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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. a silence waveform for 55516, alternating zeroes and ones.
* The channel mixing is done additively at the moment rather than * The channel mixing is done additively at the moment rather than
emulating the original multiplexer, which is actually not that hard to do 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 * 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, 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 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 of schematics. This will be a MESS to emulate since theres a lot of analog
crap involved. crap involved.
* The /INT line and related logic of the 6840 is not emulated, and should * 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 * 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; 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 The four words are used as the volumes for four 1-bit channels by inversion
of the MSB of the 8 bit value. 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: The four channels are:
CNT1 CNT0 CNT1 CNT0
0 0 6522 pin 7 output (squarewave); 'FM or AM' affects this slot only 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++) for (i = 0; i < clocks; i++)
{ {
state->m_sh6840_LFSR_clocks++; 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; state->m_sh6840_LFSR_clocks = 0;
/* shift the LFSR. finally or in the result and see if we've /* 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 += (((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 += (((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); 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
} }
} }