Re: [hatari-devel] Improved YM2149 cycle accuracy

[David Savinkoff <dsavnkff@xxxxxxxxx>]

On Sun, Jul 25, 2021 at 12:57 PM David Savinkoff wrote:

Hi Nicolas,

I looked at the code and found that some filter coefficients need to

be converted to 250 KHz. The enclosed patch does the job.

I made the patch without testing or compiling because it is

simple and I'm not set up for Hatari at the moment.

Once the patch is applied, only YM2149_Next_Resample_Nearest()

is needed (the best choice). The IIR filters are as efficient as

the YM2149_Next_Resample_Weighted_Average_N() method.

Hatari should have the same good timbre as it did when the filters

were implemented a few years ago

David Savinkoff

ps. If the sound is wrong, I'll have to double check the coefficients.

 

More comments and coefficients improved to 1% of target fc @ 250KHz

--- a/src/sound.c	2021-07-25 11:50:06.650166911 -0700
+++ b/src/sound.c	2021-07-25 12:18:56.516824122 -0700
@@ -399,17 +399,19 @@
 
 /**
  * Get coefficients for different Fs (C10 is in ST only):
- * Wc = 2*M_PI*4895.1;
- * Fs = 44100;
+ * Wc = 2*M_PI*fc;
+ * Fs = 250000 Hz;
  * warp = Wc/tanf((Wc/2)/Fs);
- * b = Wc/(warp+Wc);
- * a = (Wc-warp)/(warp+Wc);
+ * b = Wc/(Wc+warp);
+ * a = (Wc-warp)/(Wc+warp);
  *
+ * Aside: 44100 Hz coefficients example
  * #define B_z (yms32)( 0.2667*(1<<15))
  * #define A_z (yms32)(-0.4667*(1<<15))
  *
  * y0 = (B_z*(x0 + x1) - A_z*y0) >> 15;
  * x1 = x0;
+ * /Aside
  *
  * The Lowpass Filter formed by C10 = 0.1 uF
  * and
@@ -437,11 +439,11 @@
 	static	yms32 y0 = 0, x1 = 0;
 
 	if (x0 >= y0)
-	/* YM Pull up:   fc = 7586.1 Hz (44.1 KHz), fc = 8257.0 Hz (48 KHz) */
-		y0 = (3*(x0 + x1) + (y0<<1)) >> 3;
+	/* YM Pull up:   fc is approx 7693.7 Hz (at 250 KHz rate) */
+		y0 = ( 45*(x0 + x1) + 422*y0 ) >> 9; /* b= 45/512 a= 422/512 */
 	else
-	/* R8 Pull down: fc = 1992.0 Hz (44.1 KHz), fc = 2168.0 Hz (48 KHz) */
-		y0 = ((x0 + x1) + (6*y0)) >> 3;
+	/* R8 Pull down: fc is approx 2096.6 Hz (at 250 KHz rate) */
+		y0 = ( 13*(x0 + x1) + 486*y0 ) >> 9; /* b= 13/512 a= 486/512 */
 
 	x1 = x0;
 	return y0;
@@ -469,7 +471,7 @@
 		y0 = x0;
 	else
 	/* R8 Pull down */
-		y0 = (3*(x0 + x1) + (y0<<1)) >> 3;
+		y0 = ( 45*(x0 + x1) + 422*y0 ) >> 9; /* b= 45/512 a= 422/512 */
 
 	x1 = x0;
 	return y0;