dbx 161 Manuel d'instruction - Page 11

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111

or sound

pressure

ratio.) if the number of "dB's " are referenced

to a given level, then the value of the dB number becomes spe-

cific.

"breathing"

effects, whereas very slow decay times may cause

moderate-level program which follows high- level program or pro-

gram peaks to be too low in level.

dBV

expresses a vo ltage ratio.

OdBV

is usually referenced to

1.0V RMS.

Thus

OdBV

1 V RMS, +6dBV

2V RMS,

+20dBV

10V RMS, etc.

Decoder

When a circu it restores an original program from a specially

treated version of that program, the

rcuit may be said to decode

the program

The equ

pment or circuit which performs this

function

known

as a decoder. Decoders must be used only with

programs which have been encoded by complementary

encoding

circuitry.

ypical decoders

nclude: FM tuners that use multiplex

decoders to

tract

left

and right stereo signa ls

from

left -plus -right

and left -minus -right signals, matrix quadraphonic

decoders

that

extract four channels of program from the stereo program on

encoded recordings, and dbx

decoders

t hat

retrieve

wide-dynamic

range programs

from the compressed programs on dbx -encoded

recordings.

SPL

expresses a Sound

Pressure Level ratio.

dB SPL

measure of acoustic pressure (loudness), not acoustic power,

which would be measured in acoustic watts

OdB SPL is

equal to

0.0002

dynes/square centimeter

(the threshold of

human

hearing

kHz). As with dBV, an

increase

of 6dB

SPL is twice the sound pressure, and an increase of 20dB

SPL

is an increase of 10 t

es the sound pressure.

dBm

expresses a power ratio. OdBm is

lliwatt

(.00

watts), or

0.775V

rms de

ivered to a 600 -ohm load

dBm

milliwatts,

.096V into 600

ohms

(V2

times OdBm),

10dBm=10

milliwatts,

or 2.449V into 600 ohms (3 .16 times

OdBm

) , etc. dBV and

dBm

differ by

2.21

when deal ing with

600-ohm circuits.

However, when the impedance is other

than 600 ohms

the

val

ue of dBV remains the same if the

voltage is the same, whereas the value of dBm decreases with

ncreas ing impedance.

De-emphasis

Pre-emphasis

De-emphasis and pre -emphasis are related processes

that

are

usually

done

to avoid audio noise

some storage or transmission

medium.

Pre-emphasis is a boost at specif ic higher frequenc ies, the

encoding part of an encoding/decoding

system.

De-emphasis

is an

attenuat ion at the same frequencies, a reciprocal decoding that

counteracts

the pre-emphasis. In dbx noise

reduction,

de -emphasis

is performed

the decoder (the play circuitry).

The

de-emphas is

attenuates high

frequencies,

thereby

reducing

tape modu lation

noise and

restoring

the original frequency response of the program

before it was dbx encoded. There are other types of pre-emphasis

and de -emphasis. For examp le, in FM tuners,

de-emphas

is is used

to compensate fo

special equa

zation (known as 75 -microsecond

pre-emphasis) applied at the stat ion's transm itter.

alone, without

any suffix, doesn't mean anything un less

associated w ith a reference. It may express the differ

ence between two levels. Thus, the difference

between

1 OdBV

and

15dBV,

the difference

between

OdBm

and

5dBm, and the difference

between

90dB SPL and

95dB

Lare

al I

differences of 5dB.

Decay

ime

Dynamic Range

Decay time may mean different

things, depending on the con-

text.

compressor's decay t

me is also known as its re lease time

or recovery time. After a compressor (or expander) changes its

gain to accommodate an incoming signal, and the signal is then

removed, the decay time is the amount of t ime required for the

circuitry

to return to "normal."

More precise ly, the decay time

he dynamic range of a program is the range of signal levels

from the lowest to the highest level. In equipment,

the dynamic

range

is the "space," in dB, between the residual noise

level

and

the maximum

undistorted

signal level.

program with wide

dynam

c range has a large variation from the softest to the loudest

passages, and will tend to be

life

ke than programs with

narrow dynamic range.

Encod

the interva

(usua

ly measured in microseconds or

milliseconds)

during which the compressing or expanding amplifier

returns to

of the norma

gain.

Very

fast decay times can cause "pumping"

When a circu

t processes an orig ina

program

to create a

specially treated version of that program, the circu

t may be said

to encode the program.

he equipment or circuit which performs

this funct ion is known as an encoder. Encoded programs

must

decoded only with complementary

decoding circu itry.

