Marantz 5220 User Manual download pdf (Page 78)

Computer

...

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COMPUTER

USERS

TAPE

SYSTEM

THE

LACK of

standardization

the

bane

many industries.

For

example, three basic four -channel

audio

systems (SO,

QS,

and CD

-4),

in-

stead

of a universal

system,

have im-

peded progress in that field. The

same

holds

true for

computer hobbyists,

where

host

methods for

exchang-

ing

programs

or data have

been

intro-

duced, including

the HIT

system

pub-

lished in POPULAR

ELECTRONICS,

Sep-

tember 1975.

Rather

than

stifle this user -created

"program

explosion,"

a group

hobby

computer manufacturers

and

other interested

parties

(POPULAR

ELECTRONICS,

among them), met

Kansas

City,

MO,

last November

explore

standardization in general

and

hopefully

agree

on a single

method

recording

data. There

was

general

agreement that

cassette tape

represented

the best route

to go for a

hobbyist

computer

-data

exchange

system. These

tapes

are

low

-cost and

widely

available,

and cassette

machines

are

owned by

most

people.

The

use

inexpensive

cassette

re-

corders was not viewed

as a serious

limitation

as long

the record/

playback

exchange method

adopted

allowed for

certain

inherent

machine

deficiencies.

The two most

common

considerations with low

-cost cassette

machines

are:

(1) the automatic level

control

incorporated

some

machines,

and (2) variations in

aver-

age

speed,

nominally

17/s inches/

second. Both

drawbacks

could be eas-

ily

overcome,

was decided.

Another important

consideration

using low

-cost cassette tapes is

that

some tapes would likely

cause drop-

outs (momentary loss

of signal)

due to

lack

uniform

distribution of oxide

particles.

this time, the user would

have to

"certify" the best tape

brand

and model for him to

use.

There

are

also "data

cassettes" certified by tape

manufacturers.

Prices

are not too

much higher than

those

for

consumer

premium tapes.

Robert M. Marsh

Cassette

Data Recording

Meth-

ods. Various

methods

have

been

used

by computer

enthusiasts

and

manufacturers

record

data

audio

cassette

recorders.

These

fall

into five

categories:

(1) simple tone

burst, (2)

pulse

-width modulation

such

as used in the POPULAR

ELEC-

TRONICS

HIT

program,

(3)

frequency

shift

keying

(FSK)

as used in radio

teletypewriter

phone

-line commu-

nications

modems,

(4) double

-fre-

quency

pulse recording

as used

most

floppy

disc

systems,

and (5)

phase

encoding as used in

ANSI

stan-

dard magnetic

tape transports

all

major

computer manufacturers.

Most

these methods

record data

serially;

that is,

one bit after another.

Serial

recording

requires

a conversion

from parallel

to serial

form

(and vice

versa)

when

used with a computer.

Fortunately,

most

computers and ter-

minals

already have

standardized

serial

communications

channel that

transmits in

form

called "non- return

to zero"

(NRZ), shown in Fig. 1A.

Tone

-burst (orcw)

recording may

MRL

Q9rA

the

simplest way

recording

data,

where

data "1" is

the presence

of a

tone

and data

"0"

the

absence

of a

tone,

as shown

Fig. 1B. Because

this

system is

basically

amplitude

-modulation

scheme,

and

very

susceptible to noise, reliability

suffers

above

150

bits per

second.

Pulse

-width modulation may

re-

corded

in its

pure

form

(Fig.

1C)

orasa

burst

tone with varying

duration,

used in

the HIT

system

(Fig.

1D). Both

methods

are

self- synchronizing

and

are highly independent

of speed and

amplitude variations. However,

in the

original HIT proposal,

data was re-

corded

synchronously

so that

each

data word had

the

word immediately,

thus making HIT

impractical for

use with

stand -alone

asynchronous

terminals

such as

typewriters

and teleprinters. In

addi-

tion, "pure"

pulse

-width

modulation

patented

as a data recording

method,

which might

seen as a

drawback

by manufacturers.

Ordinary frequency

shift

keying

(FSK),

shown

Fig. 1E, is

far

the

most

common method

used to trans-

mit

data

over

phone

lines

and

radio

links.

It would

be a useful feature

of a

cassette

recorder

interface if

could

transmit

data

over

phone lines

as if it

were

FSK

Bell -103

compatible mod-

em. However,

while FSK is fairly in-

sensitive to

AM noise

and

level

changes, it is

susceptible to loss

data when

overall

frequency

changes

exceeding

±5% of the nominal value

occur. The

frequency,

or speed

tolerance is

not

sufficient

for reliable

data

storage on many

cassette

re-

ONE -.- ---ZERO

ZERO

-+-1-4-

ONE

raw

BURST (cw)

C. PWM

Fig. I.

Methods ttf'

reeurdiirg

data

o) enssette recorders.

D. It/7-5 PWM

FSK

000,931(

FREQ.

(OF2)

B/-PHASE-L

(PHASE

6COD/N4)

B/'PHASE-M

(MANCHEsrcK)

..7

Curs

-1 r

SCYCLE3

NCYCLES

(14001h)

(114ONz)

POPULAR

ELECTRONICS

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