Moseley associates Scg-3t User manual

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MODEL

SCG

-

3T

STEREOPHONIC

GENERATOR

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INSTRUCTION

MANUAL

MODEL

SCG-3T

STEREOPHONIC

GENERATOR

MOSELEY

ASSOCIATES,

INC.

Santa

Barbara

Research

Park

111

Castilian

Drive

Goleta,

California

93017

August,

1970

www.SteamPoweredRadio.Com

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TABLE

OF

CONTENTS

I

Page

I.

Introduction

l

I

II

.

Specifications

2

I

III.

Installation

3

IV.

Principles

of

Ope

r

ation

4

I V .

Detailed

Circuit

Description

5

I

VI.

Adjustment

9

VII.

Operational

Suggestions

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INSTRUCTION

MANUAL

MODEL

SCG-3T

STEREOPHONIC

GENERATOR

I.

INTRODUCTION

The

Moseley

Associates,

Inc.,

Model

SCG-3T

Stereo

Generator

is

intended

specifically

to

provide

the

broadcaster

with

a

high-

quality

stereophonic

composite

signal.

Intended

primarily

to

drive

the

Model

PCL-

303/

C

Studio-Transmitter

Link,

it

may

also

be

used

with

most

other

FM

exciters

using

direct

frequency

mod

-

ulation.

Because

the

radiated

signal

quality

is

dependent

on

sev-

eral

factors,

including

antenna,

transmission

line,

the

FM

transmitter

itself,

and

any

audio

processing

equipment,

the

SCG-3T

has

been

designed

to

greatly

exceed

the

system

require-

ments

in

every

respect.

Considerable

degradation

may

occur

in

other

portions

of

the

system

before

performance

fails

to

come

up

to

FCC

standards

as

set

forth

in

Section

73.

322

of

the

R

ules.

Aside

from

superior

electronic

operation,

the

SCG-

3T

also

fea-

tures

built-in

metering

of

the

audio

and

output

signals,

small

size,

rugged

construction,

and

pushbutton

mono-stereo

selection

which

can

be

remotely

controlled.

The

electronic

assemblies

are

broken

into

two

parts,

e

ach

sliding

out

of

the

main

frame

for

adjustment

or

maintenance.

On

the

right

is

the

power

supply

section,

and

on

the

left

is

the

larger

stereo

electronics

section.

Each

is

connected

to

the

main

frame

with

an

umbilical

cable

which

can

be

disconnected

easily

in

case

that

section

mus

t

be

removed

entirely

from

the

main

frame.

SCG

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II.

SPECIFICATIONS

Input

Impedance

Input

Level

Input

Program

Filters

Frequency

Response

Pre-emphasis

Harmonic

Distortion

Noise

Level

Output

Level

Minimum

Load

Resistance

Maximum

Load

Capacitance

Pilot

Carrier

Stability

Channel

Separation

,

LEFT

and

RIGHT

Cross

talk,

Main

and

Subchannels

Subcarrier

Suppressio

n

Spurious

Outputs

Above

55

kH

z

Stereo-Mono

Switching

Metering

Power

Requirements

600

n,

balanced,

floating

+10

dBm

:t:2

dB

Better

than

50

dB

down

at

18.

S

kHz

and

above

:t:l

dB,

30

Hz

to

15

kHz

75

µsecond

standard,

field

convert-

ible

to

other

values

including

flat

Less

than

0.

4%

stereo

or

mono

Better

than

68

dB

below

normal

output

3.

5

volts

peak-to-peak,

5

volts

maximum

5ooon

l

000

picofarads

Less

than

l

Hz

drift

in

6

months

35

dB

minimum,

45

dB

typical

43

dB

minimum,

SO

dB

typical

in-

cluding

the

built-in

17

kHz

low-

pass

program

filters

48

dB

minimum,

55

dB

typical,

with

or

without

modulation

Better

than

60

dB

below

normal

output

Front

Panel

illuminated

pushbutton

or

remote

momentary

contact

clo-

sure

to

ground;

modulation

level

is

the

same

in

stereo

and

mono

modes.

