<s>
A	O
voice	B-Device
modem	I-Device
is	O
an	O
analog	O
telephone	O
data	O
modem	B-Application
with	O
a	O
built-in	O
capability	O
of	O
transmitting	O
and	O
receiving	O
voice	O
recordings	O
over	O
the	O
phone	O
line	O
.	O
</s>
<s>
Voice	B-Device
modems	I-Device
are	O
used	O
for	O
telephony	O
and	O
answering	O
machine	O
applications	O
.	O
</s>
<s>
Similar	O
to	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
used	O
for	O
data	O
modems	B-Application
,	O
in	O
which	O
the	O
host	O
PC	O
commands	O
the	O
modem	B-Application
via	O
a	O
series	O
of	O
commands	O
known	O
as	O
AT	B-Device
commands	I-Device
,	O
there	O
exists	O
a	O
well-defined	O
set	O
of	O
common	O
voice	O
AT	B-Device
commands	I-Device
that	O
are	O
somewhat	O
consistent	O
throughout	O
the	O
industry	O
.	O
</s>
<s>
Because	O
voice	O
mode	O
is	O
not	O
the	O
typical	O
use	O
for	O
a	O
modem	B-Application
,	O
many	O
modems	B-Application
on	O
the	O
market	O
have	O
poor	O
or	O
buggy	O
support	O
for	O
their	O
voice	O
modes	O
.	O
</s>
<s>
Characteristics	O
of	O
a	O
good	O
voice	B-Device
modem	I-Device
depend	O
greatly	O
upon	O
the	O
intended	O
application	O
,	O
and	O
include	O
:	O
</s>
<s>
Many	O
modems	B-Application
simply	O
"	O
lock	O
up	O
"	O
or	O
crash	O
the	O
host	O
PC	O
,	O
though	O
this	O
is	O
more	O
common	O
with	O
Winmodems	B-Device
.	O
</s>
<s>
Some	O
modems	B-Application
have	O
an	O
uncorrectably	O
low	O
signal	O
volume	O
or	O
produce	O
audio	O
noise	O
.	O
</s>
<s>
Some	O
modems	B-Application
are	O
unable	O
to	O
recognize	O
all	O
but	O
the	O
best	O
DTMF	O
signals	O
.	O
</s>
<s>
Some	O
modems	B-Application
do	O
a	O
poor	O
job	O
of	O
recording	O
,	O
or	O
detecting	O
and	O
reporting	O
silence	O
or	O
the	O
end-of-call	O
voltage	O
reversal	O
,	O
which	O
some	O
applications	O
need	O
.	O
</s>
<s>
"	O
Type-1	O
caller	O
ID	O
"	O
as	O
used	O
in	O
North	O
America	O
is	O
missing	O
from	O
the	O
vast	O
majority	O
of	O
modems	B-Application
.	O
</s>
<s>
Nearly	O
all	O
modem	B-Application
chipsets	O
support	O
caller	O
ID	O
,	O
but	O
because	O
the	O
typical	O
dial-up	O
Internet	O
user	O
does	O
n't	O
need	O
caller	O
ID	O
,	O
the	O
extra	O
components	O
needed	O
to	O
support	O
caller	O
ID	O
are	O
often	O
omitted	O
for	O
cost	O
reasons	O
.	O
</s>
<s>
The	O
drivers	O
for	O
many	O
internal	O
modems	B-Application
(	O
typically	O
Winmodems	B-Device
)	O
cannot	O
tolerate	O
more	O
than	O
one	O
of	O
the	O
same	O
device	O
inside	O
a	O
single	O
computer	O
.	O
</s>
<s>
Symptoms	O
of	O
incompatibility	O
include	O
crashes	O
,	O
blue	O
screens	O
of	O
death	O
,	O
or	O
simple	O
inoperability	O
of	O
all	O
but	O
a	O
single	O
modem	B-Application
.	O
</s>
<s>
External	O
RS-232-based	O
(	O
serial	O
)	O
