Ashly DPX-100 Operating Manual - Page 11

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Operating Manual - DPX-100 Graphic Equalizer - Compressor/Limiter

9. DESIGN THEORY

Graphic Equalizers: The Basics

While most graphic equalizers look very much

the same, there are several important differences in the

circuitry used to implement various designs.

Perhaps the major differences are in the filters.

Some equalizers use a filter made of a capacitor, an in-

ductor, and a resistor, or "RLC" filter. The advantage

here is simplicity, but the real disadvantage is the induc-

tor itself. An inductor is a coil of wire with a core of

some sort. Inductors are susceptible to hum fields and

they are large and expensive.

Other equalizers use the same basic approach, but

replace the inductor with a "simulated inductor", which

is actually a circuit comprised of an amplifier, a capaci-

tor, and a couple of resistors. This adds parts but is less

expensive than a real inductor. The problem with this

approach is that simulation is less than ideal; it produces

an inductor with high resistive loss resulting in poor curve

shape when used in a filter.

Another problem with "RLC" designs is that large

capacitors must be used for the lower frequency filters,

limiting the choice to large, expensive non-polar types or

electrolytic capacitors with poor audio performance. Also,

when this filter type is combined with a potentiometer to

adjust the equalization, the resistance of this pot affects

the "Q" of the filter so that a little equalization produces

a much broader curve than a lot of equalization.

The other filter approach is a true bandpass fil-

ter. This can be made with no inductors and more practi-

cal sized capacitors; the "Q" is easily set and remains

constant, and the parts count is reasonable. there are sev-

eral types of bandpass filters suitable for this job. Ashly

uses a "Q" enhanced Wein-bridge filter. Because it is a

symmetrical design using matched tuning components,

the "Q" is easily set and is very stable.

In designing a graphic equalizer, a selection of

filter sharpness must be made. More sharpness (higher

Q) produces less filter overlap and tighter control over an

individual band, but also causes ripple in the frequency

response when many filters are boost or cut together to

produce a flat response. We feel that the graphic

equalizer's primary use is for "voicing" and tone control,

and have set our filter sharpness to produce a maximum

of 1dB ripple.

The summing system in a graphic equalizer is

also important. Since there are a number of filters which

combine to produce the overall response, it is important

that the filters not interact (they WILL overlap, but the

response of one filter should not modify the response of

another). Ashly uses an "interleaved" summing system

where every other filter uses the same summing amplifier

so that adjacent filters never share the same drive and

feedback signals. This allows the filters to maintain their

natural response.

Compressor/Limiters: The Need For Gain Control

The human ear excels in its ability to detect an

extremely wide range of loudness levels, from the quiet-

est whisper to roar of a jumbo jet. When we attempt to

reproduce this dynamic range, by means of amplifiers,

tape recorders, CD players, or radio transmitters, we run

into one of the fundamental limitations of these electronic

media: limited dynamic range. Amplifier dynamic range

is quite good, and is adequate for most musical program

material. However, some types of audio equipment, such

as cassette tape recorders, have a very narrow useful dy-

namic range.

What is it that compromises the dynamic range

of this equipment? The useful operating region of a piece

of audio equipment is squeezed in between noise and dis-

tortion. As program level decreases, it approaches what

is known as the "noise floor", and if the volume of the

program material goes lower still, it is engulfed by the

noise. The noise floor, or minimum constant noise level,

will consist of hiss, hum, transistor noise, tape hiss, buzz

and whatever noises are inherent in the medium. When

the program level is considerably higher than the noise

floor, our hearing masks the noise, and it is not a prob-

lem. However, when listening to very quiet sections of a

program for example, a pause between movements of a

string quartet the noise can become very bothersome.

At the other end of the loudness spectrum, the

limitation on dynamic range is usually distortion, either

in the form of amplifier overload, tape saturation, or A to

D clipping. In most transistorized equipment, the transi-

tion from clean, undistorted operation to severe distor-

tion is very abrupt. Therefore, it is common practice to

operate a piece of equipment at a level that is somewhat

below the distortion point, leaving a margin of safety for

unexpected, transient volume peaks in the music. This

safety margin is known as headroom, and may range from

10 to 25 dB. Lowering our standard operating level to

leave ourselves some headroom helps prevent distortion,

but at the same time it moves our average program level

closer to the noise floor, thereby compromising signal-

to-noise performance. It becomes apparent that to get most