Peak-reading

meter

calib

rat

ed

in

dB,

switchable

to

read

LEFT,

RIGHT

or

TOTAL

OUTPUT

.

1120/240

volts

AC,

50-60

Hz,

10

watts

OperatingTemperatureRange

30

to

1

40°

F

Weight

Size

Domestic

Shipping

Weight

SCG-3T

20.S

pounds

3½

11

H,

l

9''

W,

l l 11D

27

pounds

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III.

INSTALLATION

The

SCG-3T

should

be

mounted

somewhere

between

waist

and

eye

height.

This

will

enable

convenient

local

pushbutton

operation

as

well

as

selection

and

reading

of

the

meter.

The

rack

should

be

grounded.

Prior

to

application

of

the

primary

power

(

117

volts

AC,

50-60

Hz),

be

sure

that

all

transistors

are

in

their

sockets

and

that

there

is

no

obvious

physical

damage

to

the

device.

At

this

time,

the

left

and

right

audio

lines

can

be

connected,

as

well

as

the

remote

control

line.

Each

of

these

three

lines

can

use

the

same

type

of

cable,

a

typical

type

having

two

conductors

with

a

conunon

outside

shielded

jacket.

In

each

case,

the

shield

is

at

ground

potential.

The

output

line

should

be

fitted

with

a

type

BNC

connector

and

should

employ

not

more

than

20

feet

of

coaxial

cable.

The

left

and

right

audio

lines

should

be

driven

from

a

frequency-

sensitive

audio

p r

ocessing

device,

several

of

which

are

available

on

the

market.

The

remote

control

lines

for

selecting

stereo

or

monaural

operation

should

be

connected

to

a

pair

of

pushbuttons,

single-pole,

single-throw

normally

open.

Each

button

returns

to

ground.

If

a

two

conductor

plus

shield

can

be

used

as

the

ground

connection.

These

remote

buttons

can

be

used

fo

switch

from

the

stereo

mode

to

mono

and

back

again

.

In

the

mono

mode

only

one

audio

channel

is

used.

This

has

been

selected

in

accordance

with

industry

standards

as

the

left

channel.

Should

the

left

program-source

channel

fail,

routing

the

input

to

the

SCG-3T

through

a

patch panel

will

facilitate

reverting

to

the

other

audio

source

and

monaural

operation.

The

remote

control

contacts

parallel

the

switch

elements

in

the

front

panel

pushbut-

ton

assemblies

and

only

a

momentary

contact

is

required

to

switch

from

one

mode

to

another

.

The

required

audio

input

level

will

be

in

the

vicinity

of

+8

dBm

to

+12

dBm.

The

front

panel

dB

meter

may

be

relied

on

to

indicate

the

exact

level,

regardless

of

whether

sine

wave

test

signals

or

actual

program

material

is

used.

SCG-3T

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The

output

level

is

normally

3.

5

volts

peak

-to-peak,

regardless

of

whether

stereo

or

mono

signals

are

transmitted.

The

output

will

normally

go

to

the

wide-band

input

on

the

PCL-303/C

Trans-

mitter.

The

length

of

the

inter

connecting

cable

should

not

exceed

about

20

feet.

With

power

and

audio

signals

applied,

the

operation

of

the

meter

,

meter

switch,

and

pushbuttons

can

be

observed.

In

addition,

if

the

remote

switching

is

connected,

the

operation

of

this

too

can

be

confirmed.

IV.

PRINCIPLES

OF

OPERATION

Very

simply

stated,

stereophonic

transmission

is

the

electron

ic

switching

at

a

supersonic

(38

kHz)

rate

of

the

o

utput,

first

to

the

left

audio

channel

and

then

to

the

right

audio

channel.

This

takes

place

repetitively

at

a

rate

that

is

so

fast

it

is

inaudible;

the

person

with

a

monaural

re

ceive

r

hear

s

only

the

resultant

blend

between

the

two

channels.

