What does keying refer to in a transmitter context?

Keying in the context of a transmitter primarily refers to the process of turning the radio frequency (RF) output on and off. This action is fundamental for various types of communication where information is transmitted through modulation of the carrier signal. When a transmitter is keyed, it essentially allows the flow of RF signal to be activated or deactivated, which can represent different bits of information in Morse code or other digital communications.

This on-off control is crucial for generating signals that can be interpreted at the receiving end. It enables the transmission of data in a way that can vary the presence of the carrier wave, effectively encoding messages through signal presence (on) and absence (off). Thus, this aspect is integral to effective communication in radio systems, making keying a vital function within the transmitter's operation.

The other options focus on different aspects of signal processing. For instance, controlling the oscillator involves managing the frequency of the RF signal, modulating the RF signal deals with altering its characteristics to contain information, and amplifying sound waves relates to the enhancement of audio signals rather than RF transmission. Each of these plays a role in the broader context of communication technology but does not define keying directly.