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diff --git a/apps/systemlib/mixer.h b/apps/systemlib/mixer.h
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+/**
+ * @file	Generic control value mixing library.
+ *
+ * This library implements a generic mixer function that can be used
+ * by any driver or subsytem that wants to combine several control signals
+ * into a single output.
+ *
+ * Terminology
+ * ===========
+ *
+ * control
+ *	A mixer input value, typically provided by some controlling
+ *	component of the system.
+ *
+ * actuator
+ *	The mixer output value.
+ *
+ * Mixing basics
+ * =============
+ *
+ * An actuator derives its value from the combination of one or more
+ * control values. Each of the control values is scaled according to
+ * the actuator's configuration and then combined to produce the 
+ * actuator value, which may then be further scaled to suit the specific
+ * output type.
+ *
+ * Internally, all scaling is performed using floating point values.
+ * Inputs and outputs are clamped to the range -1.0 to 1.0.
+ *
+ * control    control   control
+ *    |          |         |
+ *    v          v         v
+ *  scale      scale     scale
+ *    |          |         |
+ *    |          v         |
+ *    +-------> mix <------+
+ *               |
+ *             scale
+ *               |
+ *               v
+ *              out
+ *
+ * Scaling
+ * -------
+ *
+ * Each scaler allows the input value to be scaled independently for
+ * inputs greater/less than zero. An offset can be applied to the output,
+ * as well as lower and upper boundary constraints.
+ * Negative scaling factors cause the output to be inverted (negative input
+ * produces positive output).
+ *
+ * Scaler pseudocode:
+ *
+ * if (input < 0)
+ *     output = (input * NEGATIVE_SCALE) + OFFSET
+ * else
+ *     output = (input * POSITIVE_SCALE) + OFFSET
+ *
+ * if (output < LOWER_LIMIT)
+ *     output = LOWER_LIMIT
+ * if (output > UPPER_LIMIT)
+ *     output = UPPER_LIMIT
+ *
+ *
+ * Mixing
+ * ------
+ *
+ * Mixing is performed by summing the scaled control values.  
+ *
+ *
+ * Controls
+ * --------
+ *
+ * Each mixer is presented with an array of controls from which it
+ * selects the set that will be mixed for each actuator.
+ *
+ * The precise assignment of controls may vary depending on the 
+ * application, but the following assignments should be used
+ * when appropriate.
+ *
+ * control | standard meaning
+ * --------+-----------------------
+ *     0   | roll
+ *     1   | pitch
+ *     2   | yaw
+ *     3   | primary thrust
+ */
+
+ struct MixScaler
+ {
+ 	unsigned	control;	/**< control consumed by this scaler */
+ 	float		negative_scale;	/**< scale for inputs < 0 */
+ 	float		positive_scale;	/**< scale for inputs > 0 */
+ 	float		offset;		/**< bias applied to output */
+ 	float		lower_limit;	/**< minimum output value */
+ 	float		upper_limit;	/**< maximum output value */
+ };
+
+ struct MixMixer
+ {
+ 	unsigned		control_count;	/**< number of control scalers */
+ 	struct MixScaler	output_scaler;	/**< scaler applied to mixer output */
+ 	struct MixScaler	control_scaler[0]; /**< array of control scalers */
+ };
+
+__BEGIN_DECLS
+
+/**
+ * Perform a mixer calculation.
+ *
+ * Note that the controls array is assumed to be sufficiently large for any control
+ * index in the mixer.
+ *
+ * @param mixer			Mixer configuration.
+ * @param controls		Array of input control values.
+ * @return			The mixed output.
+ */
+ __EXPORT float	mixer_mix(struct MixMixer *mixer, float *controls);
+
+ /**
+  * Check a mixer configuration for sanity.
+  *
+  * @param mixer		The mixer configuration to be checked.
+  * @param control_count	The number of controls in the system.
+  * @return			Zero if the mixer configuration is sane,
+  *				nonzero otherwise.
+  */
+ __EXPORT int	mixer_check(struct MixMixer *mixer, unsigned control_count);
+
+__END_DECLS