Boss Audio Systems BV8.5GA User Manual download pdf (Page 44)

THE FCC

now

allows FM

stations

to use

a combination

of

Dolby

B -Type

noise reduction

and reduced

pre-

emphasis in

their

signal

process-

ing.

Here's

why the new

proposal

was

accepted

and

how

it

affects the

per-

formance

of

your

FM tuner.

For years,

many

responsible

people

in the

broadcasting

field have

pri-

vately

or

publicly

suggested

a

reduc-

tion in pre-

emphasis. This

process

was

introduced

in the

early days

of

FM.

It

boosts the

level

of high -

frequency

content

by passing

the

baseband

signal through

an RC

high

-pass filter

with a

75

-ps time

con-

stant, a break frequency

of

2.123

kHz,

and

a 6 -dB

/octave

slope. In this

way,

the

S/N (signal

-to-

noise) ratio

of the

FM

channel is

increased.

At the re-

ceiver, de-

emphasis must

be intro-

duced to

balance the

spectral

content

of

the received

signal as

in

the

original

information.

The

de-

emphasis

net-

work is

a

simple low -pass

RC filter,

with a -6

-dB

/octave

slope, a 75

-ps

time

constant

and 2.123

-kHz

break

frequency.

The

time

constant

was

chosen

at a time

when

there

was

mar-

ginal

high-

frequency

content,

by

today's

standards,

in recorded

sound.

A low

break frequency

was

required

to

radically

boost

the highs

to

prevent

them

from

being

lost

in

the high -

frequency

noise

that is

characteristic

of

FM.

Today,

we have

reached

a point

where the recording

media

can give

much flatter

frequency

response.

Anyone

who has

compared

an

old and

a

recent

recording

of a certain piece

will note

a dramatic increase

in high

-

frequency

content,

or a

more

"bril-

liant"

sound.

This

presents

problems

to FM

broadcasters,

though.

They

are

constrained

to

a

frequency

deviation

of

75

kHz,

and with increased

high -

frequency

content, they must

either

lower

the

modulation

index,

or use

high- frequency

limiting.

Stations

with

integrity

will not

color the

sound

of the

modulating

signal,

and thus lose

some

of

their

potential

audience to less

scrupulous,

but

louder

competitors

who

shape the

spectral

content

of

the

baseband

signal to

achieve higher

levels

of

modulation

while

staying

within the law.

The

proposal

by Dr.

Dolby

will re-

dress the

inequities

of

the

situation. If

the level

of

pre-

emphasis

were re-

duced,

using

a

25

-ps

time

constant

and

6.36 -kHz

break frequency,

sta-

tions

could

modulate

their

carriers

more fully,

and their

signal

strengths

46

NEW

DOLBY-

ENCODED

FM

BROADCASTS

would increase

an average

of

4

dB.

However,

the

sound

from

the millions

of

FM

tuners already

in use, with

their

75 -ps de-

emphasis networks,

would

be

exceedingly

dull.

Obviously, the

FCC

could not

allow

such

incompati-

bility to

be introduced.

If FM

stations

simultaneously

change

the time

con-

stant

of

their pre-

emphasis networks

to 25

psand incorporate

Dolby

B -Type

noise reduction,

an interesting

situa-

tion

arises.

The

reduction

of high- frequency

brilliance

caused

by pre-

emphasis re-

FM

TUNER

OUTPUT

ZIK

3900pF

RECORDER

OUTPUT

75Ps 25M$

FM CAL

I0K

TAPE

CAL

10K

20K

FM

TAPE

FM

compensator provides

choice

of 75-

or 25 -ps

time

constant.

duction

is almost

completely

counter-

balanced

by

the

effects

of B -type

compression.

In low-to-

medium-

quality

equipment, there is

no notice-

able

difference,

and the

increase

in

the

highs

noticed from

medium

-to-

high-

quality

equipment

can be com-

pensated for

by a

slight

reduction

of

high response

of

the

amplifier

using

the treble

control(s).

Brilliance

is

a

subjectively meas-

ured

quantity,

and to

ascertain

the

ef-

fects

of

B -type

encoding,

several

sta-

tions

switched

unannounced

to the

new

system. Many listeners

wrote let-

ters

praising the increased

quality

of

the

sound. Not

one complaint

was

re-

ceived.

For

those

with

conventional

receiv-

ers and no

noise reduction,

there

will

be a reduction

of high- frequency

dis-

tortion,

and

an

increase

in

brilliance

and program

level.

Listeners

equipped

with 25

-ps

de- emphasis

and

Dolby

B-

Type

noise reduction,

as

produced

now

by Dolby

licensees

as per Dolby's

advice,

will receive

four

benefits:

im-

proved

signal

to noise ratio;

full

pro-

gram

dynamic range,

even

at high fre-

quencies;

better reception

in

weak

-

signal

areas; and reduced

likelihood

of SCA

interference.

Some tuners

and receivers

are

al-

ready

equipped

to

receive

the 25

-ps

Dolby

encoded FM

broadcasts. If

a

75/25

switch is

on

the

back

of the unit

it

should

be set at 25

during reception

of all

subsequent Dolby

FM

broad-

casts. Otherwise,

ordinary B -Type

noise

reduction

units

can

be easily

adapted

to take

advantage

of the new

broadcast

technique

by installing

the

FM

Compensator

shown in the

dia-

gram.

Only one channel is

illustrated;

two

will be required

for

stereo recep-

tion. Anyone

may

build a compensator

for

personal

use

or

manufacture

it for

resale

without

a

license. The

parts are

inexpensive

and may

be

obtained

from

any

electronics

supply

house.

The

unit does not

require

a power

supply.

The PLAY

CAL controls

on the noise

reduction

unit should

be turned to

TO FROM RECORDER"

INPUT

ON NOISE-

REDUCTION UNIT

maximum,

and

TAPE

CAL

control

on

the

compensator used in their place.

FM

CAL should be

set so that

the DOLBY

TONE

broadcast

by the

desired

station

gives

a DOLBY LEVEL reading

on

the meters

of

the

noise

reduction

unit.

POPULAR

ELECTRONICS

1 2 ... 39 40 41 42 43 44 45 46 47 48 49 ... 101 102

Comments to this Manuals

No comments