<s>
PSK31	B-Device
or	O
"	O
Phase	B-Device
Shift	I-Device
Keying	I-Device
,	I-Device
31	I-Device
Baud	I-Device
"	O
,	O
also	O
BPSK31	B-Device
and	O
QPSK31	B-Device
,	O
is	O
a	O
popular	O
computer-sound	O
card-generated	O
radioteletype	O
mode	O
,	O
used	O
primarily	O
by	O
amateur	O
radio	O
operators	O
to	O
conduct	O
real-time	O
keyboard-to-keyboard	O
chat	B-Protocol
,	O
most	O
often	O
using	O
frequencies	O
in	O
the	O
high	B-Algorithm
frequency	I-Algorithm
amateur	B-Algorithm
radio	I-Algorithm
bands	I-Algorithm
(	O
near-shortwave	O
)	O
.	O
</s>
<s>
PSK31	B-Device
is	O
distinguished	O
from	O
other	O
digital	O
modes	O
in	O
that	O
it	O
is	O
specifically	O
tuned	O
to	O
have	O
a	O
data	O
rate	O
close	O
to	O
typing	B-Device
speed	I-Device
,	O
and	O
has	O
an	O
extremely	O
narrow	O
bandwidth	B-Algorithm
,	O
allowing	O
many	O
conversations	O
in	O
the	O
same	O
bandwidth	B-Algorithm
as	O
a	O
single	O
voice	O
channel	O
.	O
</s>
<s>
This	O
narrow	O
bandwidth	B-Algorithm
makes	O
better	O
use	O
of	O
the	O
RF	O
energy	O
in	O
a	O
very	O
narrow	O
space	O
thus	O
allowing	O
relatively	O
low-power	O
equipment	O
(	O
5	O
watts	O
)	O
to	O
communicate	O
globally	O
using	O
the	O
same	O
skywave	O
propagation	O
used	O
by	O
shortwave	O
radio	O
stations	O
.	O
</s>
<s>
PSK31	B-Device
was	O
developed	O
and	O
named	O
by	O
English	O
amateur	O
radio	O
operator	O
Peter	O
Martinez	O
(	O
call	O
sign	O
G3PLX	O
)	O
and	O
introduced	O
to	O
the	O
wider	O
amateur	O
radio	O
community	O
in	O
December	O
1998	O
.	O
</s>
<s>
The	O
31	O
baud	O
BPSK	O
modulation	O
system	O
used	O
in	O
PSK31	B-Device
was	O
introduced	O
by	O
Pawel	O
Jalocha	O
(	O
SP9VRC	O
)	O
in	O
his	O
SLOWBPSK	O
program	O
written	O
for	O
Motorola	O
's	O
EVM	O
radio	O
.	O
</s>
<s>
PSK31	B-Device
was	O
enthusiastically	O
received	O
,	O
and	O
its	O
usage	O
spread	O
rapidly	O
worldwide	O
,	O
lending	O
a	O
new	O
popularity	O
and	O
tone	O
to	O
the	O
on-air	O
conduct	O
of	O
digital	O
communications	O
.	O
</s>
<s>
A	O
PSK31	B-Device
operator	O
typically	O
uses	O
a	O
single-sideband	B-Device
(	O
SSB	O
)	O
transceiver	O
connected	O
to	O
the	O
sound	B-Device
card	I-Device
of	O
a	O
computer	O
running	O
PSK31	B-Device
software	O
.	O
</s>
<s>
This	O
sound	O
is	O
then	O
fed	O
through	O
either	O
a	O
microphone	O
jack	O
(	O
using	O
an	O
intermediate	O
resistive	O
attenuator	O
to	O
reduce	O
the	O
sound	B-Device
card	I-Device
's	O
output	O
power	O
to	O
microphone	O
levels	O
)	O
or	O
an	O
auxiliary	O
connection	O
into	O
the	O
transceiver	O
,	O
from	O
which	O
it	O
is	O
transmitted	O
.	O
</s>
<s>
However	O
,	O
the	O
software	O
rapidly	O
shifts	O
the	O
phase	O
of	O
the	O
audio	O
signal	O
between	O
two	O
states	O
(	O
hence	O
the	O
name	O
"	O
phase-shift	B-General_Concept
keying	I-General_Concept
"	O
)	O
,	O
forming	O
the	O
character	O
codes	O
.	O
</s>
<s>
To	O
decode	O
