Which of the following is not a method used for peak limiting, in a signal processor?

Peak limiting is a technique used in transmitters to prevent overmodulation by restricting the maximum amplitude of a signal. This can be effectively achieved through AF (audio frequency) clipping, RF (radio frequency) clipping, or audio compression. AF and RF clipping physically slice off the highest peaks of the signal waveform in their respective domains, preventing the amplitude from exceeding a set threshold. Compression dynamically reduces the gain when the overall signal gets too loud, which also acts to limit peaks. 'Frequency clipping' is a nonsensical term in this context. While you can limit a signal's bandwidth through filtering, 'clipping' is strictly an amplitude-domain operation.