--- a/doc/TFO-transform	Mon Oct 07 00:25:50 2024 +0000
+++ b/doc/TFO-transform	Mon Oct 07 08:24:24 2024 +0000
@@ -36,3 +36,62 @@
 output is always emitted in the basic format, pure GSM-FR codec frames only.
 
 -d option enables DTXd, which is disabled by default.
+
+Details of FRv1 TFO transform with DTXd=0
+-----------------------------------------
+
+Our implementation of TFO transform in DTXd=0 configuration is mostly identical
+with the Rx DTX handler preprocessor stage of regular speech decoding; the
+details are covered in FR1-Rx-DTX-detail article.
+
+ThemWi implementation of TFO transform includes the feature of in-band homing:
+if the input to the transform is the spec-defined decoder homing frame (DHF),
+this DHF is passed through to the output just like any other good speech frame,
+but the internal state is reset to the initial "home" state.
+
+Details of FRv1 TFO transform with DTXd=1
+-----------------------------------------
+
+We implement the DTXd=1 version of TFO transform as a post-processor stage
+after executing the "regular" logic for DTXd=0 case; more precisely, our
+"regular" Rx DTX handler code sets some flags that are only used by the TFO
+DTXd=1 post-processor, and the latter element acts on one of those flags.
+
+The resulting visible behaviour of our TFO transform is as follows:
+
+* Whenever a valid SID frame comes in, it is re-emitted on the output in the
+  same frame position with the same parameters, even if it has different Xmaxc
+  in different subframes.  However, it is "rejuvenated" in that any possible
+  single bit error in the SID codeword is corrected, and all unused bits are
+  also cleared.  This behaviour agrees with GSM 08.62 section 8.2.2.
+
+* Also in agreement with GSM 08.62 section 8.2.2, any unusable frames or invalid
+  SID frames that come in after that valid SID (but before that cached SID
+  expires by way of two lost SID events, or a good speech frame ends the DTX
+  pause) are replaced with output that repeats the last processed valid SID.
+  This output consists of repeated SID frames just like the original, but with
+  all 4 Xmaxc parameters set to the one from the last subframe.
+
+* If an invalid SID frame is received directly after good speech, indicating a
+  need to start comfort noise insertion but lacking usable parameters for it,
+  the output from the TFO transform is just like that described in
+  FR1-Rx-DTX-detail article, but in the form of SID frames rather than "speech"
+  frames that represent CN.
+
+* If two consecutive lost SID events occur and the Rx DTX handler has to enter
+  CN muting state, our TFO transform breaks out of DTX and emits the CN muting
+  sequence as "speech" frames rather than altered SID.  This tactic is done in
+  order to produce immediate effect on the receiving end.  Once the muting fully
+  decays, the transform emits 4 silence frames of GSM 06.11 Table 1, then
+  switches to endlessly emitting SIDs derived from this silence frame (same
+  LARc, Xmaxc=0).
+
+* Any other time the Rx DTX handler is in NO_DATA state (initial reset state or
+  fully decayed state after speech muting), the TFO transform in DTXd=1 mode
+  emits SIDs derived from the silence frame instead of actual silence frames.
+
+Emission of transform-synthesized SIDs frames during muting states is done in
+order to help achieve the presumed network operator's goal of DTX maximization
+and radio interference reduction.  However, if the input to the transform is
+all good speech frames without DTX pauses, the transform does not attempt to
+apply VAD and make its own DTXd.