diff --git a/include/dsp.h b/include/dsp.h
index 62672adfa14..b4b5c740375 100644
--- a/include/dsp.h
+++ b/include/dsp.h
@@ -102,6 +102,7 @@
  *     filter = T * (2*PI) * f_c
  *
  *     phase shift = -arctan(f_in/f_c)
+ *     time constant = -T / (ln(1 - filter))
  *
  *     T    - period at which the digital filter is being calculated
  *     f_in - input frequency of the filter
diff --git a/include/dspb16.h b/include/dspb16.h
index ee9f23e50a4..1476e523b96 100644
--- a/include/dspb16.h
+++ b/include/dspb16.h
@@ -92,6 +92,7 @@
  *     filter = T * (2*PI) * f_c
  *
  *     phase shift = -arctan(f_in/f_c)
+ *     time constant = -T / (ln(1 - filter))
  *
  *     T    - period at which the digital filter is being calculated
  *     f_in - input frequency of the filter