Reading CDDA sound levels: Difference between revisions
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The CD2 [[BIOSes|BIOS]] uses the following code to reverse the bits: | The CD2 [[BIOSes|BIOS]] uses the following code to reverse the bits: | ||
< | <syntaxhighlight> | ||
;D1:Input, D0:Output | ;D1:Input, D0:Output | ||
reverse16bits: | reverse16bits: | ||
| Line 13: | Line 13: | ||
zerobit: | zerobit: | ||
dbf d7,rev | dbf d7,rev | ||
</ | </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). | 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). | ||
Revision as of 13:46, 14 March 2012
Registers REG_CDDALEFTL ($FF0188) and REG_CDDARIGHTL ($FF018A) 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,revThe 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
This does NOT work on current emulators.
Uses:
Source for AS and iso image: File:Vumeter1.zip