Reading CDDA sound levels: Difference between revisions

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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 ?).
Registers {{Reg|REG_CDDALEFTL}} and {{Reg|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 CD2 [[BIOSes|BIOS]] uses the following code to reverse the bits:
The CD [[system ROM]] uses the following code to reverse the bits:
<pre>
<syntaxhighlight>
;D1:Input
;D1:Input, D0:Output
;D0:Output
reverse16bits:
reverse16bits:
        moveq #16-1,d7
    moveq #16-1,d7
moveq #0,d0
moveq #0,d0
rev:
rev:
lsr #1,d1
lsr     #1,d1
bcc zerobit
bcc zerobit
bset d7,d0
bset d7,d0
zerobit:
zerobit:
dbf d7,rev
dbf     d7,rev
</pre>
</syntaxhighlight>


And this to get its absolute value:
It reads both registers multiple times during each frame (every 16 raster lines, by using the [[68k interrupts|raster interrupt]]) to get an average value.
<pre>
abs:
        tst    d1
bpl    notneg
not    d1
notneg:
</pre>


The CD2 BIOS reads both registers multiple times during each frame (by using the [[68k interrupts|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|right]]
This does NOT work on current emulators.


[[http://www.youtube.com/watch?v=JP5VAypJIYk Youtube video of a standalone vu-meter]] (need source for this)
Uses:
*Raster line interrupts
*[[Sprites]] (shrink and displacement)
*[[Fix layer|Fix]] (frame counter)
*[[Palettes]]


Source for AS and iso image: [[File:Vumeter1.zip]]
[[http://www.youtube.com/watch?v=JP5VAypJIYk Youtube video]]
[[Category:Code]]
[[Category:Sound Code]]
[[Category:CD systems]]
[[Category:CD systems]]
[[Category:Code]]

Latest revision as of 04:14, 26 September 2023

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.

Demo

File:Screencap cddaviz.png

This does NOT work on current emulators.

Uses:

Source for AS and iso image: File:Vumeter1.zip

[Youtube video]

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