modems	B-Application
do	O
not	O
have	O
this	O
limitation	O
because	O
each	O
modem	B-Application
contains	O
its	O
own	O
microprocessor	B-Architecture
and	O
is	O
unaware	O
of	O
other	O
modems	B-Application
on	O
the	O
same	O
host	O
.	O
</s>
<s>
USB	B-Protocol
modems	B-Application
may	O
or	O
may	O
not	O
have	O
this	O
problem	O
,	O
because	O
some	O
USB	B-Protocol
modems	B-Application
are	O
simply	O
serial	O
modems	B-Application
with	O
a	O
"	O
USB-to-serial	O
"	O
converter	O
chipset	O
(	O
in	O
which	O
case	O
there	O
should	O
be	O
no	O
problem	O
)	O
,	O
and	O
other	O
USB	B-Protocol
modems	B-Application
are	O
"	O
host-controlled	O
"	O
and	O
are	O
essentially	O
externally	O
attached	O
Winmodems	B-Device
(	O
in	O
which	O
case	O
the	O
problem	O
may	O
persist	O
)	O
.	O
</s>
<s>
Each	O
voice	B-Device
modem	I-Device
platform	O
tends	O
to	O
support	O
either	O
one	O
of	O
two	O
sets	O
of	O
voice	O
commands	O
—	O
in	O
particular	O
,	O
one	O
flavor	O
of	O
the	O
command	O
set	O
contains	O
a	O
plus	O
( +	O
)	O
sign	O
,	O
and	O
the	O
other	O
contains	O
a	O
hash	O
( #	O
)	O
sign	O
.	O
</s>
<s>
Support	O
for	O
voice	O
mode	O
can	O
be	O
detected	O
on	O
a	O
modem	B-Application
by	O
issuing	O
the	O
following	O
command	O
:	O
AT+FCLASS	O
=	O
?	O
</s>
<s>
This	O
command	O
is	O
usually	O
supported	O
containing	O
the	O
plus	O
sign	O
whether	O
a	O
modem	B-Application
supports	O
"	O
plus	O
"	O
or	O
the	O
"	O
hash	O
"	O
command	O
set	O
,	O
because	O
the	O
command	O
(	O
which	O
stands	O
for	O
"	O
fax	O
class	O
"	O
)	O
is	O
part	O
of	O
the	O
industry-standard	O
fax	O
commands	O
which	O
always	O
use	O
the	O
plus	O
.	O
</s>
<s>
A	O
modem	B-Application
supporting	O
voice	O
will	O
respond	O
with	O
a	O
comma-delimited	O
list	O
of	O
numbers	O
that	O
includes	O
the	O
number	O
8	O
.	O
</s>
<s>
A	O
modem	B-Application
not	O
supporting	O
voice	O
will	O
respond	O
with	O
ERROR	O
,	O
or	O
with	O
a	O
list	O
of	O
numbers	O
not	O
including	O
8	O
.	O
</s>
<s>
(	O
Many	O
modems	B-Application
will	O
report	O
0	O
,	O
1	O
,	O
2	O
indicating	O
support	O
for	O
data	O
(	O
0	O
)	O
,	O
and	O
class	O
1	O
and	O
2	O
faxes	O
—	O
this	O
is	O
an	O
indication	O
that	O
voice	O
support	O
is	O
not	O
present	O
.	O
)	O
</s>
<s>
Modems	B-Application
supporting	O
the	O
"	O
hash	O
"	O
command	O
set	O
usually	O
respond	O
to	O
AT	O
#CLS	O
=	O
?	O
</s>
<s>
The	O
command	O
AT+FCLASS	O
=	O
8	O
or	O
AT	O
#CLS	O
=	O
8	O
will	O
put	O
the	O
modem	B-Application
in	O
voice	O
mode	O
.	O
</s>
<s>
Most	O
modems	B-Application
still	O
remain	O
on-hook	O
and	O
respond	O
with	O
OK	O
.	O
Once	O
this	O
command	O
has	O
been	O
accepted	O
,	O
most	O
modems	B-Application
will	O
respond	O
with	O
Data	O
Link	O
Escape	O
(	O
DLE	O
)	O
