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Page 25Aphex Systems Ltd. Model 230

Appendix E: Back To BASS-ICS

by Marvin Caesar

Think of a sound system with incredible bass. Are you imagining

the sound of the bass or are you imagining how the bass would

feel? If you are imagining sound, then you are thinking too high

in frequency. The bass referred to in this article causes a visceral

response.

Ask any non-professional to listen to a sound system. Reduce the

bass, listen, then reduce the high frequencies, return the bass to

flat and listen again. The listener will probably decide that listening

to the system with attenuated highs is much preferable to listening

to the system with attenuated lows.

David J. Holman, a friend of ours and a recording engineer and

producer of several platinum albums had this to say about bass:

“Bass is the most important thing on a record. Without it you

don’t have a record. People think bass is just the bass instruments,

but it is also the “wood” in an acoustic guitar and the “chest” of

a male vocalist. With a good low end, a synthesizer sounds rich

and expensive. Without a good low end, it just sounds like more

digital nonsense.

Maintaining quality bass over a wide dynamic range is made

difficult by subjective human hearing response. As shown in the

constant-loudness curves published by Harvey Fletcher and Wilden

Munson, the threshold of hearing at the extreme low frequencies

requires approximately 60 dB higher sound level than the thresh-

old of hearing at 1kHz. The hearing curves gradually become fairly

flat at listening levels around 90dB-SPL, and remain so above that

level.

This explains the well-known phenomenon that occurs when play-

back level is reduced: the bass seems disappear.

Over the years, several methods have been used to overcome this

phenomenon. The most common method has been fixed equaliza-

tion. We’re all familiar (maybe too familiar) with the disco smile on

graphic equalizers: lowest and highest frequencies pushed up, the

midrange sagging. By adding a fixed amount of gain on the low

end, this method helps overcome the disappearing bass phenom-

enon. The problem caused by this added gain, however, may be

more serious than those solved.

The principle of superposition is that the motion of electrons in

a circuit, or of air around our ears, is single vector sum of all the

forces at work. Different electrons don’t alternate at different

frequencies; of the electrons in the circuit have the same instan-

taneous potential and direction. Figure 1 shows a high frequency

riding on a low frequency. Although frequency is not usually

shown this way, this is just the way the superposition works, in a

wire or at our eardrums.

Headroom is the range, in decibels, between present operating

levels and the crash point, those levels that would damage equip-

ment or distort signal. There is no such thing as infinite headroom,

electronic or acoustic. If the low frequencies are increased in level,

even though the boosted frequency band never reaches the crash

point, the higher frequency peaks are closer to or exceed the crash

point.

As the boosted frequency band gets lower in frequency, this phe-

nomenon gets more pronounced. Imagine adding a direct-current

offset into an amplifier, and how quickly this change would send

the signal into distortion. Subharmonic generators are sometimes

used for bass enhancement. These devices read signals in the

mid-bass frequency area and synthesize artificial signals an octave

below these signals. The use of subharmonic generators also

causes an increase in overload problems, with overloads taking

place at higher frequencies.

Many systems are biamplifted, triamplified or quad-amped. Such

systems might avoid the peak overload characteristics of wide-

band systems, but adding a fixed amount of low frequency gain

or increasing subharmonics can cause overload problems within

the bass band(s).

Let’s ignore overload problems for a moment and look at the sonic

effects of these methods. Adding bass boost through equalization

or subharmonics might be quite musical when the bass needs the

enhancement. These methods, however, have no way of determin-

ing when the bass does not need the enhancement and when it

does. If the bass content in the source material is sufficient, adding

fixed equalization or subharmonics would surely make the output

too bottom-heavy.

That brings us to yet another method of bass enhancement:

multiband gain control. The signal is divided into frequency bands,

and each band is separately processed through an automatic gain

control circuit, which narrows the dynamic range in the band.

Whether the outputs of the separate compressors are summed

again into a single band or sent directly to amplifiers, this method

helps keep the bass within a limited time dynamic range, increas-

ing level when the input is low and decreasing level when it is high.

The multiband feature reduces spectral intermodulation (one part

of the spectrum modulating another), which could be introduced

by a simple wide-band device.

Although this method maintains a limited dynamic range, by

definition it changes the dynamics of the signal. Thus the sound

of the output will depend upon compression ratios, attack and

release characteristics and threshold. Depending on how aggres-

sively the signal is being compressed, it may sound totally mashed.

Furthermore, if there is far greater gain reduction in one band than

in others, the total integrity of the entire mix may be thrown out

of balance.

A NEW SOLUTION

What to do? Enter Big Bottom (sideways, if it is a narrow door-

way).

Aphex’s Donn Werrbach, the inventor of the Compellor, the

Dominator, and several versions of the Aural Exciter, comes from a

broadcasting background. In that competitive industry, each sta-

tion tries to produce as attention-getting a sound as possible while

remaining within legal modulation limits. Given the constraint of

extremely limited headroom, Donn searched for a method that

would enhance the low frequencies musically and naturally but

without vastly increasing peak output. He found the answer in

time, literally and figuratively. He found that by making a copy of

incoming low frequency information, delaying it by a set amount,

and adding it back as a constant-level enhancement signal, he was

able to achieve a dramatic increase in the perception of the bass

without a corresponding increase in peak output.

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