The ratio of output to input as a function of frequency. The frequency-domain characteristics of a system (e.g., a filter). The complex function of frequency that changes sinusoidal inputs into outputs. Multiplying the frequency-domain transform of the input by the transfer function yields the frequency-domain transform of the output. The transfer function usually is represented by amplitude-versus-frequency and phase-versus-frequency curves; these contain the same information as the impulse response in the time domain and are convertible into the impulse response through the Fourier transform.