CAST Transforms: Brightness

revised 2/3/98

Introduction

Adjust the amplitudes of the high-frequency partials to make the sound brighter or less bright in a way that mimics the spectral change of an acoustic instrument being played with more effort or less effort. Although use of the term "loudness" opens a huge can of worms about the strict definition of loudness as a perceptual parameter and the issue of loudness matching, this transform was originally invented as a "loudness model." In any event, increasing the brightness of a sound with this transform certainly makes it louder, and it does so in a way that's much more acoustically realistic than simply increasing the gain.

The definition of "high-frequency" partials is based on the notion of a "pivot" frequency:

Partials with frequencies below the pivot are unaffected by this transform. Partials with frequencies above the pivot are made louder or quieter based on the desired increase in brightness and their distance from the pivot frequency.

This transform has the notion of specifying the original loudness of a sound so that requests for a particular loudness can be on an absolute (and possibly perceptually matched) scale rather than on a relative scale. The amount of adjustment this transform does is a function of the difference between the desired loudness and the original loudness.

Parameters

Message Argument(s) Type(s) Description
pivot Frequency in Hertz Float Sets the pivot frequency. (Default is 100.)
original Original loudness (or brightness), from 0 (ppp) to 1 (fff). Float Specifies the original loudness (or brightness) of the timbral prototype that this voice is set to. (Default is 0.5.)
desired Desired loudness (or brightness), from 0 (ppp) to 1 (fff). Float The desired loudness. (If this is less than zero, it "turns off" the transform for this voice, so that it doesn't do anything to change the sound.)

Note that the "original" loudness parameter is something that you set per-voice, even though conceptually it's a parameter of the timbral prototype that the voice is set to.

Examples

Make it a little bit brighter:

/voices/0/brightness/desired 0.6
/voices/0/tm/goto 0.0

Make it a little bit brighter, but with a higher pivot frequency:

/voices/0/brightness/desired 0.6
/voices/0/brightness/pivot 200.
/voices/0/tm/goto 0.0

Turn a pianissimo sound into a forte:

/voices/0/brightness/original 0.125
/voices/0/brightness/desired 0.75
/voices/0/tm/goto 0.0