The

listener

with

a

stereo

receiver

has

the

ability

to

decode

the

stereo

signal

so

that

he

re-

creates

the

original

left

and

right

channels.

It

has

been

deter

-

mined

in

practice

that

this

scheme

is

compatible

with

other

services

such

as

simultaneous

background

music

transmission.

In

addition

to

switching

the

output

signal

alternately

between

the

left

and

right

audio

channels,

a

pilot

tone

signal

is

also

trans

-

mitted

to

enable

the

receiver

to

synthesize

a

switching

signal

to

decode

or

demodulate

the

stereo

signal

into

its

l

eft

and

right

audio

components

.

To

prevent

interference

to

this

pilot

to

ne

(at

19

kHz),

the

SCG-3T

has

built-in

low-pass

filters

cutting

off

at

17

kHz

so

that

program

material

content

above

that

frequency

is

sharply

attenuated,

In

this

manner,

no

particular

signals

will

interfere

with

the

pilot

tone

and

the

receiver

can

extract

the

19

kHz

signa

l

with

a

minimum

of

effort.

As

in

standard

monaural

transmission

a 75

µsecond

pre-emphasis

(treble

boost)

is

employed

in

each

input

audio

channel.

After

demodulation

in

the

receiver,

de-empha~is

(treble

cut)

is

em-

ployed

to

counter

the

transmitted

signal

with

its

pre-emphasis.

The

receiver

de-emphasis

cuts

down

the

noise

on

the

signal,

as

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well

as

the

level

of

the

pilot

tone

and

other

by-products

incidental

to

the

rec

eive

r's

demodulation

process.

Details

of

some

of

the

problems

associated

with

stereophonic

transmission

will

be

touched

upon

later

in

this

manual.

The

basic

parts

required

for

stereophonic

signal

generation

are

now

seen

to

be

these:

V.

1.

A

pair

of

identical

audio

channels

incorporating

pre-emphasis.

2.

A

pair

of

17

kHz

low-pass

filters,

one

in

each

channel.

3.

An

electronic

switch

that

alternately

keys

first

the

left

and

then

the

right

c

hannel

to

the

output

ci

rcuitry.

4.

A

relay

to

select

either

the

signal

from

the

electronic

or

one

of

the

audio

channels

to

the

output

amplifier

.

5.

A

metering

system

6.

A

power

su

pply

DETAILED

CIRCUIT

DESCRIPTION

The

following

description

is

with

reference

to

the

block

diagram

shown

in

drawing

number

92A

-l

004.

Discussing

first

the

switching-signal

generation

process,

observe

the

76

kHz

crystal-controlled

oscillator

using

IC-

30

1

in

the

upper

left

corner

of

the

block

diagram.

This

oscillator

is

followed

by

a

buffer,

Q-301,

and

is

then

divided

down

to

38

kHz

with

the

integrated

circuit

binary

divider

IC-302.

The

output

of

the

divider

is

applied

to

IC-303,

where

a

push-pull

38

kHz

switching

signal

is

developed

.

This

38

kHz

signal

is

used

to

operate

th

e

diode

switch

using

CR-303, CR-304,

CR-305,

and

CR-306.

The

diode

switch

alternately

commutates

the

left

and

right

audio

chan

-

nels

to

the

following

buffer.

SCG-3T

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The

output

of

the

binary

divider

is

divided

again

m

another

section

of

IC-

302

to

develop

a

19

kHz

square

wave.

This

is

applied

to

a

buffer

and

is

then

sinusoidalized

with

a

low

-

pass

filter

using

L-301.

This

sine

wave

becomes

the

pilot

tone

and

is

mixed

with

the

buffered

output

of

the

diode

switch.

At

the

le

ft

of

the

block

diagram

is

shown

the

pair

of

audio

in

-

puts

.

These

are

applied

to

resistive

pads

to

properly

terminate

the

driving

amplifiers,

usually

a

limiter

or

clipper.

Following

the

pads

are

transformers

for

conversion

from

balanced

floating

connections

to

unbalanced

connections

suitab

le

for

application

to

the

audio

amplifiers.

