Mixer: Remove attenuation and apply saturation to mixed audio

This patch removes the down scaling of inputs to preserve the signal level
non-modified in mixing. The 64 bit mix is saturated (clamp) to prevent
overflow.

It is recommended to instead use in topology volume components with safe
gain set in the inputs of mixer (-6/-9.5/-12 dB for 2/3/4 inputs) if
saturation in mixing must be prevented.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

Author: singalsu

src/audio/mixer.c

@@ -36,6 +36,7 @@
 #include <sof/stream.h>
 #include <sof/alloc.h>
 #include <sof/audio/component.h>
+#include <sof/audio/format.h>
 
 #define trace_mixer(__e)	trace_event(TRACE_CLASS_MIXER, __e)
 #define tracev_mixer(__e)	tracev_event(TRACE_CLASS_MIXER, __e)
@@ -66,15 +67,13 @@ static void mix_n(struct comp_dev *dev, struct comp_buffer *sink,
 		val[1] = 0;
 		for (j = 0; j < num_sources; j++) {
 			src = sources[j]->r_ptr;
-
-			/* TODO: clamp */
 			val[0] += src[i];
 			val[1] += src[i + 1];
 		}
 
-		/* TODO: best place for attenuation ? */
-		dest[i] = (val[0] >> (num_sources >> 1));
-		dest[i + 1] = (val[1] >> (num_sources >> 1));
+		/* Saturate to 32 bits */
+		dest[i] = sat_int32(val[0]);
+		dest[i + 1] = sat_int32(val[1]);
 	}
 }