messages	O
instead	O
of	O
or	O
in	O
addition	O
to	O
normal	O
modem	B-Application
responses	O
.	O
</s>
<s>
For	O
example	O
,	O
instead	O
of	O
reporting	O
a	O
phone	O
line	O
ringing	O
with	O
the	O
RING	O
message	O
,	O
many	O
modems	B-Application
will	O
instead	O
send	O
the	O
DLE	O
ASCII	B-Protocol
character	I-Protocol
,	O
followed	O
by	O
the	O
letter	O
R	O
.	O
The	O
specific	O
set	O
of	O
DLE	O
events	O
reported	O
by	O
each	O
modem	B-Application
is	O
specific	O
to	O
its	O
chipset	O
and	O
documented	O
in	O
its	O
reference	O
guide	O
.	O
</s>
<s>
usually	O
returns	O
a	O
list	O
of	O
operating	O
modes	O
that	O
are	O
specific	O
to	O
each	O
modem	B-Application
.	O
</s>
<s>
Many	O
chipsets	O
offer	O
a	O
listing	O
of	O
all	O
the	O
possible	O
combinations	O
of	O
modes	O
even	O
if	O
the	O
specific	O
modem	B-Application
board	O
does	O
n't	O
support	O
them	O
all	O
.	O
</s>
<s>
from	O
a	O
modem	B-Application
on	O
the	O
market	O
in	O
2006	O
:	O
</s>
<s>
While	O
every	O
modem	B-Application
is	O
different	O
,	O
usually	O
mode	O
0	O
means	O
on-hook	O
(	O
hung	O
up	O
)	O
and	O
mode	O
1	O
is	O
sufficient	O
to	O
pick	O
up	O
the	O
phone	O
,	O
record/playback	O
audio	O
,	O
and	O
detect	O
DTMF	O
(	O
touch	O
tones	O
)	O
.	O
</s>
<s>
usually	O
returns	O
a	O
list	O
of	O
audio	O
data	O
formats	O
supported	O
by	O
the	O
modem	B-Application
.	O
</s>
<s>
Each	O
format	O
includes	O
a	O
name	O
(	O
such	O
as	O
PCM	B-Algorithm
,	O
ADPCM	B-Algorithm
,	O
μ-law	B-Algorithm
,	O
A-law	B-Algorithm
)	O
,	O
a	O
number	O
of	O
bits	O
per	O
sample	O
(	O
usually	O
2	O
,	O
3	O
,	O
4	O
,	O
8	O
,	O
or	O
16	O
)	O
and	O
an	O
audio	O
sampling	O
rate	O
(	O
usually	O
7	O
,	O
200	O
,	O
8	O
,	O
000	O
,	O
or	O
11,025	O
Hertz	O
)	O
.	O
</s>
<s>
These	O
are	O
industry-standard	O
audio	O
codecs	B-General_Concept
whose	O
implementations	O
are	O
well	O
published	O
.	O
</s>
<s>
The	O
ADPCM	B-Algorithm
standard	O
is	O
an	O
exception	O
.	O
</s>
<s>
Modems	B-Application
claiming	O
to	O
support	O
ADPCM	B-Algorithm
almost	O
always	O
support	O
Dialogic	O
ADPCM	B-Algorithm
,	O
also	O
known	O
as	O
"	O
VOX	O
"	O
,	O
which	O
is	O
similar	O
but	O
not	O
compatible	O
with	O
other	O
ADPCM	B-Algorithm
implementations	O
,	O
including	O
Interactive	O
Multimedia	O
Association	O
(	O
IMA	O
)	O
ADPCM	B-Algorithm
as	O
well	O
as	O
MS	O
ADPCM	B-Algorithm
(	O
a	O
Microsoft	O
implementation	O
used	O
in	O
WAV	O
files	O
)	O
.	O
</s>
<s>
Modems	B-Application
may	O
support	O
these	O
as	O
well	O
,	O
if	O
a	O
qualifier	O
is	O
listed	O
—	O
otherwise	O
,	O
by	O
default	O
,	O
ADPCM	B-Algorithm
means	O
Dialogic	O
.	O
</s>
<s>
from	O
a	O
modem	B-Application
on	O
the	O
market	O
in	O
2006	O
:	O
</s>
<s>
140	O
,	O
"	O
2	O
Bit	O