The

audio

amplifiers

are

similar

except

that

one

channel

has

a

set

of

three

adjustments

to

enable

its

amplitude

and

phase

re-

sponse

to

coincide

with

the

other.

The

left

channel

has

fixed

characteristics

and

the

right

channel

has

th'e

variable

controls

for

matching.

Discussing

the

left

channel

first,

this

amplifier

has

a

very

large

amount

of

negative

feedback

applied

to

it.

This

reduces

distor-

tion

and

stabilizes

the

gain

and

other

characteristics.

More

important,

it

allows

the

use

of

resistors

and

capacitors

to

achieve

an

accurate

pre-emphasis

characteristic.

To

increase

the

power

handling

ability

of

this

amplifier,

it

is

followed

by

a

buffer,

Q-307.

It

is

of

interest

to

note

that

this

amplifier

is

entirely

direct

-

coupled.

Following

the

buffered

output

of

the

audio

amplifier,

a

low-pass

filter

has

been

installed.

This

filter

is

down

about

0.

6

dB

at

15

kHz

and

down

55

dB

at

18.

5

kHz

and

above.

This

insures

that

the

stereo

receiver

will

see

no

interfering

program

com-

ponents

which

may

conflict

with

the

19

kHz

pilot.

The

output

of

the

low-pass

fi

lt

er

is

applied

to

another

buffer,

Q-

308,

which

terminates

the

filter

and

drives

the

metering

am-

plifier

as

well

as

the

mono

gain

control.

Note

that

the

SCG-3T,

in

the

mono

mode,

uses

the

left

channel

as

its

source.

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The

audio

is

then

applied

to

one

input

of

the

diode

switch

via

another

buffer,

Q-309.

This

buffer

is

needed

due

to

the

require-

ment

for

low-impedance

drive

to

the

diode

switch

audio

inputs.

The

right

audio

channel

is

electrically

similar

to

the

left

except

that

there

is

no

output

for

monaural

operation.

Pre-emp

h

as

i s

is

adjustable

to

track

with

that

of

the

left

channel

,

as

is

channel

gain.

Filtering

and

buffering

are

identica

l

with

that

in

the

left

channel.

The

pre-

e

mphasis,

including

both

phase

and

gain,

must

be

made

coincident

with

the

left

channel

to

satisfy

FCC

requirements.

This

comes

about

when

the

left

and

right

channels

are

fed

iden-

tical

signals;

under

these

conditions

the

difference

signal

must

be

suppressed

more

than

40

dB

at

any

test

frequency

from

50

Hz

to

15

kHz

.

(In

this

discussion,

occasional

reference

will

be

made

to

the

"Main,

11

"sum,

11

or

"LtR

11

signals.

These

are

all

one

and

the

same.

Likewise

the

''difference,"

"ste

r

eophonic

subca

rri

er,"

11

subchannel"

and

"L-R"

signals

are

all

one

and

the

same.)

In

a

similar

manner,

if

the

left

and

ri

ght

channels

are

fed

from

the

same

source

but

of

opposite

polarity,

then

the

resultant

main

channel

output

from

the

stereo

generator

must

be

suppressed

by

the

same

amount,

40

dB.

This

can

be

accomplished

on

·

ly

by

accurately

matching

the

two

audio

channels,

including

isolation

transformers,

low-pass

filters

and

pre-emphasis

networks,

to

within

a

tenth

of

a d

eci

bel

and

1

degree

phase

error.

Due

to

tight

manufacturing

tolerances

and

the

set

of

three

adjustments

in

corporated

in

th

e

right

channel

of

the

SCG-3T,

these

conditions

can

be

met

with

ease.

The

diode

switch,

driven

from

the

push-pull

38

kHz

square-wave

switching

signal,

deliv

e

rs

an

output

which

may

be

termed

square-

wave

stereo.

The

pilot

tone

is

absent

at

this

point.

