What is a 'notch filter' used for in digital signal processing?

A notch filter is specifically designed to attenuate a specific frequency while allowing other frequencies to pass through with minimal impact. This is particularly useful in various applications, such as in audio processing where certain unwanted frequencies, like hum or noise from electrical equipment, may interfere with the desired signal. By targeting and reducing the amplitude of that specific frequency, a notch filter helps to clean up the signal, ensuring that clarity and quality are improved without affecting the surrounding frequencies. This selective attenuation is what makes notch filters invaluable in signal processing contexts where precision is crucial.

The other options focus on broader concepts that do not accurately describe the specialized function of a notch filter: reducing overall signal level or amplifying all frequencies does not highlight the specific targeting capacity of a notch filter, and enhancing a specific frequency goes against its primary function of attenuation.