Reading CDDA sound levels: Difference between revisions
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Registers | 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 ?). | ||
The CD2 [[BIOSes|BIOS]] uses the following code to reverse the bits: | The CD2 [[BIOSes|BIOS]] uses the following code to reverse the bits: | ||
Revision as of 01:06, 14 August 2012
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 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