The

diode

switch

also

has

a

carrier

balance

control

to

enable

an

excellent

rejection

of

residual

38

kHz

signal,

either

in

the

presence

or

absence

of

audio

.

SCG-3T

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The

output

of

the

diode

switch

is

applied

to

a

buffer,

part

of

IC-306,

and

the

output

of

this

buffer

is

combined

with

the

sinus-

oidal

pilot

from

L -

301.

The

combined

signal

is

applied

to

a

phase-linear

low-pass

filter

to

remove

harmonics

of

the

stereo

signal.

The

result

is

a

standard

stereo

signal

with

sine-wave

switching.

The

filtering

process

yields

an

unwanted

byproduct

in

the

form

of

an

excess

of

level

of

the

38

kHz

difference

out

-

put

component.

This

is

corrected

by

adding

a

small

amount

of

the

L+R

signa

l

with

the

aid

of

the

SEPARATION

control.

The

output

of

the

left

audio

amplifier

or

the

output

of

the

phase-

linear

low-pass

filter

is

selected

by

the

stereo-mono

relay

K-201.

The

output

of

the

relay

is

then

applied

to

the

output

buffer,

IC-307.

Metering

signals

are

derived

from

the

audio

amplifiers

Q-305

and

Q-

308

for

the

right

and

left

channels

respectively.

The

total

or

composite

output

signal

for

metering

purposes

is

derived

from

the

output

buffer

IC-

307.

The

metering

amplifier

has

sufficient

ga

in

and

bandwidth

to

give

a

good

indication

not

only

with

monaural

operation

but

also

with

stereo

operation.

The

output

stage

of

this

amplifier

has

the

current

handling

ability

to

enable

the

peak

detector

to

respond

to

very

short

pulses

of

signal,

i.e.,

stereo

waveforms.

The

am-

plifier

deliveres

its

signal

to

the

meter

located

on

the

power

supply

module.

This

meter

is

peak

reading,

and

is

ca

libra

ted

in

decibels

rather

than

volume

units

(VU)

b

ecause

it

res

ponds

not

to

the

rms

or

average

value

of

the

waveform

in

question

but

rather

to

the

peak

instantaneous

value.

Complex

waveforms,

as

encountered

in

typical

music

and

speech

waveforms

are

ac-

curately

indicated.

This

meter

should

not

be

us

ed

as

a

modula-

tion

monitor,

but

may

be

used

as

a

valuable

maintenance

aid.

The

power

supply

for

the

SCG-

3T

is

conventional

,

with

a

split

prima

ry

on

the

power

transformer

to

enable

re

wiring

in

the

field

for

240

VAC

operation.

Operation

on

frequencies

below

50

Hz

should

be

avoided.

The

40

volt

output

is

used

solely

to

operate

the

lamps

and

the

relay;

the

22

volt

rectifie

r

outputs

are

regu-

lated

down

to

15

volts

both

positive

and

negative.

Additionally,

10

volt

regulators

located

at

the

stereo

board

are

used

for

final

SCG-3T

- 8-

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www.SteamPoweredRadio.Com

I

..

I

••

I

regulation

and

hum

reduction.

This

also

offers

protection

against

both

transient

voltages

should

the

module

be

plugged

in

while

power

is

on

or

should

the

15

volt

regulators

fail.

VI.

ADJUSTMENT

Operation

of

the

SCG-3T

should

present

few

problems.

The

simplicity

of

installation,

the

elementary

input

and

output

con-

nections,

and

the

self-explanatory

external

lab

elling

contribute

to

this.

However,

should

the

monitoring

apparatus

indic

ate

that

there

is

a

problem

in

the

system,

and

further

should

this

trou-

ble

positively

be

traced

to

the

SCG-3T,

then

readjustment

may

be

in

order.

Because

of

the

fact

that

aging

of

components

in

solid

-

state

equip-

ment

is

greatly

reduced

compared

to

similar

tube

equipment,

readjustment

should

be

considered

undesirable

unless

virtually

proven

necessary.