ADPCM	B-Algorithm
"	O
,	O
2	O
,	O
0	O
,	O
8000	O
,	O
</s>
<s>
Answering	O
calls	O
is	O
usually	O
done	O
with	O
either	O
the	O
AT+VLS	O
=	O
n	O
or	O
AT	O
#VLS	O
=	O
n	O
commands	O
,	O
where	O
n	O
is	O
a	O
number	O
representing	O
the	O
modem	B-Application
's	O
mode	O
.	O
</s>
<s>
For	O
the	O
vast	O
majority	O
of	O
modems	B-Application
,	O
this	O
number	O
will	O
be	O
1	O
to	O
answer	O
a	O
telephone	O
call	O
,	O
and	O
0	O
to	O
hang	O
up	O
;	O
other	O
numbers	O
activate	O
other	O
functionality	O
when	O
present	O
,	O
such	O
as	O
speakerphone	O
.	O
</s>
<s>
Some	O
modems	B-Application
answer	O
in	O
response	O
to	O
ATA	O
—	O
the	O
standard	O
data-mode	O
answer	O
command	O
—	O
but	O
other	O
modems	B-Application
will	O
interpret	O
this	O
as	O
a	O
command	O
to	O
actually	O
answer	O
in	O
data	O
and	O
not	O
voice	O
mode	O
.	O
</s>
<s>
This	O
results	O
in	O
a	O
response	O
from	O
the	O
modem	B-Application
of	O
CONNECT	O
or	O
VCON	O
.	O
</s>
<s>
(	O
Modems	B-Application
using	O
the	O
"	O
plus	O
"	O
command	O
set	O
usually	O
respond	O
CONNECT	O
,	O
while	O
those	O
using	O
the	O
"	O
hash	O
"	O
set	O
respond	O
VCON	O
,	O
which	O
stands	O
for	O
voice	O
connect	O
.	O
)	O
</s>
<s>
From	O
then	O
on	O
,	O
the	O
modem	B-Application
interprets	O
any	O
data	O
sent	O
from	O
the	O
computer	O
as	O
wave	O
audio	O
data	O
,	O
using	O
the	O
codec	B-General_Concept
selected	O
by	O
the	O
AT+VSM	O
or	O
AT	O
#VSM	O
command	O
.	O
</s>
<s>
The	O
audio	O
data	O
is	O
always	O
sent	O
to	O
the	O
modem	B-Application
slightly	O
faster	O
than	O
it	O
can	O
play	O
it	O
,	O
so	O
the	O
modem	B-Application
may	O
buffer	B-General_Concept
a	O
small	O
portion	O
of	O
it	O
and	O
play	O
it	O
smoothly	O
with	O
no	O
clicks	O
or	O
pops	O
caused	O
by	O
delays	O
in	O
the	O
computer	O
's	O
operating	O
system	O
.	O
</s>
<s>
For	O
example	O
,	O
during	O
playback	O
of	O
an	O
8kHz	O
audio	O
file	O
at	O
8-bit	O
resolution	O
(	O
which	O
creates	O
8,000	O
bytes	O
,	O
or	O
80,000	O
bits	O
when	O
including	O
start/stop	O
bits	O
,	O
per	O
second	O
)	O
,	O
the	O
data	O
must	O
travel	O
over	O
the	O
serial	B-Protocol
port	I-Protocol
at	O
a	O
minimum	O
of	O
115,200	O
bits	O
per	O
second	O
.	O
</s>
<s>
(	O
115,200	O
bit/s	O
is	O
the	O
first	O
setting	O
of	O
a	O
typical	O
computer	O
serial	B-Protocol
port	I-Protocol
that	O
's	O
greater	O
than	O
80,000	O
.	O
)	O
</s>
<s>
In	O
addition	O
,	O
due	O
to	O
some	O
extra	O
overhead	O
involved	O
in	O
doubling	O
DLE	O
bytes	O
in	O
the	O
stream	O
(	O
mentioned	O
below	O
)	O
,	O
a	O
small	O
amount	O
of	O
extra	O
bandwidth	B-Algorithm
is	O
mandatory	O
to	O
allow	O
for	O
this	O
.	O
</s>
<s>
When	O
the	O
modem	B-Application
wants	O
the	O
computer	O
to	O
temporarily	O
pause	O
