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Adds note on monic filter

--- a/silk/fixed/noise_shape_analysis_FIX.c

+++ b/silk/fixed/noise_shape_analysis_FIX.c

@@ -34,6 +34,9 @@

 /* Compute gain to make warped filter coefficients have a zero mean log frequency response on a     */

 /* non-warped frequency scale. (So that it can be implemented with a minimum-phase monic filter.)   */

+/* Note: A monic filter is one with the first coefficient equal to 1.0. In Silk

+   we omit the first coefficient in an array of coefficients, for monic filters.

+*/

 static inline opus_int32 warped_gain( /* gain in Q16*/

     const opus_int32     *coefs_Q24,

     opus_int             lambda_Q16,

--- a/silk/float/noise_shape_analysis_FLP.c

+++ b/silk/float/noise_shape_analysis_FLP.c

@@ -34,6 +34,9 @@

 /* Compute gain to make warped filter coefficients have a zero mean log frequency response on a     */

 /* non-warped frequency scale. (So that it can be implemented with a minimum-phase monic filter.)   */

+/* Note: A monic filter is one with the first coefficient equal to 1.0. In Silk

+   we omit the first coefficient in an array of coefficients, for monic filters.

+*/

 static inline silk_float warped_gain(

     const silk_float     *coefs,

     silk_float           lambda,