Sharp PG-C45S Operation Instrukcja Użytkownika Pobierz pdf (Strona 12)

N E W N E W S F R O M T H E I N D U S T R YT E C H N I C A L L Y S P E A K I N G . . .

12 ExtroNews 15.1 Spring 2004

tal signal processing, or DSP. Within the vast

possibilities afforded by digital processing,

the creation of ﬁlters capable of replicating

a near-exact compensating response is not

only possible, but generally straightforward.

DSP-based algorithms allow for continuously

adaptable functions that will compensate in

real time as sound pressure level is varied over

its normal excursion.

High speed digital signal processing, in all

its forms, provides the means for the best

implementation of loudness compensation

contouring in today’s sophisticated audio

systems. With tools like this, engineers must

return and study the fundamental knowledge

base developed by researchers like Fletcher

and Munson; et al. Taking a fresh look at

“what once was” will ensure us the best

shot at developing digital-based products

that perform to the closest approximation of

the original concept. But, no matter what, all

that you and I really care about is that when

we push that loudness button, the system

“sounds good.”

References

1. Fletcher-Munson deﬁnition. Rane

Corporation at http://www.rane.com/par-

f.html

2. Weik, M. H., Communications Standard

Dictionary, 1997, Chapman & Hall

3. Lord, H., Gatley, W., Evensen, H., Noise

Control for Engineers, Robert Krieger

Publishing, 1987

4. Ballou, Glen M., Handbook for Sound

Engineers, Third edition,

2002, Butterworth-Heinemann, Chapter 2,

Psychoacoustics, F. Alton Everest

low and high frequencies require additional

ampliﬁcation so the audio “sounds good.”

Since the ear’s frequency response is rela-

tively ﬂat at high sound levels, the compensat-

ing effect of the loudness contouring control

is not required. The loudness feature is a kind

of equalizing function that, ideally, should ad-

just itself to have greater compensation effect

at low sound pressure levels and less effect as

sound pressure increases.

From Figure 4, you can see that the amount

of power needed (green shaded area bounded

by L

curve) to compensate for low frequencies

is signiﬁcant. For this reason, in home theater

audio system design, it is not uncommon to

use fairly large, separate ampliﬁcation just for

the low frequency channel. The shaded area

within the high frequency range indicates rela-

tive compensation required for this portion of

the spectrum when at a lower volume level. At

high loudness levels, where the ear’s response

is nearly ﬂat, compensation requirements de-

crease to nearly zero as shown by the L

curve.

Loudness

Made Too Simple

The issue is whether the implementation

of the loudness control feature merely boosts

lows and highs using one

ﬁxed setting as some simplis-

tic designs might do; or is it

dynamic and capable of modi-

fying the amount of equaliza-

tion depending on the setting

of the volume control?

Historically, most loudness

controls were analog imple-

mentations using discrete re-

sistors, capacitors, and even inductors intended

to approximate the compensation curve (curve

in Figure 4) for the A-weighting function.

Most were designed around the volume con-

trol. Figure 5 illustrates one simple approach

using a volume control incorporating a fourth

tap located about halfway through rotation.

Resistor-capacitor networks, when switched

into the volume control circuit, provided ampli-

tude compensation. For really low cost circuits,

only the low end frequencies may have been

boosted. Or, perhaps the midrange was “cut”

to make it sound more like the level of the low

end. Certainly, analog implementations of the

loudness feature vary widely. Full compensation

for the A-weighted response requires a relatively

complex compensation network.

The basic approach with the circuit in Figure

5 is: 1) use C1 to boost high fre-

quencies where it is connected

across the top half of the volume

control when the loudness switch

is ON; 2) select the value of C2 so

its reactance is lower at high and

mid frequencies, and; 3) select R

so that high and mid frequencies

are attenuated; but, as frequency

decreases, the reactance of C2 will

rise and reduce attenuation of low

frequencies. This is a simple, low-

cost design built totally around per-

formance trade offs.

DSP: Just Made

for Loudness

Modern implementations of

loudness equalization circuits fall

comfortably into the realm of digi-

Figure 5: Simple analog loudness circuit example

Approx.

Loudness

Control

Dynamic

Range

Compensation Level

Decreasing as Listening

Level Increases

Loudness Contr

Compensation at

Low Levels

Very Little

or No,

Compensation

at High Levels

Frequency, Hz

-50

20 50 100 200 500 1000 2000 5000 10,000 20,000

-45

-40

-35

-30

-25

-20

-15

-10

-5

+10

+15

+20

+25

+30

+35

+40

+45

+50

Relative

Response, dB

Figure 4: Loudness function compensation range

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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