so	O
the	O
playback	O
can	O
catch	O
up	O
,	O
it	O
temporarily	O
lowers	O
the	O
CTS	O
(	O
Clear	O
to	O
Send	O
)	O
signal	O
on	O
the	O
RS-232	O
serial	B-Protocol
port	I-Protocol
.	O
</s>
<s>
The	O
modem	B-Application
re-raises	O
the	O
signal	O
in	O
time	O
for	O
the	O
computer	O
to	O
resume	O
sending	O
audio	O
data	O
before	O
the	O
playback	O
buffer	B-General_Concept
becomes	O
completely	O
empty	O
.	O
</s>
<s>
When	O
the	O
computer	O
wants	O
to	O
signal	O
the	O
end	O
of	O
audio	O
data	O
,	O
most	O
modems	B-Application
expect	O
to	O
see	O
an	O
ASCII	B-Protocol
DLE	O
character	O
(	O
0x10	O
)	O
,	O
followed	O
by	O
the	O
!	O
</s>
<s>
Because	O
the	O
DLE	O
byte	O
can	O
and	O
often	O
does	O
occur	O
in	O
normal	O
audio	O
data	O
,	O
it	O
must	O
be	O
sent	O
twice	O
to	O
the	O
modem	B-Application
when	O
it	O
is	O
to	O
be	O
interpreted	O
as	O
a	O
byte	O
of	O
audio	O
data	O
.	O
</s>
<s>
Most	O
modems	B-Application
also	O
accept	O
a	O
sequence	O
of	O
DLE	O
+	O
CAN	O
(	O
cancel	O
)	O
as	O
a	O
signal	O
to	O
cancel	O
audio	O
playback	O
.	O
</s>
<s>
The	O
distinction	O
is	O
that	O
the	O
modem	B-Application
is	O
to	O
understand	O
that	O
it	O
is	O
to	O
immediately	O
abort	O
playback	O
now	O
,	O
rather	O
than	O
let	O
remaining	O
data	O
in	O
the	O
playback	O
buffer	B-General_Concept
run	O
to	O
completion	O
.	O
</s>
<s>
When	O
the	O
modem	B-Application
is	O
done	O
playback	O
,	O
it	O
responds	O
OK	O
.	O
</s>
<s>
During	O
playback	O
,	O
it	O
is	O
necessary	O
to	O
send	O
the	O
audio	O
data	O
at	O
a	O
rate	O
that	O
keeps	O
the	O
audio	O
playing	O
smoothly	O
,	O
but	O
without	O
sending	O
it	O
faster	O
than	O
the	O
modem	B-Application
can	O
handle	O
it	O
.	O
</s>
<s>
It	O
is	O
also	O
desirable	O
to	O
make	O
sure	O
the	O
modem	B-Application
can	O
always	O
abort	O
playback	O
and	O
discard	O
any	O
buffered	O
audio	O
in	O
case	O
a	O
message	O
is	O
to	O
be	O
canceled	O
.	O
</s>
<s>
There	O
are	O
several	O
ways	O
to	O
keep	O
the	O
computer	O
sending	O
audio	O
data	O
to	O
the	O
modem	B-Application
at	O
a	O
rate	O
to	O
keep	O
up	O
with	O
playback	O
without	O
overrunning	O
the	O
audio	O
buffer	B-General_Concept
.	O
</s>
<s>
Some	O
voice	B-Device
modems	I-Device
have	O
bugs	O
in	O
their	O
implementation	O
of	O
flow	O
control	O
.	O
</s>
<s>
Conexant	O
is	O
a	O
hugely	O
popular	O
chipset	O
in	O
voice	B-Device
modems	I-Device
today	O
and	O
they	O
otherwise	O
implement	O
voice	O
commands	O
well	O
,	O
making	O
it	O
worthwhile	O
to	O
consider	O
working	O
around	O
this	O
bug	O
.	O
</s>
<s>
Some	O
Conexant	O
chipsets	O
will	O
also	O
not	O
bring	O
CTS	O
back	O
up	O
if	O
the	O
"	O
playback	O
abort	O
"	O
command	O
is	O
sent	O
or	O
processed	O
by	O
the	O
modem	B-Application
