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+Libgsmfr2 general usage
+=======================
+
+The external public interface to Themyscira libgsmfr2 consists of a single
+header file <tw_gsmfr.h>; it should be installed in some system include
+directory.
+
+The dialect of C used by all Themyscira GSM codec libraries is ANSI C (function
+prototypes), const qualifier is used where appropriate, and the interface is
+defined in terms of <stdint.h> types; <tw_gsmfr.h> includes <stdint.h>.  The
+use of old libgsm defined types (gsm_byte, gsm_frame and gsm_signal) has been
+abolished in the migration from libgsm+libgsmfrp to libgsmfr2.
+
+GSM 06.10 encoder and decoder
+=============================
+
+Both the encoder and the decoder are stateful; each running instance of either
+element needs its own state structure.  However, this GSM 06.10 component of
+libgsmfr2 shares a peculiar property with old libgsm from which it was derived:
+the same state structure (struct gsmfr_0610_state) is used by both entities.
+Needless to say, each given instance of struct gsmfr_0610_state must be used
+for only one purpose, either for the encoder or for the decoder; mixing calls
+to encoder and decoder functions with the same state structure is an invalid
+operation with undefined results.
+
+State structures for the basic encoder or decoder are allocated with this
+function:
+
+struct gsmfr_0610_state *gsmfr_0610_create(void);
+
+This function allocates dynamic memory for the state structure with malloc()
+(the size of the struct is internal to the library and not exposed) and returns
+a pointer to the allocated and initialized struct if successful, or NULL if
+malloc() fails.  The state structure is malloc'ed as a single chunk, hence when
+you are done with it, simply free() it.
+
+The initialization or reset portion of gsmfr_0610_create() operation can always
+be repeated with this function:
+
+void gsmfr_0610_reset(struct gsmfr_0610_state *state);
+
+Immediately after gsmfr_0610_create() or gsmfr_0610_reset(), the "virgin" state
+structure can be used either for the encoder or for the decoder; however, once
+that state struct has been passed to functions of either group, it can only be
+used for that functional group.
+
+Encoder specifics
+-----------------
+
+The most elementary single-frame processing function of libgsmfr2 GSM 06.10
+encoder is:
+
+void gsmfr_0610_encode_params(struct gsmfr_0610_state *st, const int16_t *pcm,
+			      struct gsmfr_param_frame *param);
+
+The input is an array of 160 linear PCM samples (left-justified in int16_t),
+and the output is this structure:
+
+struct gsmfr_param_frame {
+	int16_t	LARc[8];
+	int16_t	Nc[4];
+	int16_t	bc[4];
+	int16_t	Mc[4];
+	int16_t	xmaxc[4];
+	int16_t	xMc[4][13];
+};
+
+Most of the time the following wrapper function is more useful:
+
+void gsmfr_0610_encode_frame(struct gsmfr_0610_state *st, const int16_t *pcm,
+			     uint8_t *frame);
+
+The output is a 33-byte buffer, filled with the encoded GSM-FR speech frame in
+the RTP format specified in ETSI TS 101 318 and IETF RFC 3551.
+
+If the optional encoder homing feature is desired, call this function right
+after the call to gsmfr_0610_encode_frame() or gsmfr_0610_encode_params():
+
+void gsmfr_0610_encoder_homing(struct gsmfr_0610_state *st, const int16_t *pcm);
+
+This function checks to see if the PCM frame (160 linear PCM samples) is an EHF;
+if the input frame is indeed EHF, the function calls gsmfr_0610_reset().
+
+Decoder specifics
+-----------------
+
+The internal native form of the 06.10 decoder once again uses
+struct gsmfr_param_frame:
+
+void gsmfr_0610_decode_params(struct gsmfr_0610_state *st,
+			      const struct gsmfr_param_frame *param,
+			      int16_t *pcm);
+
+The more commonly used RTP-format version is:
+
+void gsmfr_0610_decode_frame(struct gsmfr_0610_state *st, const uint8_t *frame,
+			     int16_t *pcm);
+
+Please note:
+
+1) The basic GSM 06.10 decoder is just that: there is no SID recognition or DTX
+   handling, every possible input bit pattern will be interpreted and decoded
+   as a GSM 06.10 speech frame.
+
+2) There is no decoder homing function at this layer, and no check for DHF.
+
+3) The RTP signature nibble 0xD is ignored (not checked) by
+   gsmfr_0610_decode_frame().
