Paul White

Previous article in this issue:

> Retro Rocker

Next article in this issue:

> Hard & Fast!

Help Support The Things You Love

mu:zines is the result of thousands of hours of effort, and will require many thousands more going forward to reach our goals of getting all this content online.

If you value this resource, you can support this project - it really helps!

Donations for December 2021
Issues donated this month: 0

New issues that have been donated or scanned for us this month.

Funds donated this month: £4.00

All donations and support are gratefully appreciated - thank you.

If you're enjoying the site, please consider supporting me to help build this archive...

...with a one time Donation, or a recurring Donation of just £2 a month. It really helps - thank you!

Small Print

Terms of usePrivacy