while	O
CTS	O
is	O
down	O
.	O
</s>
<s>
Some	O
voice	B-Device
modems	I-Device
offer	O
a	O
very	O
large	O
transmit	O
buffer	B-General_Concept
(	O
for	O
example	O
,	O
4	O
seconds	O
 '	O
worth	O
of	O
audio	O
)	O
coupled	O
with	O
a	O
bug	O
that	O
prevents	O
the	O
host	O
from	O
requesting	O
an	O
"	O
abort	O
playback	O
"	O
.	O
</s>
<s>
It	O
is	O
reasonable	O
to	O
assume	O
that	O
the	O
PC	O
needs	O
to	O
stay	O
ahead	O
of	O
the	O
playback	O
by	O
a	O
couple	O
of	O
hundred	O
bytes	O
and	O
that	O
the	O
modem	B-Application
will	O
buffer	B-General_Concept
this	O
.	O
</s>
<s>
(	O
The	O
commands	O
AT+VBQ	O
or	O
AT	O
#VBQ	O
on	O
voice	B-Device
modems	I-Device
will	O
often	O
reveal	O
the	O
size	O
of	O
the	O
buffer	B-General_Concept
in	O
bytes	O
,	O
and	O
1	O
to	O
2	O
kilobytes	O
is	O
a	O
typical	O
response	O
.	O
)	O
</s>
<s>
A	O
third	O
way	O
to	O
throttle	O
playback	O
involves	O
inserting	O
dummy	O
DLE	O
messages	O
into	O
the	O
output	O
stream	O
such	O
that	O
the	O
audio	O
data	O
takes	O
a	O
known	O
amount	O
of	O
time	O
to	O
transmit	O
through	O
the	O
serial	B-Protocol
port	I-Protocol
,	O
and	O
the	O
playback	O
is	O
essentially	O
clocked	O
by	O
the	O
UART	O
in	O
the	O
serial	B-Protocol
port	I-Protocol
.	O
</s>
<s>
The	O
trick	O
involves	O
inserting	O
enough	O
meaningless	O
DLE	O
messages	O
into	O
the	O
bytes	O
that	O
the	O
modem	B-Application
will	O
discard	O
(	O
that	O
is	O
,	O
a	O
DLE	O
followed	O
by	O
a	O
byte	O
without	O
any	O
specific	O
meaning	O
)	O
so	O
that	O
there	O
are	O
exactly	O
11,520	O
bytes	O
(	O
assuming	O
a	O
serial	B-Protocol
port	I-Protocol
locked	O
at	O
115,200	O
bit/s	O
)	O
which	O
will	O
take	O
exactly	O
1	O
second	O
to	O
transmit	O
through	O
the	O
serial	B-Protocol
port	I-Protocol
.	O
</s>
<s>
Although	O
it	O
is	O
possible	O
that	O
interrupt	O
latency	O
on	O
the	O
host	O
PC	O
may	O
cause	O
slightly	O
less	O
than	O
11,520	O
bytes	O
to	O
be	O
sent	O
per	O
second	O
,	O
most	O
voice	B-Device
modems	I-Device
will	O
buffer	B-General_Concept
enough	O
bytes	O
before	O
actually	O
starting	O
playback	O
to	O
permit	O
a	O
small	O
skew	O
here	O
.	O
</s>
<s>
Also	O
the	O
PC	O
can	O
be	O
programmed	O
to	O
convert	O
a	O
second	O
of	O
audio	O
into	O
slightly	O
fewer	O
than	O
11,520	O
bytes	O
(	O
all	O
voice	B-Device
modems	I-Device
will	O
buffer	B-General_Concept
a	O
small	O
overrun	B-General_Concept
without	O
the	O
need	O
for	O
flow	O
control	O
as	O
long	O
as	O
it	O
is	O
no	O
more	O
than	O
a	O
few	O
hundred	O
bytes	O
)	O
.	O
</s>
<s>
Dummy	O
DLE	O
stuffing	O
is	O
unlikely	O
to	O
work	O
with	O
"	O
Winmodems	B-Device
"	O
that	O
have	O
no	O
physical	O
UART	O
.	O
</s>
<s>
It	O
makes	O
sense	O
only	O
with	O
external	O
serial	O
modems	B-Application
