Moseley associates Scg-3t User manual

www.SteamPoweredRadio.Com

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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www.SteamPoweredRadio.Com

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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Moseley Associates SCG-3T User manual

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