Toyota 2008 Prius Quick Reference Manual page 292

3-3. Using the audio system
Setting DSP
The following features can be set as desired by touching
screen.
Type B only
n Turning Automatic Sound Levelizer (ASL)
Type A
Touch
ASL automatically adjusts the volume and tone quality according to the
noise level as you drive your vehicle.
Type B
Touch
ASL adjusts the volume and tone quality according to the noise level as
you drive your vehicle.
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