Half-duplex vs. full-duplex

On a regular telephone, you can talk and listen at the same time: the telephone is a full-duplex 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 half-duplex: it can communicate in both directions but only one way at a time. (A radio broadcast station is simplex: you can't respond to its transmissions at all, except by calling the DJ on the phone.)

Audio hardware on the Sun and Silicon Graphics workstations which first ran Speak Freely for Unix 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.

Speak Freely 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 Speak Freely detected your hardware to be half- or full-duplex by displaying the Help/About Speak Freely or Help/Extended Status dialogue boxes.

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 Options/Break Input 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 send sound files to hosts even while they're transmitting to you but, of course, if their own hardware is half-duplex they won't hear it.

If your hardware is half-duplex, use the double click 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 Help/About Speak Freely dialogue box.

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.