diff --git a/src/emu/sound/disc_flt.c b/src/emu/sound/disc_flt.c
index c1189867cd8..420443f44d7 100644
--- a/src/emu/sound/disc_flt.c
+++ b/src/emu/sound/disc_flt.c
@@ -833,8 +833,7 @@ static DISCRETE_STEP( dst_rcdisc5)
 		diff = u - node->output[0];
 
 		if(diff < 0)
-			//diff = diff - (diff * exp(discrete_current_context->sample_time / context->exponent0));
-			diff = -node->output[0] + (node->output[0] *  context->exponent0);
+			diff = diff * context->exponent0;
 		node->output[0] += diff;
 	}
 	else
diff --git a/src/emu/sound/disc_mth.c b/src/emu/sound/disc_mth.c
index 244f3ea09c2..f0bf3c0970e 100644
--- a/src/emu/sound/disc_mth.c
+++ b/src/emu/sound/disc_mth.c
@@ -1680,6 +1680,9 @@ static DISCRETE_STEP(dst_transform)
 			case 'P':
 				dst_transform_push(trans_stack, &trans_stack_ptr, top);
 				break;
+			case 'a':	/* absolute value */
+				top = fabs(top);
+				break;
 			case 'i':	/* * -1 */
 				top = -top;
 				break;
diff --git a/src/emu/sound/discrete.h b/src/emu/sound/discrete.h
index 68db0bb2dd8..9accc318e97 100644
--- a/src/emu/sound/discrete.h
+++ b/src/emu/sound/discrete.h
@@ -1650,6 +1650,7 @@
  *   + - Pop two values from stack, add and push result to stack
  *   / - Pop two values from stack, divide and push result to stack
  *   * - Pop two values from stack, multiply and push result to stack
+ *   a - Pop one value from stack, multiply -1 if less than 0 and push result to stack
  *   i - Pop one value from stack, multiply -1 and push result to stack
  *   ! - Pop one value from stack, logical invert, push result to stack
  *   = - Pop two values from stack, logical = and push result to stack