VOX and PTT
| VOX and PTT | |
|---|---|
| PTT | Push-to-talk: a switch, usually on the microphone, that puts the radio into transmit while pressed |
| VOX | Voice-operated transmit: the radio switches to transmit when it hears the operator's voice |
| VOX controls | VOX gain, VOX delay and anti-VOX |
| Other ways to key | Footswitch, CAT command, RTS or DTR serial line |
| Related | |
| CW equivalent | Semi break-in and full break-in (QSK) |
| Time-out timer | Limits the length of a single transmission |
| Remote stations (US) | Transmissions limited to 3 minutes if the control link fails |
PTT (push-to-talk) and VOX (voice-operated transmit, often expanded as voice-operated exchange or switch) are the two ways a voice operator switches a transceiver between receive and transmit. With PTT, the operator holds down a switch, usually on the microphone, for as long as they want to talk. With VOX, the radio listens to the microphone and switches to transmit by itself when the operator speaks, then returns to receive when they stop. The same ideas carry over to computer-controlled digital modes, where software keys the transmitter, and to the timers that stop a radio from transmitting indefinitely.
Push-to-talk
The PTT switch on a hand microphone, a desk microphone or a handheld is the most common method and the easiest to understand: transmit while pressed, receive when released. Many operators prefer a footswitch, which leaves both hands free for tuning and logging and is a fixture of contest stations. Headsets made for amateur use generally work with either a hand switch or a footswitch.
PTT also names the control signal itself, the line that tells the radio and anything connected to it to switch to transmit. Most transceivers have a separate keying output, often labeled SEND or TX, that switches an external amplifier, transverter or preamplifier at the same moment. Equipment with relays needs a few milliseconds to change over. If RF arrives before a relay's contacts have closed, called hot switching, it can damage the relay or the equipment behind it. Many radios have a transmit delay setting for this reason, and stations with several devices in line often use a sequencer, which switches them in a fixed order on transmit and the reverse order on receive.
VOX
VOX lets an operator talk hands-free, much as on a telephone. Three settings control it:
- VOX gain, or sensitivity, sets how loud a sound must be to trigger transmit. Set too high, the radio keys on a cough, a closing door or the hum of a fan; set too low, the first syllable of each sentence is lost.
- VOX delay, or hang time, sets how long the radio stays in transmit after the voice stops. Too short, and the transmitter chatters in and out between words; too long, and the operator misses the start of the other station's reply.
- Anti-VOX keeps sound from the station's speaker from tripping the VOX. It feeds a sample of the receiver audio to the VOX circuit, which reduces its sensitivity to that sound, so only the operator's voice keys the transmitter. Wearing headphones removes the problem altogether.
VOX is popular for SSB ragchews and for operators who type a log while they talk. Its drawback is that it transmits whatever the microphone hears, including side conversations and household noise, so the room needs to be reasonably quiet.
On CW, many transceivers offer a related feature. In semi break-in, the transmitter switches on when the key closes and stays on for an adjustable delay after the last character, much like VOX. In full break-in, or QSK, it switches between every dot and dash, so the operator can hear the other station between characters.
Digital modes
For FT8 and other digital modes, the computer has to key the radio. There are three common methods. The software can send a transmit command over the radio's CAT (computer aided transceiver) control link; it can raise the RTS or DTR line on a serial port wired to the radio's PTT input; or the radio, or an interface, can use VOX and key whenever audio arrives from the computer. CAT and serial-line keying are generally the most reliable. VOX keying is the simplest to wire, but it transmits any sound the computer makes, including notification chimes. When the radio is keyed by CAT command, it usually must be set to take its transmit audio from the data or USB input rather than the microphone.
Time-out timers
A time-out timer (TOT) limits how long a single transmission can last. Many radios, especially VHF and UHF mobiles and handhelds, have one in the menu, adjustable in minutes. Its main job is to catch a stuck microphone, a PTT switch held down by accident, as when a microphone slips under a car seat or someone sits on a handheld. Without a timer, the radio would block a frequency, or a whole repeater, until the battery ran down or the operator noticed.
Repeaters have timers of their own, usually two or three minutes, that cut off a station that talks too long. Software has similar safeguards: WSJT-X has a transmit watchdog that stops automatic repeated transmissions after a set time without any action from the operator. Under §97.213 of Part 97, a station operated by remote control must be able to limit its transmissions to no more than three minutes if the control link fails.