Reading CDDA sound levels
Registers REG_CDDALEFTL and REG_CDDARIGHTL hold the values of the currently streamed CD audio data, in a reversed 16bit 2's complement format suited for the LC78815 DAC. They were only used to display the vu-meters in the CD player interface.
The CD system ROM uses the following code to reverse the bits:
;D1:Input, D0:Output reverse16bits: moveq #16-1,d7 moveq #0,d0 rev: lsr #1,d1 bcc zerobit bset d7,d0 zerobit: dbf d7,rev
It reads both registers multiple times during each frame (every 16 raster lines, by using the raster interrupt) to get an average value.
This does NOT work on current emulators.
Source for AS and iso image: File:Vumeter1.zip