dbx 165A Manual de instruções - Página 15

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logarithm

of a

power ratio

20 t

mes the logar

thm of a voltage

or sound pressure

ratio.)

If the

number of "dB's"

are

referenced to

given level,

then the

value of the dB number becomes

specific.

dBv

expresses a

voltage

ratio. OdBv is

usually

referenced

0. 775V rms.

Thus,

OdBv=O. 775V, +6dBv=1.55V

(twice OdBv), +20dBv=7.75V

(ten times

OdBv),

etc.

dBV

expresses a voltage ratio and

similar

dBv, but

OdBV is

usually

referenced to 1 V rms. Thus, OdBV is 2.2dB

higher than OdBv.

dB SPL

expresses a Sound

Pressure Level

ratio. dB SPL is a

measure of acoustic

pressure

(loudness), not acoustic povver,

which

would be measured in acoustic watts.

OdB

SP Lis

equal to 0.0002 dynes/square centimeter (the threshold of

human

hearing

1 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 times

the sound pressure.

dBm

expresses a power ratio. OdBm is 1 milliwatt

(.001

watts),

0.775V

rms delivered to a 600-ohm load. +3

dBm=2 milliwatts,

1.55V into 600

ohms (twice OdBm),

+10dBm =10 milliwatts,

or 7.75V

into

600 ohms (ten

times

OdBm),

etc. dBV and dBm are numerically

equal

when dealing with 600-ohm circuits. However.

when

the

impedance is other than 600 ohms, the value of

dBV

remains the same if the voltage

the same, whereas the

value of dBm decreases with increasing impedance.

alone, without

any suffix, doesn't mean anything

unless it is associated with a reference.

may express the

difference between two levels.

Thus,

the difference

between 1 OdBV and 15d BV. the difference between

OdBm and 5dBm, and the difference between 90dB SPL

and 95dB SPL are all differences of 5dB.

Decay Time

Decay time may mean different things, depending on the con-

text. A compressor's decay time is also known as its release time

recovery

time. After a compressor (or expander) changes its

gain to accommodate an incoming signal, and the signal

then

removed,

the decay time is the amount of time required for the

circuitry

to return to "normal."

More precisely, the decay time

is the

interval

(usually measured in microseconds) during which

the compressing or expanding amplifier returns to

90%

of the

normal gain.

Very

fast decay times can cause "pumping"

"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.

Decoder

When

a circuit restores an original program from a specially

treated version of that program, the circuit

may

said to

decode

the program. The equipment or circuit which performs this

function

is known

as a decoder. Decoders must be used only with

programs which

have

been encoded by complementary

encoding

circuitry.

Typical decoders include: FM tuners that use multiplex

decoders to extract left and right stereo signals 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 that retrieve wide-dynamic

range programs from the compressed programs on dbx-encoded

recordings.

D&-emphasis

Pre-emphasis

De-emphasis and pre-emphasis are related processes that are

usually done to avoid audio

noise

in some storage or transmission

medium. Pre-emphasis

a boost at specific higher frequencies, the

encoding part of an encoding/decoding

system. De-emphasis is an

attenuation at the same frequencies, a reciprocal decoding that

counteracts the pre-emphasis.

dbx noise reduction, de-emphasis

is performed by the decoder (the play circuitry).

The de-emphasis

attenuates high frequencies, thereby reducing tape modulation

noise and restoring the original frequency response of the program

before

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

and de-emphasis. For example, in FM tuners, de-emphasis is used

to compensate for special equalization

(known as 75-microsecond

pre-emphasis) applied at the station's transmitter.

Dynamic Range

The 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 thr residual noise

level

and

the maximum undistorted signal

level.

A program with wide

dynamic range has a

large

variation from the softest to the loudest

passages, and

will

tend to be more lifelike than programs with

narrow dynamic range.

Encoder

When

a circuit processes an or iginal program to create a

specially treated version of that program, the circuit

may

said

to encode the program. The equipment or circuit which performs

this function

known as an encoder. Encoded programs must

decoded only with complementary

decoding circuitry.

Typical

encoded programs include: FM multiplex

broadcasts, matrix

quadraphonic

recordings, and dbx encoded recordings.

Envelope

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

,.,,. ....... '""'

,m,lope,

refer

frequency.

theJl.t,,.i i1w

ilhin

~en11tlope

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

all audio frequencies rise and fall in instantaneous level from 40 to

40,000

times per second, an envelope may take many milliseconds,

seconds or even minutes to

rise

and fall. In dbx processing, the

envelope is what "cues" therms level detection circuitry

to com-

press and expand the signal; the peak or average level of individual

cycles of a note

would

be useless for level detection because the

gain would change

much

too rapidly for audibly pleasing sound

reproduction.

(Equalization)

EQ or equalization, is an intentional

change in the frequency

response of a circuit. EQ may be used for boosting (increasing) or

cutting (decreasing) the relative level of a portion of the audible

spectrum. Some EQ is used for achieving sound to suit personal

listening tastes, while other types of

are specifically designed

to correct for non-linearities

in the system; these corrective EQ

"curves" include tape (NAB or CCIR) equalization, 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

EQualization

curves

(characteristics):

PEAKING and SHELVING.

Shelving EQ is

used in most Hi-Fi bass and treble tone controls. Peaking EQ is

used in

Hi-Fi midrange tone controls, in graphic equalizers, and

many types of professional sound mixing equipment.

EQ is performed by an equalizer, which may be a specially built

piece of equipment, or it may be no more than the tone control

section of an amplifier. Graphic equalizers have many controls,

each affecting one octave, one-half octave, or one-third octave of

the audio spectrum. (An octave is the interval between a given tone

and

its

repetition

eight tones above or below on the musical scale;

a note which is an octave higher than another note is twice the

frequency of the first note.)

Expander

An expander is an amplifier that increases

its

gain as the

level

the input signal increases, a characteristic that "stretches"

the

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

An expander may

operate over the entire range of input levels, or it may operate only

on signals above and/or below a given

level

(the threshold level).

Expansion

Expansion is a process whereby the dynamic range of program

material is

increased.

In other words, the difference between the

lowest and highest audio

levels is

"stretched"

into a wider 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 is an integral

part

of com-

pander-type noise reduction systems, including dbx. Expansion is

achieved with an expander, a special type of amplifier

that increases

its gain as the level of the

input

signal increases. The amount of

expansion is expressed as a

ratio

of the input dynamic range to

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

input

with 50dB of dynamic range and yields an output program

of 1 OOdB dynamic range may be said to have a

compression

ratio.

Fundamental

A musical note is usually comprised of a

basic

frequency,

plus one or more whole-number

multiples of that frequency.

The basic frequency is known as the fundamental,

and the

multiples are known as harmonics or overtones. A pure tone

would consist of only the fundamental.

Ground

Compensated

Output

This

is a sophisticated 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

connected.

Ideally, the dbx unit and the input of the following

device should

be at the same

level

(potential).

However, where grounding

not

"right"

(where so-called "ground loops" exist), this circuit calculates

the ground error and adds a correction signal to the high side of the

output, thereby cancelling much of the hum,

buzz

and noise that

might otherwise

have

been introduced by ground

loops.