Marantz 5220 Manuel d'utilisateur télécharger pdf (Page 39)

BY LEE

FELSENSTEIN

Build

"PENNYWHISTLE"

The

Hobbyist's

Modem

Under -$100

interface

for

digital

data transmission

/reception

over

channels

such

as telephone

or cassette

tape.

TRANSMIT

and

/or

receive dig-

ital data

-as

between two com-

puters,

between

terminal and com-

puter,

etc.

-over telephone

lines

and

other

limited-

bandwidth

channels,

you

need a modem (an acronym

for

modulator /demodulator). Most such

channels cannot cope

with the

steep -

sided,

flat- topped digital

pulse with-

out

distorting it,

much

less pass

the

high

baud rate (bits

per

second) used

in

computers.

The

modem translates

the

pulses

into

a usable

form.

MARCH

1976

Digital pulses

going

into a modem at

one

end emerge as

different

fre-

quency audio tones

that

are

fed

into

the transmitting channel.

At the other

end, another

modem translates

the

tones

back

to the appropriate

digital

pulses.

Modems

can

operate in one direc-

tion

only

(half duplex) or

in

both

direc-

tions

(full

duplex).

The

under -$100

Pennywhistle modem project pre-

sented

here

offers

the

user

both

modes with the flip

of a switch.

Although

some

modems

can oper-

ate

at up

to 9600 baud

over

telephone

lines, full

-duplex operation

usually

re-

stricts

the

maximum speed

to

300

baud,

or 30

characters /second.

Mo-

dems

in

this speed

range

generally

operate

with frequency -shift

keying

(FSK), using

four

frequencies

in two

"bands."

The

1070

-to-

1270

-Hz

low

band

is

used

for

transmission

from a

terminal

to

a central system,

while

the

2025

-to-

2225 -Hz

high

band

is for

transmission

in the opposite

direc-

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