Commit optional code for high-precision EQ which will almost certainly not make a difference on 16 bit output targets.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@12451 a1c6a512-1295-4272-9138-f99709370657

1 files changed, 33 insertions, 7 deletions

diff --git a/apps/eq_cf.S b/apps/eq_cf.S
index c9458cd..75bfcaf 100644
--- a/apps/eq_cf.S
+++ b/apps/eq_cf.S
@@ -7,7 +7,7 @@
  *                     \/            \/     \/    \/            \/
  * $Id$
  *
- * Copyright (C) 2006 Thom Johansen
+ * Copyright (C) 2006-2007 Thom Johansen
  *
  * All files in this archive are subject to the GNU General Public License.
  * See the file COPYING in the source tree root for full license agreement.
@@ -17,14 +17,27 @@
  *
  ****************************************************************************/
 
+/* uncomment this to make filtering calculate lower bits after shifting.
+ * without this, "shift" - 1 of the lower bits will be lost here.
+ */
+/* #define HIGH_PRECISION */
+
+/*
+ * void eq_filter(int32_t **x, struct eqfilter *f, unsigned num,
+ *                unsigned channels, unsigned shift)
+ */
     .text
     .global eq_filter
 eq_filter:
     lea.l (-11*4, %sp), %sp 
     movem.l %d2-%d7/%a2-%a6, (%sp)    | save clobbered regs
     move.l (11*4+8, %sp), %a5         | fetch filter structure address
-    movem.l (11*4+16, %sp), %d6-%d7   | load num. channels and shift count
+    move.l (11*4+20, %sp), %d7        | load shift count
     subq.l #1, %d7                    | EMAC gives us one free shift
+#ifdef HIGH_PRECISION
+    moveq.l #8, %d6
+    sub.l %d7, %d6                    | shift for lower part of accumulator
+#endif
     movem.l (%a5), %a0-%a4            | load coefs
     lea.l (5*4, %a5), %a5             | point to filter history
 
@@ -34,11 +47,16 @@ eq_filter:
     move.l (%a6), %a6
     move.l (11*4+12, %sp), %d5        | number of samples
     movem.l (%a5), %d0-%d3            | load filter history
+
+    /* d0-r3 = history, d4 = number of channels, d5 = sample count,
+     * d6 = lower shift amount, d7 = upper shift amount, a0-a4 = coefs,
+     * a5 = history pointer, a6 = x[]
+     */
 .loop:
     /* Direct form 1 filtering code. We assume DSP has put EMAC in frac mode.
-       y[n] = b0*x[i] + b1*x[i - 1] + b2*x[i - 2] + a1*y[i - 1] + a2*y[i - 2],
-       where y[] is output and x[] is input. This is performed out of order
-       to do parallel load of input value.
+     * y[n] = b0*x[i] + b1*x[i - 1] + b2*x[i - 2] + a1*y[i - 1] + a2*y[i - 2],
+     * where y[] is output and x[] is input. This is performed out of order
+     * to do parallel load of input value.
      */
     mac.l %a2, %d1, %acc0               | acc = b2*x[i - 2]
     move.l %d0, %d1                     | fix input history
@@ -47,15 +65,23 @@ eq_filter:
     mac.l %a3, %d2, %acc0               | acc += a1*y[i - 1]
     mac.l %a4, %d3, %acc0               | acc += a2*y[i - 2]
     move.l %d2, %d3                     | fix output history
-    movclr.l %acc0, %d2                 | fetch and write result
+#ifdef HIGH_PRECISION
+    move.l %accext01, %d2               | fetch lower part of accumulator
+    move.b %d2, %d4                     | clear upper three bytes
+    lsr.l %d6, %d4                      | shift lower bits
+#endif
+    movclr.l %acc0, %d2                 | fetch upper part of result
     asl.l %d7, %d2                      | restore fixed point format
+#ifdef HIGH_PRECISION
+    or.l %d2, %d4                       | combine lower and upper parts
+#endif
     move.l %d2, (%a6)+                  | save result
     subq.l #1, %d5                      | are we done with this channel?
     jne .loop
     
     movem.l %d0-%d3, (%a5)              | save history back to struct
     lea.l (4*4, %a5), %a5               | point to next channel's history
-    subq.l #1, %d6                      | have we processed both channels?
+    subq.l #1, (11*4+16, %sp)           | have we processed both channels?
     jne .filterloop
 
     movem.l (%sp), %d2-%d7/%a2-%a6