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saa1099.cpp: Use the correct LFSR polynomial for the SAA1099 noise generation, and add some notes about an unemulated undocumented feature of the noise clock. [Jepael, Lord Nightmare]

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This commit is contained in:
Lord-Nightmare 2017-01-25 05:45:25 -05:00
parent 0e73860daa
commit c11c22b3e8
2 changed files with 7 additions and 5 deletions
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8
src/devices/sound/saa1099.cpp
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@ -221,7 +221,7 @@ void saa1099_device::sound_stream_update(sound_stream &stream, stream_sample_t *
case 0: m_noise[ch].freq = m_master_clock/256.0 * 2; break;
case 1: m_noise[ch].freq = m_master_clock/512.0 * 2; break;
case 2: m_noise[ch].freq = m_master_clock/1024.0 * 2; break;
case 3: m_noise[ch].freq = m_channels[ch * 3].freq; break;
case 3: m_noise[ch].freq = m_channels[ch * 3].freq; break; // todo: this case will be m_master_clock/[ch*3's octave divisor, 0 is = 256*2, higher numbers are higher] * 2 if the tone generator phase reset bit (0x1c bit 1) is set.
}
}
@ -280,12 +280,14 @@ void saa1099_device::sound_stream_update(sound_stream &stream, stream_sample_t *
for (ch = 0; ch < 2; ch++)
{
/* check the actual position in noise generator */
/* update the state of the noise generator
* polynomial is x^18 + x^11 + x (i.e. 0x20400) and is a plain XOR, initial state is probably all 1s
* see http://www.vogons.org/viewtopic.php?f=9&t=51695 */
m_noise[ch].counter -= m_noise[ch].freq;
while (m_noise[ch].counter < 0)
{
m_noise[ch].counter += m_sample_rate;
if( ((m_noise[ch].level & 0x4000) == 0) == ((m_noise[ch].level & 0x0040) == 0) )
if( ((m_noise[ch].level & 0x20000) == 0) != ((m_noise[ch].level & 0x0400) == 0) )
m_noise[ch].level = (m_noise[ch].level << 1) | 1;
else
m_noise[ch].level <<= 1;

4
src/devices/sound/saa1099.h
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@ -56,12 +56,12 @@ struct saa1099_noise
saa1099_noise() :
counter(0.0),
freq(0.0),
level(0) {}
level(0xFFFFFFFF) {}
/* vars to simulate the noise generator output */
double counter;
double freq;
int level; /* noise polynomial shifter */
uint32_t level; /* noise polynomial shifter */
};