remotely operable electronic apparatus are applied to distance measurement and picture / sound recording wherein , for example , a remote unit may be equipped with a microphone and an active range finder coupled to an auto - focussing capability . in one embodiment a camera is adapted to transmit a signal to the remote unit which returns a signal enabling the camera to accurately calculate distance and adjust focus in accordance with time - of - flight . the transponder preferably forms part of a wireless microphone , with the signal transmitted by the camera being preferably acoustical in nature . the acoustical signal transmitted by the camera is preferably modulated onto a return rf signal , enabling at least a portion of the audio circuitry associated with voice modulation to be used for the purpose of modulating the ranging signal , thereby realizing a very efficient and economical approach to remote sound detection and active ranging finding .