Typical

encoded programs

include: FM

multip lex broadcasts, matrix

quadraphonic

recordings, and dbx encoded record

ings.

Envelope

In music, the envelope of a note

describes the change in average signal

level

from initial attack

to peak level,

to decay time, to sustain, to release

time. In other words, the envelope

describes the level of the note as a

function of time. Envelope does not

Thtouth

ntt ,stheenn lOPt,

refer

frequency.

1he

9nllo1 n

w1t

h1n th• en velop

In fact, any audio signal may be said to have an envelope.While

audio frequencies rise and fall

in instantaneous

level

from

40,000 times per second, an envelope may take many mi

liseconds,

seconds or even minutes to rise and fall.

dbx

processing,

the

envelope is what

"cues"

the rms level detection c ircuitry

to com

press and expand the signal; the peak or average

level

of ind ividual

cycles of a note would be useless for level detection because

the

gain wou ld change

much

too rapidly for audibly

pleasi

ng sound

reproduct

on.

{Equalization)

EQ or equa

zation, is

intentional

change in the frequency

response of a circuit.

may be used for boosting

(in

creasing) or

cutting (decreas ing) t he

relative

leve

of a port ion of the aud ible

spectrum. Some EQ

used for ach ieving sound to suit persona

listening tastes, whi le other types of EQ are specifica

lly

designed

to correct for non

-Ii

nearities

in the system; these correct ive EQ

"curves"

include

tape

(NAB

IR) eq ualizat ion, and phono-

graph (RIAA)

equalization.

In a sense, the

pre-emphasis

and de-

emphasis used in dbx processing are special forms of equalization.

There are two common types of

EOualization

curves

(characteristics):

PEAKING

and

SHELVING.

Shelving

EQ is

used in

most

Hi -Fi

bass and treb le tone contro ls.

Peaking

EQ is

used in Hi-Fi midrange

tone

controls, in graphic equalizers, and

many

types of professiona

sound mixing equ

ment.

EQ is performed

equalizer, which may be a special

built

piece of equipme

t, or it may be no more than the tone contro

section of an amp

fier.

Graphic

equalizers

have

many contro ls,

each affecting o ne octave, one

-half

octave, or one-third octave of

the audio spectrum. (An octave is the interva

between a given tone

and its repet it ion eight tones above or be low on t he musical scale;

a note which

an octave

higher

t han another note is

twice the

frequency of the first note.)

Expander

An expander is an ampl

fier that increases its gain as the level of

the

input

signal

increases,

a characteristic

t hat "stretches"

the

dynamic range of the program (see "expansion").

An expander may

operate over the entire range of

nput

levels,

may operate only

on signals above and/or below a given level (the thresho ld leve

Expansion

Expansion is a process whereby the dynamic

range

program

material is increased. In other words, the

difference

between the

lowest and

highest

audio levels is "stretched"

into a w ider

dynamic

range. Expansion is sometimes used to restore dynamic range that

has been lost through compression

or limiting

done in the original

recording or broadcast; expansion

integral

part of com

pander-type noise

reduction

systems, including dbx.

pansion is

achieved with an expander, a

special

type of amplifier

that

increases

its

ga in as the leve

of the input signal

increases. The

amount of

expans ion is expressed as a ratio of the input dynam ic range

the output dynamic range; thus, an expander that takes a program

input with 50d

of dynamic range and yields an output

program

1 OOdB dynamic

range may be said to

have

a 1 :2 compress

ratio.

Fund

amental

A musical note

usual

ly comprised of a basic frequency,

plus

one or

whole-

num

ber

ltip les of that frequency.

The basic

frequency

is known

as the fundamenta

, and

the

mult

ipl

es are

known

harmonics

or overtones. A

pure

tone

would cons ist of o

nly

the fundamental.

Ground Compens

ated

Output

This

a sophist icated output circuit that senses the potential

difference

between the ground of

the dbx unit

and the shield

ground of unbalanced inputs to which the

dbx

unit is connected.

Ideally, the dbx unit and the input of t he following

device should

be at

the

same level (potential).

However, where grounding is not

"right"

(where

so-call

"ground

loops"

exi

st),

this

circuit calculates

the ground error and adds a correct ion signal to the high side of the

output,

thereby cancel ling

much

of the hum, buzz and

noise

that

might otherw ise have been introduced

ground

loops.