<s>
It	O
continued	O
to	O
be	O
the	O
only	O
type	O
of	O
radio	O
transmission	O
during	O
the	O
first	O
few	O
decades	O
of	O
radio	O
,	O
called	O
the	O
"	O
wireless	O
telegraphy	O
era	O
"	O
up	O
until	O
World	O
War	O
I	O
,	O
when	O
the	O
development	O
of	O
amplitude	B-Device
modulation	I-Device
(	O
AM	O
)	O
radiotelephony	O
allowed	O
sound	O
(	O
audio	O
)	O
to	O
be	O
transmitted	O
by	O
radio	O
.	O
</s>
<s>
Beginning	O
about	O
1908	O
,	O
powerful	O
transoceanic	O
radiotelegraphy	O
stations	O
transmitted	O
commercial	O
telegram	B-Application
traffic	O
between	O
countries	O
at	O
rates	O
up	O
to	O
200	O
words	O
per	O
minute	O
.	O
</s>
<s>
Wireless	O
telegraphy	O
or	O
radiotelegraphy	O
,	O
commonly	O
called	O
CW	O
(	O
continuous	O
wave	O
)	O
,	O
ICW	O
(	O
interrupted	O
continuous	O
wave	O
)	O
transmission	O
,	O
or	O
on-off	B-Device
keying	I-Device
,	O
and	O
designated	O
by	O
the	O
International	O
Telecommunication	O
Union	O
as	O
emission	B-Device
type	I-Device
A1A	I-Device
or	I-Device
A2A	I-Device
,	O
is	O
a	O
radio	O
communication	O
method	O
.	O
</s>
<s>
It	O
was	O
transmitted	O
by	O
several	O
different	O
modulation	B-Application
methods	O
during	O
its	O
history	O
.	O
</s>
<s>
The	O
primitive	O
spark-gap	O
transmitters	O
used	O
until	O
1920	O
transmitted	O
damped	B-Device
waves	I-Device
,	O
which	O
had	O
very	O
wide	O
bandwidth	B-Algorithm
and	O
tended	O
to	O
interfere	O
with	O
other	O
transmissions	O
.	O
</s>
<s>
To	O
receive	O
CW	O
transmissions	O
,	O
the	O
receiver	O
requires	O
a	O
circuit	O
called	O
a	O
beat	B-Application
frequency	I-Application
oscillator	O
(	O
BFO	O
)	O
.	O
</s>
<s>
The	O
third	O
type	O
of	O
modulation	B-Application
,	O
frequency-shift	O
keying	O
(	O
FSK	O
)	O
was	O
used	O
mainly	O
by	O
radioteletype	O
networks	O
(	O
RTTY	O
)	O
.	O
</s>
<s>
In	O
a	O
minor	O
legacy	O
use	O
,	O
VHF	B-Device
omnidirectional	I-Device
range	I-Device
(	O
VOR	O
)	O
and	O
NDB	O
radio	O
beacons	O
in	O
the	O
aviation	O
radio	O
navigation	O
service	O
still	O
transmit	O
their	O
one	O
to	O
three	O
letter	O
identifiers	O
in	O
Morse	O
code	O
.	O
</s>
<s>
As	O
of	O
2021	O
,	O
licence	O
Class	O
A	O
in	O
Belarus	O
and	O
Estonia	O
,	O
or	O
the	O
General	O
class	O
in	O
Monaco	O
,	O
or	O
Class	O
1	O
in	O
Ukraine	O
require	O
Morse	O
proficiency	O
to	O
access	O
the	O
full	O
amateur	O
radio	O
spectrum	O
including	O
the	O
high	B-Algorithm
frequency	I-Algorithm
(	O
HF	O
)	O
bands	O
.	O
</s>
<s>
In	O
the	O
1890s	O
inventor	O
Nikola	O
Tesla	O
worked	O
on	O
an	O
air	O
and	O
ground	O
conduction	O
wireless	B-Application
electric	I-Application
power	I-Application
transmission	I-Application
system	I-Application
,	O
similar	O
to	O
Loomis	O
 '	O
,	O
which	O
he	O
planned	O
to	O
include	O
wireless	O
telegraphy	O
.	O
</s>
<s>
Tesla	O
's	O
experiments	O
had	O
led	O
him	O
to	O
incorrectly	O
conclude	O
that	O
he	O
could	O
use	O
the	O
entire	O
globe	O
of	O
the	O
Earth	O
to	O
conduct	O
electrical	O
energy	O
and	O
his	O
1901	O
large	O
scale	O
application	O
of	O
his	O
ideas	O
,	O
a	O
high-voltage	O
wireless	B-Application
power	I-Application
station	O
,	O
now	O
called	O
Wardenclyffe	O
Tower	O
,	O
lost	O
funding	O
and	O
was	O
abandoned	O
after	O
a	O
few	O
years	O
.	O
</s>
<s>
During	O
the	O
Great	O
Blizzard	O
of	O
1888	O
,	O
this	O
system	O
was	O
used	O
to	O
send	O
and	O
receive	O
wireless	O
messages	O
from	O