that	O
are	O
physically	O
clocked	O
to	O
a	O
specific	O
bit	O
rate	O
by	O
a	O
clock	O
generator	O
behind	O
the	O
external	O
serial	B-Protocol
port	I-Protocol
.	O
</s>
<s>
The	O
method	O
for	O
recording	O
audio	O
data	O
is	O
the	O
same	O
,	O
except	O
that	O
the	O
command	O
is	O
AT+VRX	O
,	O
or	O
AT	O
#VRX	O
,	O
and	O
the	O
modem	B-Application
transmits	O
audio	O
data	O
while	O
the	O
computer	O
receives	O
it	O
.	O
</s>
<s>
The	O
RTS/CTS	O
flow	O
control	O
are	O
not	O
used	O
here	O
(	O
the	O
computer	O
must	O
accept	O
all	O
the	O
audio	O
data	O
it	O
receives	O
,	O
and	O
the	O
modem	B-Application
automatically	O
paces	O
its	O
transmission	O
to	O
match	O
the	O
audio	O
sampling	O
rate	O
)	O
.	O
</s>
<s>
The	O
modem	B-Application
never	O
stops	O
transmitting	O
until	O
the	O
computer	O
tells	O
it	O
to	O
stop	O
,	O
which	O
is	O
usually	O
with	O
CTRL-C	O
.	O
</s>
<s>
Before	O
,	O
during	O
,	O
and	O
after	O
recording	O
,	O
the	O
modem	B-Application
may	O
notify	O
the	O
computer	O
host	O
of	O
specific	O
events	O
including	O
,	O
but	O
not	O
limited	O
to	O
,	O
the	O
following	O
:	O
</s>
<s>
When	O
the	O
modem	B-Application
wants	O
to	O
tell	O
the	O
host	O
about	O
these	O
,	O
it	O
sends	O
a	O
DLE	O
byte	O
,	O
plus	O
a	O
(	O
usually	O
)	O
1-byte	O
message	O
describing	O
the	O
event	O
.	O
</s>
<s>
The	O
list	O
of	O
supported	O
events	O
varies	O
by	O
modem	B-Application
,	O
but	O
usually	O
a	O
digit	O
(	O
as	O
well	O
as	O
*	O
and	O
#	O
)	O
mean	O
touch-tones	O
pressed	O
,	O
and	O
the	O
letter	O
"	O
s	O
"	O
means	O
silence	O
detected	O
.	O
</s>
<s>
Some	O
modems	B-Application
report	O
only	O
one	O
event	O
for	O
each	O
touch-tone	O
keypress	O
,	O
while	O
others	O
report	O
a	O
keypress	O
repeatedly	O
until	O
the	O
key	O
is	O
released	O
,	O
and	O
then	O
a	O
special	O
"	O
key	O
released	O
"	O
event	O
.	O
</s>
<s>
Any	O
of	O
the	O
following	O
commands	O
usually	O
cause	O
the	O
modem	B-Application
to	O
hang	O
up	O
and	O
terminate	O
a	O
voice	O
call	O
:	O
AT+VLS	O
=	O
0	O
,	O
AT	O
#VLS	O
=	O
0	O
,	O
ATH	O
,	O
ATZ	O
.	O
</s>
<s>
The	O
modem	B-Application
remains	O
in	O
voice	O
mode	O
(	O
except	O
in	O
the	O
case	O
of	O
ATZ	O
)	O
.	O
</s>
<s>
Voice	B-Device
modems	I-Device
do	O
not	O
automatically	O
hang	O
up	O
even	O
when	O
the	O
caller	O
on	O
the	O
other	O
end	O
does	O
.	O
</s>
<s>
If	O
when	O
the	O
modem	B-Application
is	O
recording	O
,	O
the	O
caller	O
hangs	O
up	O
and	O
the	O
computer	O
does	O
n't	O
react	O
,	O
the	O
modem	B-Application
will	O
continue	O
providing	O
the	O
audio	O
recording	O
everything	O
else	O
heard	O
on	O
the	O
line	O
,	O
such	O
as	O
dial	O
tones	O
,	O
telephone	O
company	O
error	O
messages	O
,	O
and	O
so	O
forth	O
.	O
</s>