PSK31	B-Device
,	O
the	O
audio	O
whistle	O
received	O
from	O
the	O
transceiver	O
's	O
headphone	O
output	O
is	O
fed	O
into	O
a	O
computer	B-Device
sound	I-Device
card	I-Device
's	O
audio	O
input	O
,	O
and	O
software	O
decodes	O
it	O
.	O
</s>
<s>
Because	O
PSK31	B-Device
was	O
developed	O
for	O
use	O
through	O
a	O
computer	O
's	O
sound	B-Device
card	I-Device
,	O
many	O
programs	O
have	O
since	O
been	O
created	O
to	O
use	O
the	O
same	O
technology	O
for	O
other	O
modes	O
,	O
such	O
as	O
RTTY	O
,	O
Hellschreiber	O
,	O
and	O
Olivia	B-Device
MFSK	I-Device
.	O
</s>
<s>
So	O
,	O
once	O
it	O
has	O
been	O
set	O
up	O
to	O
run	O
PSK31	B-Device
,	O
a	O
computer	O
can	O
be	O
used	O
for	O
a	O
variety	O
of	O
digital	O
message	O
transmission	O
modes	O
.	O
</s>
<s>
Aside	O
from	O
a	O
standard	O
radio	O
transceiver	O
and	O
a	O
computer	O
with	O
a	O
sound	B-Device
card	I-Device
,	O
very	O
little	O
equipment	O
is	O
required	O
to	O
use	O
PSK31	B-Device
.	O
</s>
<s>
Normally	O
,	O
an	O
older	O
computer	O
and	O
a	O
few	O
cables	O
will	O
suffice	O
,	O
and	O
many	O
PSK31	B-Device
software	O
applications	O
are	O
free	B-License
and	I-License
open	I-License
source	I-License
.	O
</s>
<s>
These	O
devices	O
incorporate	O
the	O
necessary	O
impedance	O
matching	O
and	O
sound	O
level	O
adjustment	O
to	O
permit	O
the	O
sound	B-Device
card	I-Device
output	O
to	O
be	O
injected	O
into	O
the	O
microphone	O
input	O
,	O
send	O
the	O
radio	O
's	O
audio	O
output	O
to	O
the	O
sound	B-Device
card	I-Device
input	O
,	O
and	O
handle	O
the	O
radio	O
's	O
transmit-receive	O
switching	O
.	O
</s>
<s>
Many	O
interfaces	O
also	O
incorporate	O
their	O
own	O
sound	B-Device
card	I-Device
and	O
can	O
be	O
powered	O
and	O
run	O
from	O
the	O
computer	O
via	O
a	O
single	O
USB	O
connection	O
.	O
</s>
<s>
Like	O
other	O
narrow	O
band	O
digital	O
modes	O
,	O
PSK31	B-Device
can	O
often	O
overcome	O
interference	O
and	O
poor	O
propagation	O
conditions	O
in	O
situations	O
where	O
voice	O
or	O
other	O
methods	O
of	O
communication	O
fail	O
.	O
</s>
<s>
However	O
,	O
PSK31	B-Device
was	O
designed	O
only	O
for	O
leisure	O
use	O
by	O
amateurs	O
,	O
and	O
due	O
to	O
its	O
relatively	O
slow	O
speed	O
and	O
limited	O
error	B-Error_Name
control	I-Error_Name
,	O
is	O
not	O
suitable	O
for	O
transmitting	O
large	O
blocks	O
of	O
data	O
or	O
text	O
,	O
or	O
critical	O
data	O
requiring	O
high	O
immunity	O
from	O
errors	O
.	O
</s>
<s>
PSK31	B-Device
works	O
well	O
over	O
propagation	O
paths	O
that	O
preserve	O
phase	O
,	O
and	O
resists	O
fading	O
(	O
QSB	O
)	O
well	O
.	O
</s>
<s>
Conversely	O
,	O
PSK63	B-Device
is	O
increasingly	O
used	O
for	O
faster	O
exchanges	O
,	O
especially	O
during	O
amateur	O
radio	O
contest	O
operating	O
.	O
</s>
<s>
PSK31	B-Device
is	O
typically	O
created	O
by	O
software	O
that	O
generates	O
an	O
amplitude	O
-	O
