Spectral Transformers:

IMPORTANT: this filtering technique is crude but computationally efficient. It can introduce significant distortion, depending on the spectral and temporal nature of the audio material being processed. This can lead to interesting audio effects, but may be unacceptable for a given application. If accurate filtering is required, refer to the Digital Filters category of the Component Library.

The attributes of the filter are adjustable via the Parameter Window, as summarised in the following table. Stereo signals have separate adjustments for left and right channels.

Parameter	Purpose
"Filter type" selection	Selects between a low-pass or high-pass filter. The idealised frequency response is displayed in the plot window. The actual frequency response will, in general, be highly distorted due to the approximate nature of the filtering method.
"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 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.
"Output gain" slider	Adjusts the overall gain of the output signal, after the filtering has been applied.
"Lower freq" slider	Adjusts the filter's lower cut-off frequency over the allowable range from 0 to the Nyquist frequency (defined as half the sample rate).
"Upper freq" slider	Adjusts the filter's upper cut-off frequency over the allowable range from the lower cut-off frequency to the Nyquist frequency (defined as half the sample rate).
"Band gain" slider	Adjusts the stop-band gain of the filter.
"Window type" selection	Selects the profile of the windowing function applied to the input data.

Algorithm

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.

Signal Implementations

• Brick Wall 1
• Brick Wall 2
• Brick Wall 3
• Controllable Brick Wall 1
• Controllable Brick Wall 2
• Controllable Brick Wall 3
• Controllable Brick Wall 4
• Controllable Spectral Morph 1
• Controllable Spectral Shift 1
• Convolution
• Spectral 3-Peak Detector
• Spectral Averaging
• Spectral Compressor
• Spectral Expander
• Spectral Gate
• Spectral Morph 1
• Spectral Peak Detector
• Spectral Shift 1
• Spectral Shift 2
• Spectral X-Synth 1
• Spectrum Shaper
• Tracking 3-Peak Detector
• Tracking Peak Detector

• ApproximateBrickWallFiltering4