Description

of

the

various

internal

controls

and

their

functions

will

enable

the

competent

engineer

to

quickly

correct

minor

discrepancies,

possibly

without

loss

of

air

time.

Some

tests

require

momentary

removal

of

program,

but

in

any

event

the

unique

packaging

of

the

SCG

-

3T

enables

fast

access

to

the

circuitry

from

the

front

of

the

rack

while

maintaining

a

clean

front

panel

appearance.

The

location

of

the

controls

and

test

points

is

shown

in

·Drawing

SK.A-6222.

The

labelling

of

these

controls

is,

of

necessity,

abbreviated.

The

following

information

should

clarify

their

functions.

PILOT

-

Adjusts

the

level

of

the

pilot

tone.

FCC

requires

that

this

level

should

be

set

to

between

8%

and

10%

modulation.

CARRIER

BALANCE

-

This

cont

rol

enables

the

diode

switch

modulator

to

operate

in

a

balanced

condition

such

that

there

is

a

minimum

amount

of

38

kHz

switching

sig-

nal

appearing

in

the

output.

Normal

adjustment

procedure

is

to

remove

modulation

and

adjust

this

control

for

a

null

when

the

stereo

monitor

is

set

to

read

the

38

kHz

level.

SCG-3T

-9-

www.SteamPoweredRadio.Com

SUB

TO

MAIN

-

This

control

affects

the

signal

known

as

s;

ross

talk

from

the

subchannel

to

the

main

channel.

Feed

both

stereo

channels

with

the

same

signal

but

of

opposite

polarity.

Under

these

conditions,

the

signal

in

the

main

channel

(or

as

heard

on

an

ordinary

monaural

receive

r)

will

be

cancelled

out.

This

control

will

enable

the

cross-

talk

cancellation.

SEPARATION

-

As

its

name

implies,

this

control

affects

the

separation

between

the

left

and

right

stereophonic

channels.

While

feeding

a

signal

into

one

channel,

moni-

tor

the

other

stereo

channel

with

the

stereo

monitor

and

adjust

the

SEPARATION

control

fo

r

maximum

separation.

~

• "

MONO

GAIN

-

This

control

insur

es

that

when

switching

from

the

stereo

to

the

mono

mode

of

operation,

the

modu-

lation

percentage

is

equal.

This

control

is

adjusted

to

make

the

mono

agree

with

the

previously

adjusted

stereo

signal

level.

...

50

HZ

GAIN

-

This

control

affects

the

gain

of

one

of

the

audio

channels

at

low

frequencies

only.

It

is

necessary

for

the

two

channels

to

have

identical

gain

at

all

frequen-

cies

in

order

to

have

minimum

cross

talk.

Feeding

both

channels

in

phase

with

a

low

frequency,

this

control

is

adjusted

to

produce

a

minimum

amount

of

signal

in

·

the

subchannel.

5

KHZ

GAIN

-

This

control

is

similar

to

the

above

men-

tioned

control

but

affects

eras

s

ta

lk

in

the

5

kHz

region

only.

l S

KHZ

GA

IN -

This

is

the

third

and

final

control

for

cross-talk

adjustment

and

affects

cross

talk

in

the

15

kHz

region

only.

FILTER

TERM

-

This

control

is

used

to

properly

ter-

minate

the

phase-linear

low-pass

filter.

It

1s

set

for

maximum

separation

between

audio

channels

at

a

fre-

quency

of

about

13

kHz.

SCG-3T

-10-

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I

Immediately

behind

the

PILOT

LEVEL

control

is

a

variable

inductor

.

This

inductor

adjusts

the

pilot

phase

relative

to

the

38

kHz

switching

signal.

While

monitoring

the

19

kHz

level

with

the

stereo

monitor,

this

coil

may

be

tuned

for

maximum

level.

Then

it

sho

uld

be

varied

slightly

in

whatever

direction

enables

best

stereo

separation.

About

one

inch

behind

Q-301,

a

small

mica

capacitor

will

be

seen.

This

capacitor

is

us

e d

to

adjust

the

pilot

frequency.

