Volume: Mitigate zipper noise by adjusting gain ramp parameters

Fixes: #103 "volume: zipper noise with fast changes".

The volume ramp length is increased from 64 ms to 250 ms. The vol_work
rate to update gain value is increased from every 2 ms to 1 ms. The
volume ramp macros are redefined to enter directly the desired ramp
length in milliseconds to ease the tuning.

Also the volume gain Qxy notation (currently Q2.16) is defined with macros
to avoid magic numbers in code in ramp computation.

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

Author: singalsu

src/audio/volume.h

@@ -64,19 +64,34 @@
 /** \brief Volume trace error function. */
 #define trace_volume_error(__e)	trace_error(TRACE_CLASS_VOLUME, __e)
 
+//** \brief Volume gain Qx.y integer x number of bits including sign bit. */
+#define VOL_QXY_X 2
+
+//** \brief Volume gain Qx.y fractional y number of bits. */
+#define VOL_QXY_Y 16
+
 /**
- * \brief Volume ramp time in microseconds.
- *
- * This should ramp from 0dB to mute in 64ms.
- * i.e. 2^16 -> 0 in 32 * 2048 steps each lasting 2ms.
+ * \brief Volume ramp update rate in microseconds.
+ * Update volume gain value every 1 ms.
  */
-#define VOL_RAMP_US	2000
+#define VOL_RAMP_US 1000
 
-/** \brief Volume ramp step. */
-#define VOL_RAMP_STEP	(1 << 11)
+/**
+ * \brief Volume linear ramp length in milliseconds.
+ * Use linear ramp length of 250 ms from mute to unity gain. The linear ramp
+ * step in Q1.16 to use in vol_work function  is computed from the length.
+ */
+#define VOL_RAMP_LENGTH_MS 250
+#define VOL_RAMP_STEP Q_CONVERT_FLOAT(1.0 / 1000 * \
+				      VOL_RAMP_US / VOL_RAMP_LENGTH_MS, \
+				      VOL_QXY_Y)
 
-/** \brief Volume maximum value. */
-#define VOL_MAX		(1 << 16)
+/**
+ * \brief Volume maximum value.
+ * TODO: This should be 1 << (VOL_QX_BITS + VOL_QY_BITS - 1) - 1 but
+ * the current volume code cannot handle the full Q1.16 range correctly.
+ */
+#define VOL_MAX		(1 << VOL_QXY_Y)
 
 /** \brief Volume minimum value. */
 #define VOL_MIN		0