--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/HR-codec-limits	Thu Mar 19 04:13:45 2026 +0000
@@ -0,0 +1,27 @@
+GSM-HR speech codec is currently deemed by us to have no practical utility:
+
+* GSM networks that need to use half-rate channels for capacity reasons will
+  preferentially use AMR-HR (superior speech quality to HRv1), and may even
+  have to reject GSM MS that don't support it;
+
+* GSM networks that aren't so constrained in capacity will use one of AMR-FR,
+  EFR or FRv1, all of which provide better speech quality than HRv1.
+
+Because of this lack of practical utility, some important limits have been
+placed on the scope of work to support this codec in the present Themyscira
+Wireless GSM codec libraries and utilities suite:
+
+* Between speech encoder & decoder functions on the one hand and the TFO
+  transform function (TS 28.062 section C.3.2.1.1) on the other hand, the
+  latter received more development attention.  For completeness, TFO transform
+  implementations need to be provided for all 3 codecs for which this transform
+  is defined, and the implementation of TFO transform for GSM-HR serves as a
+  stepping stone toward the much more important one for EFR.
+
+* Librified forms of speech encoder and decoder engines from GSM 06.06 GSM-HR
+  reference code have been included in libgsmhr1 for the sake of completeness,
+  as well as command line utilities built on top of these library engines.
+  However, unlike the situation with EFR and AMR codecs, no effort is being
+  expended to make these operations fast or computationally efficient - the
+  slow and grossly inefficient code from ETSI remains mostly unchanged except
+  for elimination of global variables and introduction of state structures.