📄 hardduplex.htm
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<H2><A NAME="hardduplex"></A>Half-duplex vs. full-duplex</H2>
<P>On a regular telephone, you can talk and listen at the same
time: the telephone is a <I>full-duplex</I> device. With a
portable radio walkie-talkie, on the other hand, as long as you
hold down the "Talk" button, you can't hear anybody else who's
trying to talk to you--that's why radio users say "Over" at the
end of a transmission--so the other person knows you've
finished and they can talk now. The walkie-talkie is
<I>half-duplex</I>: it can communicate in both directions but
only one way at a time. (A radio broadcast station is
<I>simplex:</I> you can't respond to its transmissions at all,
except by calling the DJ on the phone.)</P>
<P>Audio hardware on the Sun and Silicon Graphics workstations
which first ran <A HREF="sfunixmanpage.htm"><i>Speak Freely</i> for Unix</A>
was full-duplex but, unfortunately, many inexpensive sound
boards installed in early Windows machines were half-duplex, intended
for "recording" and "playing" like a tape recorder instead of
real-time conversation.</P>
<P><i>Speak Freely</i> copes with half-duplex audio hardware in the
following manner. When launched, it immediately attempts to
open both audio input and output simultaneously, input first.
If the output open fails, but then succeeds on a second try
after input has been closed, the hardware is marked
half-duplex. You can see whether <i>Speak Freely</i> detected your
hardware to be half- or full-duplex by displaying the <A
HREF="hardabout.htm">Help/About Speak Freely</A> or <A
HREF="propellerhead.htm">Help/Extended Status</A> dialogue
boxes.</P>
<P>If a half-duplex card is installed, pressing the mouse
button to send live audio immediately mutes any sound you're
receiving and discards any that arrives while you're talking
unless you've checked the <A
HREF="commreceive.htm">Options/Break Input</A> menu item, in
which case arriving sound disrupts your transmission. When you
release the mouse button, output of sound from other hosts
resumes. You can <A HREF="commsendfile.htm">send sound
files</A> to hosts even while they're transmitting to you but,
of course, if their own hardware is half-duplex they won't hear
it.</P>
<P>If your hardware is half-duplex, use the <A
HREF="commsend.htm">double click</A> feature with great care.
If you accidentally leave a connection in constant
transmission, output will remain muted and nobody will be able
to speak to you. If an input-output conflict has caused your
audio hardware to be treated as half-duplex, an indication to
that effect appears in the <A HREF="hardabout.htm">Help/About
Speak Freely</A> dialogue box.</P>
<P>Some users with full-duplex audio hardware routinely
transmit continuously. If you're talking to somebody
who does this, consider asking them to indicate, perhaps by
saying "Over", when they're done speaking and expect a response
from you and, ideally, switch to push-to-talk mode when
communicating with you.</P>
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