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Performance Implications of Frequency Domain Crossfading for Time-Varying FIR Filtering

A common strategy to avoid audible artifacts when exchanging impulse responses used for convolution with audio signals at runtime is to use a simple crossfade. This often requires more than double the amount of operations by filtering the input signal with both impulse responses separately (often in the frequency domain), and blending the resulting signals in the time-domain. If the crossfade was applied in the frequency domain, one IFFT operation could be eliminated, at the cost of two direct convolutions between the two signal spectra with the blending functions. Most of the time however, transforming each filtered signal separately back into the time domain before blending is still significantly faster than applying the blending function through convolution in the frequency domain. With a carefully chosen blending function, direct convolution of the spectra could be faster than the time-domain multiplication. We look at why and how this technique might work and the associated performance implications.

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