and	O
phase-modulated	O
waveform	O
that	O
is	O
converted	O
to	O
an	O
audio	O
frequency	O
analog	O
signal	O
by	O
a	O
sound	B-Device
card	I-Device
.	O
</s>
<s>
In	O
the	O
most-commonly-used	O
variant	O
,	O
BPSK31	B-Device
,	O
binary	O
information	O
is	O
transmitted	O
by	O
either	O
imparting	O
a	O
180-degree	O
phase	O
shift	O
(	O
a	O
binary	O
"	O
zero	O
"	O
)	O
or	O
no	O
phase	O
shift	O
(	O
a	O
binary	O
"	O
one	O
"	O
)	O
in	O
each	O
32ms	O
symbol	O
interval	O
.	O
</s>
<s>
All	O
subsequent	O
amplification	O
of	O
the	O
signal	O
must	O
be	O
linear	O
to	O
preserve	O
the	O
modulation	O
waveform	O
and	O
ensure	O
minimum	O
occupied	O
bandwidth	B-Algorithm
.	O
</s>
<s>
The	O
Varicode	B-General_Concept
is	O
a	O
kind	O
of	O
Fibonacci	B-Algorithm
code	I-Algorithm
where	O
the	O
boundaries	O
between	O
character	O
codes	O
are	O
marked	O
by	O
two	O
or	O
more	O
consecutive	O
zeros	O
.	O
</s>
<s>
Like	O
all	O
Fibonacci	B-Algorithm
codes	I-Algorithm
,	O
since	O
no	O
character	O
code	O
contains	O
more	O
than	O
one	O
consecutive	O
zero	O
,	O
the	O
software	O
can	O
easily	O
identify	O
the	O
spaces	O
between	O
characters	O
,	O
regardless	O
of	O
the	O
length	O
of	O
the	O
character	O
.	O
</s>
<s>
He	O
named	O
this	O
encoding	O
scheme	O
"	O
varicode	B-General_Concept
"	O
.	O
</s>
<s>
PSK31	B-Device
's	O
symbol	O
rate	O
of	O
31.25Hz	O
was	O
chosen	O
because	O
a	O
normal	O
typing	B-Device
speed	I-Device
of	O
about	O
50	O
words	B-Device
per	I-Device
minute	I-Device
requires	O
a	O
bit	O
rate	O
of	O
about	O
32	O
bits	O
per	O
second	O
,	O
and	O
specifically	O
because	O
31.25Hz	O
could	O
easily	O
be	O
derived	O
from	O
the	O
8kHz	O
sample	O
rate	O
used	O
in	O
many	O
DSP	B-General_Concept
systems	O
,	O
including	O
those	O
used	O
in	O
the	O
computer	B-Device
sound	I-Device
cards	I-Device
commonly	O
used	O
for	O
PSK31	B-Device
operation	O
(	O
31.25Hz	O
is	O
8kHz	O
divided	O
by	O
256	O
,	O
and	O
so	O
can	O
be	O
derived	O
from	O
8kHz	O
by	O
halving	O
the	O
frequency	O
eight	O
times	O
in	O
succession	O
)	O
.	O
</s>
<s>
Colloquial	O
usage	O
of	O
the	O
term	O
'	O
PSK31	B-Device
 '	O
in	O
amateur	O
radio	O
usually	O
implies	O
the	O
use	O
of	O
the	O
most	O
commonly	O
used	O
variant	O
of	O
PSK31	B-Device
:	O
binary	B-General_Concept
phase	I-General_Concept
shift	I-General_Concept
keying	I-General_Concept
(	O
BPSK	O
)	O
.	O
</s>
<s>
The	O
BPSK	O
variant	O
of	O
PSK31	B-Device
uses	O
no	O
error	B-Error_Name
control	I-Error_Name
.	O
</s>
<s>
QPSK31	B-Device
,	O
the	O
variant	O
based	O
on	O
quadrature	B-General_Concept
phase	I-General_Concept
shift	I-General_Concept
keying	I-General_Concept
(	O
QPSK	O
)	O
,	O
uses	O
four	O
phases	O
instead	O
of	O
two	O
.	O
</s>
<s>
It	O
is	O
simple	O
to	O
switch	O
from	O
BPSK	O
to	O
QPSK	O
if	O
difficulties	O
arise	O
during	O
a	O
contact	O
;	O
QPSK31	B-Device
has	O
the	O
same	O
