AURAL EXCITER:
The Aural Exciter is based on a patent by Curt A. Knoppel, conceived in 1975. His basic idea was to restore or recreate in a controllable way, the high-frequency harmonics that were missing from a sound material due to Dolby systems or due to degraded signal paths and audio devices. Unlike Equalization (EQ) that can change the balance of the existing high-frequency components, the Aural Exciter simply restores the missing harmonics and delivers more clarity and details to the recordings. This is happening by extracting a further low amplitude signal from the original signal, with frequency and amplitude dependent, phase-shifted, odd and even harmonics (http://patft.uspto.gov/). This new signal is added to the original signal before the output.
The whole system works by splitting the input signal into two separate paths, one path being passed directly to the output while the other being passed through an exciter circuit and an amplitude attenuator. The signal traveling in the exciter circuit, is first passed through a high-pass filter and then through a harmonic creator. The high-pass filter is adjustable and used to determine the frequency range from which the extra harmonics are generated, usually between the range of 800Hz and 6 KHz. The harmonic creator circuit, presumably a distortion device, is designed to create low order harmonics of frequency already present, in accordance with the amplitude level of the output signal from the high pass filter. These new harmonics do not increase significantly to the output level as they are low in amplitude and phase-shifted relative to the original signal. The amount by which the excited signal is attenuated is dependent by the nature of the audio information being processed (http://patft.uspto.gov/). Usually the attenuation is between 20% and 70% of the amplitude of the original signal. Lastly, both signals, the excited and the original, are being combined to an internal mixer and then send to an amplifier to reach the output. Below is a simplified diagram of the Aural Exciter circuit.
The operation of an Aural Exciter is rather straight-forward. There are basically three controls in the front panel. The Tune control, the Harmonics control and the Mix control. The Tune control essentially controls the high-pass filter, thus sets the range over which the new harmonics will be generated. The range can be set between 800 Hz and 6 KHz. For example, a setting of 4 KHz will only generate harmonics above the 4 KHz frequency range. The Harmonics control relates to the amount of harmonics generated. Usually, complex sounds like complete audio mixes benefit from low settings while for individual sounds like a guitar or a snare, a higher setting could be more sufficient to make the instrument more ‘present’ in the final mix. The Mix control determines the amount of harmonics introduced to the output. A very high setting might result to an over-processed or even distorted output. If this is not what we are aiming for, then a moderate setting will be more suitable for most sound materials. An additional very helpful feature is the bypass function which can be found in the shape of a button. By pressing it, it is possible to switch the signal flow from being passed through the exciter or go directly to the output without any processing. This is quite important because as listeners we normally like music being played loud, thus is easy to go off limits with the Exciter. With the bypass function we can check easily and at anytime the amount of enhancement we apply to the sound material and correct our mix if necessary.
Despite the original idea of using the Aural Exciter to compensate in loss of quality of analogue recordings, through the years the Exciter found many other uses in audio applications and at the same time expanded its qualities. The latest model from Aphex now includes a low frequency enhancement also, using the same technique as the early models but with a Low Pass filter http://www.soundonsound.com/sos/Feb02/articles. Aural Exciters are particularly good to use with individual instruments. They can fix mistakes made during the recording process such as bad microphone placements or even, as some people suggests, enhance the quality of originally poor sounding instruments like cheap guitars or violins. Voices could sound crispier and fuller, brass instruments could cut better within a song mix and drums could gain more clarity and space. Any instrument, even old analogue synthesizers, with a low frequency bandwidth could come to life by passing it through an Aural Exciter.
Another less obvious use of the Aural Exciter can be found in relation with data-reduction music formats like MP3s. A music material passed through an Exciter, preferably before encoded to a data-reduction format, could retain more details from the original recording and increase its quality on playback. The same effect is possible when copy or playback music material from analogue tapes. Although analogue tapes are not very common in our days, there are lots of recordings made in the past that could benefit from an Aural Exciter to improve their sonic quality in the future. As Aphex suggests (www.aphex.com) the Aural Exciter could also be used in live sound situations as an additional device to control sound feedback. The fact that the Exciter can increase the perception of loudness without a significant increase of the overall level, allow us to increase the sound level more than the normal thus gaining more headroom before feedback. It is also suggested that in certain live sound situations it could replace some of the functions of EQ in order to design a sonic space that could penetrate more effectively to a live audience [due to emphasized HF].
The Aural Exciter is a clever device that if used wisely can result to a great increase of sound quality in audio recordings or playback. However, extreme settings can seriously alter the original sound and sometimes even moderate settings could leave an unwanted sound signature to the sound processed. Nevertheless, it is an easy to use and very effective tool that although it has been designed in the era of analogue recording, it can still find use in many applications in the digital age of today.
Dimitris Barnias 2004
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