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trivial comment updates to beezer audio, no whatsnew

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This commit is contained in:
Jonathan Gevaryahu 2011-04-03 11:42:20 +00:00
parent 68958f72db
commit ca94190eb7
1 changed files with 6 additions and 3 deletions
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9
src/mame/audio/beezer.c
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@ -16,13 +16,14 @@
a silence waveform for 55516, alternating zeroes and ones.
* The channel mixing is done additively at the moment rather than
emulating the original multiplexer, which is actually not that hard to do
but adds a bit of complexity to the render loop.
* The 'FM OR AM' output of the audio via (pb1) appears to control some sort
of suppression or filtering change of the post-DAC amplifier when enabled,
only during the TIMER1 OUT time-slot of the multiplexer, see page 1B 3-3
of schematics. This will be a MESS to emulate since theres a lot of analog
crap involved.
* The /INT line and related logic of the 6840 is not emulated, and should
be hooked to the audio 6809
be hooked to the audio 6809.
* Convert this to a modern device instead of a deprecated old style device
@ -31,6 +32,8 @@
of 74ls670 4x4 register files wired up as four 8-bit words;
The four words are used as the volumes for four 1-bit channels by inversion
of the MSB of the 8 bit value.
NOISE is the output of an MM5837 whitenoise generator, a self-clocked (at
around 100khz) LFSR with taps on bits (base-0) 16 and 13.
The four channels are:
CNT1 CNT0
0 0 6522 pin 7 output (squarewave); 'FM or AM' affects this slot only
@ -211,7 +214,7 @@ INLINE int sh6840_update_noise(beezer_sound_state *state, int clocks)
for (i = 0; i < clocks; i++)
{
state->m_sh6840_LFSR_clocks++;
if (state->m_sh6840_LFSR_clocks >= 10) // about 10 clocks per 6840 clock
if (state->m_sh6840_LFSR_clocks >= 10) // about 10 clocks per 6840 clock, as MM5837 runs at around 100kHz, while clock is 1MHz
{
state->m_sh6840_LFSR_clocks = 0;
/* shift the LFSR. finally or in the result and see if we've
@ -360,7 +363,7 @@ static STREAM_UPDATE( beezer_stream_update )
sample += (((sample1*0x80)^(state->m_sh6840_volume[1]&0x80))?-1:1)*(state->m_sh6840_volume[1]&0x7F);
sample += (((sample2*0x80)^(state->m_sh6840_volume[2]&0x80))?-1:1)*(state->m_sh6840_volume[2]&0x7F);
sample += (((sample3*0x80)^(state->m_sh6840_volume[3]&0x80))?-1:1)*(state->m_sh6840_volume[3]&0x7F);
*buffer++ = sample*64; // adding 3 numbers ranging from -128 to 127 yields a range of -512 to 508; to scale that to '-32768 to 32767' we multiply by 64
*buffer++ = sample*64; // adding 4 numbers ranging from -128 to 127 yields a range of -512 to 508; to scale that to '-32768 to 32767' we multiply by 64
}
}