Reading CDDA sound levels

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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 (Sample rate =~ 100kHz ?).

The CD2 System ROM uses the following code to reverse the bits:

;D1:Input, D0:Output
        moveq	#16-1,d7
	moveq	#0,d0
	lsr	#1,d1
	bcc	zerobit
	bset	d7,d0
	dbf	d7,rev

The CD2 system ROM reads both registers multiple times during each frame (every 16 raster lines, by using the raster interrupt) to get an average value. The displayed vu-meters use a double buffer to avoid getting updated with the average of only the first few register reads (those that were made before the vu-meters were actually drawn to the screen).


Screencap cddaviz.png

This does NOT work on current emulators.


Source for AS and iso image:

[Youtube video]