SPUX: New stretcher: Should be slightly more stable now, compared to r4840.

On 4840 I added dynamic tuning (which may have slight negative effect on stability) to adapt to different iterations/sec of different systems, but I didn't like this system too much. On 4840 I also made a small change which, I now understand, factors out the differences between systems in this regard, essentially making dynamic tuning not required anymore. So now dynamic tuning is disabled.

If you enable the SPU2X messages (and the overrun messages), you would see log messages similar to this:

buffers:   44 ms ( 89%), tempo: 1.000000, comp: 1.000, iters: 764, (N-IPS:750 -> avg:50, minokc:50, div:100)

If the iters value (764 at this example) on your system when running at 100% speed (50/60fps) is higher than 1000 or lower than 500, then please comment here and post few of the log lines.

git-svn-id: http://pcsx2.googlecode.com/svn/trunk@4843 96395faa-99c1-11dd-bbfe-3dabce05a288

plugins/spu2-x/src/Timestretcher.cpp

@@ -61,16 +61,24 @@ float SndBuffer::GetStatusPct()
 
 
 //Alternative simple tempo adjustment. Based only on the soundtouch buffer state.
-//Base algorithm: aim at specific % filled of that buffer, and adjust tempo simply by current/target of % fill.
+//Base algorithm: aim at specific average number of samples at the buffer (by GUI), and adjust tempo simply by current/target.
 //An extra mechanism is added to keep adjustment at perfect 1:1 ratio (when emulation speed is stable around 100%)
 //  to prevent constant stretching/shrinking of packets if possible.
 //  This mechanism is triggered when the adjustment is close to 1:1 for long enough (defaults to 100 iterations within hys_ok_factor - defaults to 3%).
 //  1:1 state is aborted when required adjustment goes beyond hys_bad_factor (defaults to 20%).
 //
-//To compensate for wide variation of the current buffer ratio due to relatively small size of the buffer,
+//To compensate for wide variation of the <num of samples> ratio due to relatively small size of the buffer,
 //  The required tempo is a running average of STRETCH_AVERAGE_LEN (defaults to 50) last calculations.
 //  This averaging slows down the respons time of the algorithm, but greatly stablize it towards steady stretching.
 //
+//Keeping the buffer at required latency:
+//  This algorithm stabilises when the actual latency is <speed>*<required_latency>. While this is just fine at 100% speed,
+//  it's problematic especially for slow speeds, as the number of actual samples at the buffer gets very small on that case,
+//  which may lead to underruns (or just too much latency when running very fast fast).
+//To compensate for that, the algorithm has a slowly moving compensation factor which will eventually bring the actual latency to the required one.
+//compensationDivider defines how slow this compensation changes. By default it's set to 100,
+//  which will finalize the compensation after about 200 iterations.
+//
 // Note, this algorithm is intentionally simplified by not taking extreme actions at extreme scenarios (mostly underruns when speed dtops sharply),
 //  and let's the overrun/underrun protections do what they should (doesn't happen much though in practice, even at big FPS variations).
 //
@@ -79,6 +87,15 @@ float SndBuffer::GetStatusPct()
 
 int targetIPS=750;
 
+//Dynamic tuning changes the values of the base algorithm parameters (derived from targetIPS) to adapt, in real time, to
+//  diferent number of invocations/sec (mostly affects number of iterations to average).
+//  Dynamic tuning can have a slight negative effect on the behavior of the algorithm, so it's preferred to have it off.
+//Currently it looks like it's around 750/sec on all systems when playing at 100% speed (50/60fps),
+//  and proportional to that speed otherwise.
+//If changes are made to SPU2X which affects this number (but it's still the same on all systems), then just change targetIPS.
+//If we find out that some systems are very different, we can turn on dynamic tuning by uncommenting the next line.
+//#define NEWSTRETCHER_USE_DYNAMIC_TUNING
+
 
 //running average can be implemented in O(1) time.
 //For the sake of simplicity, this average is calculated in O(<buffer-size>). Possibly improve later.
@@ -121,6 +138,7 @@ void SndBuffer::UpdateTempoChangeSoundTouch2()
 
 	float bufferFullness=(float)m_data;///(float)m_size;
 
+#ifdef NEWSTRETCHER_USE_DYNAMIC_TUNING
 	{//test current iterations/sec every 0.5s, and change algo params accordingly if different than previous IPS more than 30%
 		static long iters=0;
 		static wxDateTime last=wxDateTime::UNow();
@@ -138,6 +156,7 @@ void SndBuffer::UpdateTempoChangeSoundTouch2()
 		}
 		iters++;
 	}
+#endif
 
 	//Algorithm params: (threshold params (hysteresis), etc) 
 	const float hys_ok_factor=1.04;