QuickTime 4 Reference

Using Slope and Intercept

The slope parameter must be greater than zero. The slope and intercept parameters define the meaning of the sample levels in the audio data described by a sound sample description.

A sample in the audio data equal to intercept corresponds to the zero amplitude voltage of an analog input or output, and to the zero amplitude Y-coordinate of a graphical waveform display.

A sample in the audio data equal to intercept + slope or intercept - slope corresponds to full amplitude voltage on an audio input or output, and to full amplitude (i.e., maximum Y coordinate excursion) on a graphical waveform display.

The audio data itself may take any value regardless of the setting of slope and intercept . These parameters do not define a limit for the audio data, they only define how its values map onto the real-world concepts of zero amplitude and full amplitude. That means it is possible to have a sample value that is more than full amplitude.

A program which modifies data in a QuickTime file by scaling it or offsetting it, but does not intend to change the final amplitude or DC offset of the data when it is played or displayed, should modify slope and offset accordingly.