Reading CDDA sound levels

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Revision as of 22:49, 20 April 2011 by Furrtek (talk | contribs)
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Registers REG_CDDALEFTL ($FF0188) and REG_CDDARIGHTL ($FF01AA) 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 BIOS 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

The CD2 BIOS 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).

Demo

File:Screencap cddaviz.png

Uses:

  • Raster line interrupts
  • Sprites (shrink and displacement)
  • Fix (displays hex value)
  • Palettes

[Source and ISO image]

[Youtube video]