The

oscillators

have

proven

to

be

quite

stable;

they

drift

about

1

Hz

low

after

about

a

year

of

operation

and

then

should

remain

on

-

frequency

thereafter.

Hence,

when

the

equipment

is

first

re-

ceived,

and

if

the

oscillator

is

somewhat

high

(perhaps

1

Hz),

do

not

adjust

the

oscillator

frequency.

It

will

age

to

the

target

frequency

representing

a

19

kH z

pilot

tone

within

several

months.

The

crystal

is

a

low-temperature

coefficient

unit,

with

a

zero

temperature

coefficient

at

room

temperature.

No

other

com-

ponents

significantly affect

frequency.

At

the

right

rear

of

the

main

stereo

board

will

be

noted

the

output

buffe

r

amplifier

usin

g

IC-

307.

This

is

a

direct

coupled

amplifier

with

a

few

external

components

for

phase

compensation

and

parasitic

suppression.

There

may

also

be

seen

a

few

blank

holes

in

this

area.

These

are

for

the

purpose

of

pre-distortion

of

the

st

ereo

signal

should

this

be

required

in

unusual

circumstances.

VII

.

OPERATIONAL

SUGGESTIONS

It

may

be

of

interest,

in

automated

operations,

to

consider

the

use

of

auxiliary

trip

tones

to

switch

automatically

from

stereo

to

monaural

transmission.

Selected

items

during

the

day,

for

example

station

identification,

could

be

transmitted

in

stereo

but

using

one

chan

nel

only

(the

left

channel).

This

will

allow

the

use

of

your

station

for

accurate

receiver

alignment.

When

stereo

signals

are

being

transmitted,

the

entire

frequency

spectrum

from

50

Hz

(or

below)

through

about

53

kHz

is

occu-

pied.

The

only

remaining

spectrum

for

SCA

operation

is

the

band

between

53

kHz

and

75

kHz.

An

industry

standard

for

SCA

operation

is

at

a

center

frequency

of

67

kHz.

Thi

s

frequency,

SCG-3T

-11

-

www.SteamPoweredRadio.Com

when

modulated

either

excessively

or

with

program

signals

above

about

5

kHz,

can

still

interfere

with

stereo

programming.

Hence,

when

SCA

program.ming

is

attempted

at

67

kHz,

it

is

highly

advis

-

able

to

limit

both

the

instantaneous

excursion

of

the

SCA

subcar-

rier

and

to

restrict

the

bandwidth

of

the

program

material.

These

two

things,

when

done

simultaneously,

will

normally

prevent

side-

band

interference

to

the

stereo

signal.

Refer

to

the

FCC

Rules,

Sec.

73.

319

for

technical

standards

concerning

SCA

operation.

Another

form

of

interf

erence

between

the

SCA

and

the

stereo

sig-

nals

which

may

occur

in

some

systems

is

the

splatter

from

the

stereo

signal

into

the

SCA

region.

The

design

philosophy

of

the

SCG-3T

Stereo

Generator

is

to

use

audio

filters

in

each

channel,

cutting

off

the

response

at

17

kHz.

This

positively

prevents

sideband

components

in

the

stereo

subchannel

from

being

generated

above

55

kHz

in

the

first

place.

This

completely

cleans

the

spec-

trum

above

55

kHz

and

the

SCA

receiver

then

has

no

interference

with

which

to

contend.

On

occasion,

there

may

be

noted

a

whistle

in

the

stereo

receiver

when

SCA

signals

are

being

transmitted

on

67

kHz

along

with

the

stereophonic

progranuning.

This

whistle,

at

a

frequency

of

10

kHz,

is

generated

in

the

stereo

demodulator

as

a

byproduct

of

the

demodulation

process.

The

19

kHz

pilot

tone

itself

is

''demodulated,

11

and

a

large

third

harmonic

component

is

developed.

This

57

kHz

signal,

which

is

not

transmitted

but

which

is

gener-

ated

in

the

demodulator,

is

then

mixed

in

the

demodulator

circuit

in

a

non-linear

manner

with

the

67

kHz

SCA

signal.

