[hatari-devel] LMC1992 and Microwire emulation

David Savinkoff dsavnkff at telus.net
Tue Apr 6 21:31:04 CEST 2010


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Hi,

What my patches do:

1)  Makes changes to (sound.c) and (dmaSnd.c) to allow for Falcon, STE,
    TT, and ST mixing.
2)  Sets levels so that clipping doesn't occur when bass and treble are
    boosted by 12dB.
3)  Mixes sound in the proper proportion to get the levels right.

My observations:

1)  Emulated YM sound puts out values that are 0-32767 and centered.
2)  The DMA DAC puts out 0 to 255.
3)  The Yamaha sound chip represents 1Vpp or 0.5Vpeak (non-inverted).
4)  The DAC represents 5Vpp or 2.5Vpeak from its inverted output.
5)  Therefore, the DMA sound is -5 times louder than the YM Sound ic.
6)  YM Sound is amplified by -10000/1500 = -6.667 (from schematics).
7)  0.5V x -6.66667 = -3.333 volts for YM sound compared to -2.5 volts
    for the DAC: -2.5/-3.333 = 0.75
8)  Thus, the DMA is 0.75 times as loud as the YM sound ic.
9)  With -12dB mixing the DMA is 4 x 0.75 = 3 times as loud as the YM.
10) Values noted in (1) and (2) must be reconciled with (3) through (9)
    on an emulator.
11) The DAC will always have an output voltage (the last value). This
    value can be left alone, or slowly decayed to zero (sub audio freq)

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