FILTERS: Detailed Information

by Archer Endrich

Key Features of the CDP FILTER set

• time-varying Q – the tightness, the degree of focus on the centre pitch of the various filter bands, can vary over time.
• time-varying user-defined filter bands – which frequencies are used as the centres of the filter bands, can vary over time.
• automatic generation – of filter bands of various types: harmonic, subharmonic etc.
• specialist effects – such as sweeping and phasing filters, and repeating the sound with ever-increasing filtering (iterative).

Some possible musical results with FILTER

• Standard Low-pass, High-pass, Band-pass and Band-reject filtering
• Filtering with pre-defined (in the program) types of filter bank: harmonics, alternate harmonics, subharmonics, harmonics with offset, or spaced at equal intervals, possibly with time-varying Q. One can also create such filter bank files with BANKFREQS and then edit them to suit, or use USERBANK to make use of an original filter bank written with a text editor.
• Filters which sweep through a specified range of frequencies, which may vary over time, with High-pass, Low-pass, Band-pass and Band-reject options, as well as the possibility of time-varying Q: SWEEP.
• A PHASING filter which passes all frequencies while changing their phase relationships. Softly wavering effects are created.
• Repeating the sound for a specified duration with cumulative (ever-increasing) filtering. The delay time between iterations can be defined, and may vary over time.

Other forms of filtering in CDP

• All the the main forms of filtering (Hi-pass etc.) are also available in the spectral dimension, i.e., operating directly on analysis data.
• In general terms, filtering is a form of data-reduction, and there are many different forms of this in the CDP System. Here are some of them:
• HILITE TRACE retains a specified number of the loudest partials, while BLUR SUPPRESS removes them.
• COMBINE MAX retains the loudest windows of several soundfiles on a window-to-window basis, thus serendipitously mixing the files.
• PITCH ALTHARMS retains either the odd or the even harmonics, removing the others.
• DISTORT DELETE in the Distortion packages time-contracts a sound by deleting wavecycles.

General observations

• Lo-pass and High-pass filters can be used to remove unwanted high or low components from a sound. They can also be used to muffle or brighten a sound.
• Developing a sound by other means of sound transformation is an important part of preparing it before introducing filter effects.
• Change over time, by means of time-varying Q or filter bands, creates timbral variation. A low Q allows more of the original sound through, while a high Q creates resonant pitched tones.
• It is difficult to ascertain just which frequencies to invoke, but as a general rule, timbral variation results from playing with the upper harmonics, rather than with the area near the fundamental.
• There is often significant loss of volume when filtering. The gain parameter should be set as high as possible, just below a value at which overload occurs. Some judicious use of GAIN (as in our example) may be needed after filtering.

Last updated: 4 December 2003