The

result-

ant

10

kHz

intermodulation

component

is

passed

by

the

audio

system

and

is

heard

as

an

annoying

whistle.

Depending

on

the

individual

receiver

or

monitor,

it

is

generally

about

55

to

65

dB

below

full

modulation.

The

importnat

thing

to

remember

is

that

this

whistle

is

probably

developed

in

the

stereo

demodulator

and

is

not

being

transmitted.

This

can

be

confirmed

by

noting

that

the

modulation

monitor

baseband

(wideband)

output,

as

measured

by

a

tuneable

voltmeter,

does

not

contain

th

e

10

kHz

signal.

Should

there

be

an

apparent

problem

in

properly

demodulating

the

output

of

the

SCG-3T,

bear

in

mind

that

the

fault

may

not

necessarily

lie

in

the

generator.

Remember

that

stereo

moni-

tors

essentially

reverse

the

switching

process

used

in

the

SCG-3T

-12-

I

J

I

•:

I

www.SteamPoweredRadio.Com

I

..

I

••

I

to

generate

the

stereo

signal.

They

process

the

composite

stereo

signal

back

into

the

original

left

and

right

audio

channels.

Most

of

the

same

techniques

used

in

the

SCG-3T

are

common

in

stereo

monitors,

and

so

it

can

be

seen

that

the

monitor

can

be

eas

i

ly

adjusted

to

compensate

for

an

incorrect

condition

i n

the

SCG-

3T.

Likewise,

if

the

monitor

is

out

of

adjustment

then

the

SCG-3T

will

require

compensating

(and

incorrect)

adjustments

mad

e

to

it

to

give

correct

readings.

Generally

speaking

such

adjustments

can

be

confirmed

as

being

erroneous

by

carefully

running

through

the

entire

setup

procedure

for

the

stereo

generator.

Sume

moni-

tors

can

be

checked

out

in

the

field

without

external

equipment

while

some

require

more

test

equipment.

The

most

valuable

single

piece

of

equipment

the

broadcaster

c

an

use

to

visually

check

a

suspected

condition

is

an

industrial-grade

wideband

DC

coupled

oscilloscope.

It

is

beyond

the

scope

of

this

manual

to

give

complete

procedures

in

analyzing

stereophonic

waveforms,

but

an

oscilloscope

can

give

results

whose

accuracy

will

compete

with

the

finest

monitors.

SCG-3T

-

13-

www.SteamPoweredRadio.Com

I

SCG-3T

STEREO

GENERATOR

FINAL

CHECKOUT

SHEET

Date

December

1,

1972

D.

B.

#

3551

Tester

H.A.

Station

Cal

1

KTHO

Serial

No.

6354

__

;..._"--

NOTE:

All

performance

data

measurements

referenced

to

3.

5

vol

t

s,

p-p

output

for

100

%

Modulation

.

Pilot

Frequency

19

0

-~=--

·

;;;,.__

_

Pilot

Injection

9. 0

Signa

l-

to

-

Noise

Ratio

D

emodulated

Left

-

68

db

Demodulated

Right

-68

db

Inp

ut

Level

for

1

00%

Mod.

@

100

Hz

+10.

5

dbm

Separation

Hz

L i

nto

R R

into

L

50

-45

-42

400

-48

-42

1000

-46 -43

5000

-44

-41

1

0000

-

42

-40

1

5000

-36 -36

COMMENTS:

7/70

Moseley

Associates,

Inc

,

ml

38

kHz

Carrier

Suppression

-63

db

with

no

mod.

-60

db

with

100

%,

10

kHz

Distortion

@

400

Hz

O.

18

%

Mono

Left

0.

20

%

Stereo

L,

0.

21

%

Stereo

R

Cross

Talk

L/Mono

L+R

to

L-R

to

L +R

Freq.

R e

sp,

-53

- 51 + . 2

-54

-5

1 0

-54

-5

1 -. 8

-54

-51

-

7.5

-53

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