Voice-operated switch

Lore & Background

VOX circuits include a delay between the sound stopping and switching direction to avoid turning off during short pauses in speech. A special case exists where a microphone may send a voltage high enough to directly operate a transmitter, providing enough energy to power the system directly. VOX was used by some of the first users, including pilots and astronauts, who would habitually preface every transmission with 'uh' to overcome the short activation time that could clip the beginnings of phrases. Older VOX systems had finite activation times, but modern circuits can eliminate this problem by recording or transmitting a delayed version of the input signal.

Reader's Guide

VOX offers the advantage of hands-free operation compared to manual push-to-talk (PTT), but it has significant disadvantages that explain why PTT remains common. Most VOX circuits have a sensitivity adjustment, yet unwanted triggering can occur on background noise, heavy breathing, or side conversations, while speech that is too weak may not activate it. In two-way radios, VOX can be triggered by the loudspeaker carrying the other side of the conversation, a problem minimized with an 'anti vox' feature that decreases sensitivity when the receiver is active. The hang timer, typically 1–3 seconds, keeps the switch engaged during brief pauses but means the last several seconds of each transmission or recorded segment are always silence. A VOX-activated recorder can delete the end of each segment, but a user of a VOX-activated half-duplex radio must wait for the timer to expire before receiving again. These limitations have kept PTT in use despite VOX's convenience.

Did You Know?

• VOX is also called a voice-operated exchange.
• A microphone can send a voltage high enough to directly operate a transmitter in a special case.
• Pilots and astronauts were among the first users of VOX.
• The hang timer typically lasts 1–3 seconds.