trains	B-Application
buried	O
in	O
snowdrifts	O
.	O
</s>
<s>
The	O
disabled	O
trains	B-Application
were	O
able	O
to	O
maintain	O
communications	O
via	O
their	O
Edison	O
induction	O
wireless	O
telegraph	O
systems	O
,	O
perhaps	O
the	O
first	O
successful	O
use	O
of	O
wireless	O
telegraphy	O
to	O
send	O
distress	O
calls	O
.	O
</s>
<s>
The	O
primitive	O
spark-gap	O
transmitters	O
used	O
until	O
1920	O
transmitted	O
by	O
a	O
modulation	B-Application
method	O
called	O
damped	B-Device
wave	I-Device
.	O
</s>
<s>
Damped	B-Device
wave	I-Device
had	O
a	O
large	O
frequency	B-Algorithm
bandwidth	I-Algorithm
,	O
meaning	O
that	O
the	O
radio	O
signal	O
was	O
not	O
a	O
single	O
frequency	O
but	O
occupied	O
a	O
wide	O
band	O
of	O
frequencies	O
.	O
</s>
<s>
Damped	B-Device
wave	I-Device
transmitters	O
had	O
a	O
limited	O
range	O
and	O
interfered	O
with	O
the	O
transmissions	O
of	O
other	O
transmitters	O
on	O
adjacent	O
frequencies	O
.	O
</s>
<s>
After	O
1905	O
new	O
types	O
of	O
radiotelegraph	O
transmitters	O
were	O
invented	O
which	O
transmitted	O
code	O
using	O
a	O
new	O
modulation	B-Application
method	O
:	O
continuous	O
wave	O
(	O
CW	O
)	O
(	O
designated	O
by	O
the	O
International	O
Telecommunication	O
Union	O
as	O
emission	O
type	O
A1A	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
radio	O
receivers	O
used	O
for	O
damped	B-Device
wave	I-Device
could	O
not	O
receive	O
continuous	O
wave	O
.	O
</s>
<s>
In	O
his	O
"	O
heterodyne	B-Algorithm
"	O
receiver	O
,	O
the	O
incoming	O
radiotelegraph	O
signal	O
is	O
mixed	O
in	O
the	O
receiver	O
's	O
detector	B-Algorithm
crystal	O
or	O
vacuum	O
tube	O
with	O
a	O
constant	O
sine	O
wave	O
generated	O
by	O
an	O
electronic	O
oscillator	O
in	O
the	O
receiver	O
called	O
a	O
beat	B-Application
frequency	I-Application
oscillator	O
(	O
BFO	O
)	O
.	O
</s>
<s>
In	O
the	O
detector	B-Algorithm
the	O
two	O
frequencies	O
subtract	O
,	O
and	O
a	O
beat	B-Application
frequency	I-Application
(	O
heterodyne	B-Algorithm
)	O
at	O
the	O
difference	O
between	O
the	O
two	O
frequencies	O
is	O
produced	O
:	O
.	O
</s>
<s>
If	O
the	O
BFO	O
frequency	O
is	O
near	O
enough	O
to	O
the	O
radio	O
station	O
's	O
frequency	O
,	O
the	O
beat	B-Application
frequency	I-Application
is	O
in	O
the	O
audio	O
frequency	O
range	O
and	O
can	O
be	O
heard	O
in	O
the	O
receiver	O
's	O
earphones	O
.	O
</s>
<s>
In	O
later	O
superheterodyne	O
receivers	O
from	O
the	O
1930s	O
on	O
,	O
the	O
BFO	O
signal	O
was	O
mixed	O
with	O
the	O
constant	O
intermediate	O
frequency	O
(	O
IF	O
)	O
produced	O
by	O
the	O
superheterodyne	O
's	O
detector	B-Algorithm
.	O
</s>
<s>
CW	O
became	O
the	O
standard	O
method	O
of	O
transmitting	O
radiotelegraphy	O
by	O
the	O
20s	O
,	O
damped	B-Device
wave	I-Device
spark	O
transmitters	O
were	O
banned	O
by	O
1930	O
and	O
CW	O
continues	O
to	O
be	O
used	O
today	O
.	O
</s>
<s>
The	O
transmission	O
of	O
sound	O
(	O
radiotelephony	O
)	O
began	O
to	O
displace	O
radiotelegraphy	O
by	O
the	O
1920s	O
for	O
many	O
applications	O
,	O
making	O
possible	O
radio	O
broadcasting	B-Operating_System
.	O
</s>
<s>
Wireless	O
telegraphy	O
continued	O
to	O
be	O
used	O
for	O
private	O
person-to-person	O
business	O
,	O
governmental	O
,	O
and	O
military	O
communication	O
,	O
such	O
as	O
telegrams	B-Application
and	O
diplomatic	O
communications	O
,	O
and	O
evolved	O
into	O
radioteletype	O
networks	O
.	O
</s>
