add SAASound as an alternate SAA1099 core

DOES NOT WORK YET

extern/SAASound/src/SAANoise.cpp

@@ -0,0 +1,180 @@
+// Part of SAASound copyright 1998-2018 Dave Hooper <dave@beermex.com>
+//
+// SAANoise.cpp: implementation of the CSAANoise class.
+// One noise generator
+//
+// After construction, it's important to SetSampleRate before
+// trying to use the generator.
+// (Just because the CSAANoise object has a default samplerate
+//  doesn't mean you should rely on it)
+//
+//////////////////////////////////////////////////////////////////////
+
+#include "SAASound.h"
+
+#include "types.h"
+#include "SAANoise.h"
+#include "defns.h"
+
+
+//////////////////////////////////////////////////////////////////////
+// Construction/Destruction
+//////////////////////////////////////////////////////////////////////
+
+CSAANoise::CSAANoise()
+:
+m_nCounter(0),
+m_nCounter_low(0),
+m_nOversample(0),
+m_nCounterLimit_low(1),
+m_bSync(false),
+m_nSampleRate(SAMPLE_RATE_HZ),
+m_nSourceMode(0),
+m_nRand(1)
+{
+	_SetClockRate(EXTERNAL_CLK_HZ);
+	m_nAdd = m_nAddBase;
+}
+
+CSAANoise::CSAANoise(unsigned long seed)
+:
+m_nCounter(0),
+m_nCounter_low(0),
+m_nOversample(0),
+m_nCounterLimit_low(1),
+m_bSync(false),
+m_nSampleRate(SAMPLE_RATE_HZ),
+m_nSourceMode(0),
+m_nRand(seed)
+{
+	_SetClockRate(EXTERNAL_CLK_HZ);
+	m_nAdd = m_nAddBase;
+}
+
+CSAANoise::~CSAANoise()
+{
+	// Nothing to do
+}
+
+void CSAANoise::_SetClockRate(int nClockRate)
+{
+	// at 8MHz the clock rate is 31.250kHZ
+	// This is simply the clock rate divided by 256 i.e. 2^8
+	// We then shift this by 2^12 (like the Freq) for better
+	// period accuracy.  So that's the same as shifting by (12-8)
+	m_nAddBase = nClockRate << (12 - 8);
+}
+
+void CSAANoise::Seed(unsigned long seed)
+{
+	m_nRand = seed;
+}
+
+void CSAANoise::SetSource(int nSource)
+{
+	m_nSourceMode = nSource;
+	m_nAdd = m_nAddBase >> m_nSourceMode;
+}
+
+void CSAANoise::Trigger(void)
+{
+	// Trigger only does anything useful when we're
+	// clocking from the frequency generator - i.e
+	// if bUseFreqGen = true (i.e. SourceMode = 3)
+	
+	// So if we're clocking from the noise generator
+	// clock (ie, SourceMode = 0, 1 or 2) then do nothing
+
+//	No point actually checking m_bSync here ... because if sync is true,
+//	then frequency generators won't actually be generating Trigger pulses
+//	so we wouldn't even get here!
+	// EXCEPT - cool edge case:  if sync is set, then actually the Noise Generator
+	// is triggered on EVERY CLOCK PULSE (i.e. 8MHz noise).  So indeed it is correct
+	// to not check for sync here.  NEEDS TEST CASE.
+
+	if (m_nSourceMode == 3)
+	{
+		ChangeLevel();
+	}
+}
+
+void CSAANoise::Tick(void)
+{
+	// Tick only does anything useful when we're
+	// clocking from the noise generator clock
+	// (ie, SourceMode = 0, 1 or 2)
+	
+	// So, if SourceMode = 3 (ie, we're clocking from a
+	// frequency generator ==> bUseFreqGen = true)
+	// then do nothing
+	if ( (!m_bSync) && (m_nSourceMode!=3) )
+	{
+		m_nCounter += m_nAdd;
+		while (m_nCounter >= (m_nSampleRate<<12))
+		{
+			m_nCounter -= (m_nSampleRate<<12);
+			m_nCounter_low++;
+			if (m_nCounter_low >= m_nCounterLimit_low)
+			{
+				m_nCounter_low = 0;
+				ChangeLevel();
+			}
+		}
+	}
+}
+
+void CSAANoise::Sync(bool bSync)
+{
+	if (bSync)
+	{
+		m_nCounter = 0;
+		m_nCounter_low = 0;
+	}
+	m_bSync = bSync;
+}
+
+
+void CSAANoise::_SetSampleRate(int nSampleRate)
+{
+	m_nSampleRate = nSampleRate;
+}
+
+
+void CSAANoise::_SetOversample(unsigned int oversample)
+{
+	// oversample is a power of 2 i.e.
+	// if oversample == 2 then 4x oversample
+	// if oversample == 6 then 64x oversample
+	if (oversample < m_nOversample)
+	{
+		m_nCounter_low <<= (m_nOversample - oversample);
+	}
+	else
+	{
+		m_nCounter_low >>= (oversample - m_nOversample);
+	}
+
+	m_nCounterLimit_low = 1<<oversample;
+	m_nOversample = oversample;
+}
+
+inline void CSAANoise::ChangeLevel(void)
+{
+	/*
+	https://www.vogons.org/viewtopic.php?f=9&t=51695
+	SAA1099P noise generator as documented by Jepael
+	18-bit Galois LFSR
+	Feedback polynomial = x^18 + x^11 + x^1
+	Period = 2^18-1 = 262143 bits
+	Verified to match recorded noise from my SAA1099P
+	*/
+
+	if (m_nRand & 1)
+	{
+		m_nRand = (m_nRand >> 1) ^ 0x20400;
+	}
+	else
+	{
+		m_nRand >>= 1;
+	}
+}