Spectral cross-synthesis based on direct manipulation of FFT spectra. The audio output is derived from a "blend" of the input audio spectra. Specifically, the phase of the first input spectrum is combined with the magnitude of the second. The characteristics of the spectral cross-synthesis are adjustable via the Parameter Window, as summarised in the following table. Parameter Purpose "Buffer length" slider Adjusts the length of the input data buffer, which also defines the "Latency" (overall delay) of the process. The FFT buffer size is computed from the "Buffer length" rounded up to the nearest power of 2 (for efficient FFT computation). If "double zero-padding" is selected, the FFT buffer size is doubled (after the rounding) to improve the smoothness of the spectrum between successive FFT bins (but without increasing the underlying frequency resolution). The input data buffer is windowed (using a selected profile), then extended to the length of the FFT buffer by padding with zeros (on either side). Successive input data buffers are overlapped by a factor of 2, and the ouputs of the overlapped inverse transformed buffers are summed, in order to avoid discontinuities at the window boundaries. "Blend factor" slider Adjusts the extent of the spectral blending. "Output gain" slider Adjusts the overall gain of the output signal, after the spectral manipulations have been applied. "Window type" selection Selects the profile of the windowing function applied to the input data.
Each successive input buffer is transformed to the frequency domain using the windowed-FFT with double overlapping. The spectra of the two input signals are blended on a point-by-point basis by combining the phase of the first input with the magnitude of the second. The modified spectrum is then re-converted to the time domain (using the inverse FFT), and the overlapped output buffers are summed to yield the output signal. For an introduction to the Discrete Fourier Transform and the FFT, see, for example, [St] sections 4.1 and 4.2. For further introductory information (with emphasis on audio applications), and for discussions on spectral measurements, zero-padding, windowing, the overlap-add re-synthesis method, and the Short Time Fourier Transform (STFT) for audio applications, see [Roa] p. 1084-1112 and [Moore] p. 61-111.
