Marantz 5220 User Manual download pdf (Page 79)

corders.

In addition,

FSK

is more

ex-

pensive

implement than

many

other

methods.

Double

-frequency

recording (DFR),

shown

in Fig. 1F,

often used on

disc

memories at

high data rates. When

used

a cassette,

however, it re-

quires a relatively

high bandwidth for

a given data

rate. This method is in-

sensitive

to speed

variation

since each

bit

self

-clocked, but

it is

only

mod-

erately

free from problems created by

noise and amplitude changes.

DFR is,

therefore,

not

reliable as other

methods at data rates higher than 500

bits /second,

making future

expansion

and

improvement difficult.

Phase

encoding

has many variants

and

has

been

in use

many different

types of

magnetic tape data systems

for many years.

The most

common

forms

are

Bi Phase -L, usually called

"phase encoding,"

and Bi Phase -M,

often called

"Manchester" code. Both

methods

are self

-clocking and, at

first

glance, resemble simplified

FSK.

fact, phase modulation does create a

form

frequency

modulation. All

phase- encoded

methods

are

inde-

pendent of

frequency

changes

over a

wide

range,

and can

be made

highly

resistant to AM noises and

level

shifts.

Bi Phase

-L

shown

in Fig. 1G. You

can see

that there is a transition in the

middle

each bit cell and

that the

polarity of

the transition determines

whether

the

bit

logic 1

0. Bi

Phase -M, or

Manchester, shown in

Fig. 1H, has a transition at the begin-

ning

of each

bit cell.

Logical 1's have

another transition

in the middle

the

cell, whereas

logical 0's do

not.

Manchester

code

is extremely easy

generate,

decode, and

synchronize,

and is the basis

for the

CUTS

(Compu-

ter Users

Tape System) recording

method proposed as

outgrowth

the

meeting in Kansas City.

The

CUTS

method

employs a

varia-

tion of

the Manchester code

which a

COMPARISON CHART

Level,

Noise

Tolerance

Frequency,

Speed

Tolerance

Self

clock-

ing Cost*

Future

Upgrading **

Remarks

(Fig.

1B)

Poor Poor No Very

Low

Susceptible

to noise.

Reliability

suffers

over 150 baud.

PWM

(Fig. 1C)

Good Very

Good

Yes Low

To 1500

Baud

Patented. Requires

higher bandwith

than

Bi -phase for

same

baud

rate.

HITS

(Fig. 1D)

Good

Very

Good

Yes Low

To 600

Baud

Requires higher

bandwidth

than

CUTS

for

same

baud rate

(see

PE,

Sept.

1975).

FSK * **

(Fig. 1E)

Very

Good

Poor-

Fair

Moderate

To 450

baud

(with Bell

103 tones)

Can

be transmitted

over

phone lines

to any

modem.

DFR

(Fig. 1F)

Moderate

Good

Yes

Low to

Moderate

To 800

Baud

See "Computer

Hobbyist"

Vol. 1,

No.

5, 6, 1975.

Bi-

Phase

-L

(Fig. 1G)

Very

Good

Very

Good

Yes Low to

Moderate

To 1500

Baud

Proposed

ANSI

standard. Subject

phase inversion.

Bi-

Phase

-M

(Fig. 1H)

Very

Good

Very

Good

Yes Low to

Moderate

To 1500

Baud

Widely used.

Easily decoded.

CUTS

(Fig. 1J)

Very

Good

Very

Good

Yes

Low

1200

Baud

Very

easily

decoded.

Can

be transmitted by

phone to

other CUTS

units.

*Cost

estimated

for

devices which do not require a

CPU

for reading and writing. Cost

may

be quite

low if

CPU decodes mostly by

software.

**Future upgrading implies minimal hardware modification

low

cost.

-*Especially Bell -103

compatible.

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Interfacing to timers,

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I/O

Plus much more!

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Now, Microcomputer

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The

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CIRCLE

NO. 32

ON FREE

INFORMATION

CARO

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