number	O
of	O
symbols	O
per	O
second	O
,	O
and	O
hence	O
the	O
same	O
bandwidth	B-Algorithm
as	O
the	O
BPSK	O
variant	O
.	O
</s>
<s>
In	O
a	O
coherent	O
receiver	O
,	O
the	O
bit	O
error	O
probability	O
of	O
QPSK	O
is	O
the	O
same	O
as	O
for	O
BPSK	O
operating	O
at	O
the	O
same	O
power	O
,	O
making	O
QPSK31	B-Device
the	O
generally	O
preferable	O
mode	O
from	O
a	O
robustness	O
,	O
and	O
thus	O
reach	O
,	O
point	O
of	O
view	O
.	O
</s>
<s>
Using	O
four	O
instead	O
of	O
two	O
constellation	O
points	O
provides	O
twice	O
the	O
physical	O
layer	O
bit	O
rate	O
,	O
which	O
allows	O
addition	O
of	O
redundant	O
information	O
to	O
provide	O
a	O
degree	O
of	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
.	O
</s>
<s>
When	O
QPSK	O
is	O
used	O
,	O
after	O
encoding	O
into	O
varicode	B-General_Concept
,	O
the	O
bits	O
of	O
the	O
binary	O
data	O
signal	O
is	O
subject	O
to	O
a	O
rate-1/2	O
channel	O
code	O
,	O
which	O
means	O
that	O
for	O
every	O
information	O
bit	O
,	O
two	O
code	O
bits	O
are	O
calculated	O
and	O
transmitted	O
.	O
</s>
<s>
For	O
that	O
,	O
a	O
convolutional	B-Error_Name
code	I-Error_Name
with	O
constraint	B-Error_Name
length	I-Error_Name
5	O
(	O
i.e.	O
</s>
<s>
At	O
the	O
receiver	O
,	O
a	O
decoder	O
for	O
the	O
convolutional	B-Error_Name
code	I-Error_Name
needs	O
to	O
be	O
used	O
,	O
typically	O
the	O
Viterbi	B-Algorithm
Algorithm	I-Algorithm
,	O
which	O
is	O
able	O
to	O
reconstruct	O
the	O
most	O
likely	O
sent	O
sequence	O
,	O
even	O
if	O
multiple	O
symbols	O
were	O
received	O
incorrectly	O
.	O
</s>
<s>
Optimal	O
decoding	O
must	O
take	O
into	O
account	O
the	O
same	O
constraint	B-Error_Name
length	I-Error_Name
of	O
information	O
bits	O
as	O
encoding	O
,	O
yielding	O
a	O
5-symbol	O
decoding	O
delay	O
,	O
which	O
corresponds	O
to	O
160	O
ms	O
of	O
delay	O
.	O
</s>
<s>
PSK31	B-Device
's	O
efficiency	O
and	O
narrow	O
bandwidth	B-Algorithm
make	O
it	O
highly	O
suitable	O
for	O
low-power	O
and	O
crowded-band	O
operation	O
.	O
</s>
<s>
PSK31	B-Device
contacts	O
can	O
be	O
conducted	O
at	O
less	O
than	O
100Hz	O
separation	O
,	O
so	O
with	O
disciplined	O
operation	O
at	O
least	O
twenty	O
simultaneous	O
PSK31	B-Device
contacts	O
can	O
be	O
carried	O
out	O
side-by-side	O
in	O
the	O
2.5kHz	O
bandwidth	B-Algorithm
required	O
for	O
just	O
one	O
SSB	O
voice	O
contact	O
.	O
</s>
<s>
The	O
following	O
amateur	O
radio	O
frequencies	O
are	O
commonly	O
used	O
for	O
transmitting	O
and	O
receiving	O
PSK31	B-Device
signals	O
.	O
</s>
<s>
PSK31	B-Device
operators	O
generally	O
use	O
upper	O
sideband	O
(	O
USB	O
)	O
,	O
even	O
on	O
frequencies	O
below	O
10MHz	O
where	O
the	O
convention	O
normally	O
calls	O
for	O
lower	O
sideband	O
.	O
</s>
<s>
The	O
was	O
revised	O
in	O
March	O
2009	O
to	O
reflect	O
the	O
expanded	O
40	B-Algorithm
meter	I-Algorithm
band	I-Algorithm
.	O
</s>
