Reading CDDA sound levels: Difference between revisions

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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 (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:
<syntaxhighlight>
<syntaxhighlight>
;D1:Input, D0:Output
;D1:Input, 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
</syntaxhighlight>
</syntaxhighlight>


The CD2 BIOS reads both registers multiple times during each frame (every 16 raster lines, 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).
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.


==Demo==
==Demo==
Line 32: Line 32:


[[Category:Code]]
[[Category:Code]]
[[Category:Sound Code]]
[[Category:CD systems]]

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]