+
+Rx DTX preprocessor block
+=========================
+
+The Rx DTX preprocessor is its own stateful element, independent from the 06.10
+decoder to which it is usually coupled.  Libgsmfr2 provides a "fulldec" wrapper
+that incorporates both elements, but the ability to use the Rx DTX preprocessor
+by itself still remains, unchanged from our previous libgsmfrp offering.  One
+potential application for this preprocessor by itself, without immediately
+following it with the GSM 06.10 decode step, is the possibility of implementing
+the TFO/TrFO transform of 3GPP TS 28.062 section C.3.2.1.1 for GSM-FR: our Rx
+DTX preprocessor does exactly what that section calls for, specifically in
+"case 1" where the input UL frame stream may contain SIDs and BFI frame gaps,
+but the output must be 100% valid frames and SID-free.
+
+The state structure for this block is struct gsmfr_preproc_state, and it is
+allocated with this function:
+
+struct gsmfr_preproc_state *gsmfr_preproc_create(void);
+
+Like other state structures in Themyscira GSM codec libraries, this opaque
+state is malloc'ed as a single chunk and can be simply freed afterward.  A
+reset function is also provided:
+
+void gsmfr_preproc_reset(struct gsmfr_preproc_state *state);
+
+Preprocessing good frames
+-------------------------
+
+For every good traffic frame (BFI=0) you receive from the radio subsystem, you
+need to call this preprocessor function:
+
+void gsmfr_preproc_good_frame(struct gsmfr_preproc_state *state,
+				uint8_t *frame);
+
+The second argument is both input and output, i.e., the frame is modified in
+place.  If the received frame is not SID (specifically, if the SID field
+deviates from the SID codeword by 16 or more bits, per GSM 06.31 section 6.1.1),
+then the frame (considered a good speech frame) will be left unmodified (i.e.,
+it is to be passed unchanged to the GSM 06.10 decoder), but preprocessor state
+will be updated.  OTOH, if the received frame is classified as either valid or
+invalid SID per GSM 06.31, then the output frame will contain comfort noise
+generated by the preprocessor using a PRNG, or a silence frame in one particular
+corner case.
+
+GSM-FR RTP (originally libgsm) 0xD magic: the upper nibble of the first byte
+can be anything on input to gsmfr_preproc_good_frame(), but the output frame
+will always have the correct magic in it.
+
+Handling BFI conditions
+-----------------------
+
+If you received a lost/missing frame indication instead of a good traffic frame,
+call this preprocessor function:
+
+void gsmfr_preproc_bfi(struct gsmfr_preproc_state *state, int taf,
+			uint8_t *frame_out);
+
+TAF is a flag defined in GSM 06.31 section 6.1.1; if you don't have this flag,
+pass 0 - you will lose the function of comfort noise muting in the event of
+prolonged SID loss, but all other Rx DTX functions will still work the same.
+
+With this function the 33-byte frame buffer is only an output, i.e., prior
+buffer content is a don't-care and there is no provision for making any use of
+erroneous frames like in EFR.  The frame generated by the preprocessor may be
+substitution/muting, comfort noise or silence depending on the state.
+
+GSM-FR full decoder
+===================
+
+The full decoder is a high-level feature of libgsmfr2, incorporating both the
+Rx DTX preprocessor block and the GSM 06.10 decoder block.  The state structure
+for the full decoder (struct gsmfr_fulldec_state) internally incorporates both
+struct gsmfr_0610_state and gsmfr_preproc_state, but because it is implemented
+inside libgsmfr2, it is still malloc'ed as a single chunk and can thus be
+released with a single free() call.  The functions for allocating and
+initializing this state follow the established pattern:
+
+struct gsmfr_fulldec_state *gsmfr_fulldec_create(void);
+
+void gsmfr_fulldec_reset(struct gsmfr_fulldec_state *state);
+
+The reset function internally calls gsmfr_0610_reset() and
+gsmfr_preproc_reset(), initializing both processing blocks.
+
+Frame processing functions are also straightforward:
+
+void gsmfr_fulldec_good_frame(struct gsmfr_fulldec_state *state,
+				const uint8_t *frame, int16_t *pcm);
+
+void gsmfr_fulldec_bfi(struct gsmfr_fulldec_state *state, int taf,
+			int16_t *pcm);
+
+These functions follow the same pattern as gsmfr_preproc_good_frame() and
+gsmfr_preproc_bfi(), but the output is a 160-sample linear PCM buffer.  Also
+note that the frame input to gsmfr_fulldec_good_frame() is const, unlike the
+situation with gsmfr_preproc_good_frame() - the copying into a scratchpad
+buffer (on the stack) happens inside this "fulldec" wrapper.
+
+The "fulldec" layer also adds the decoder homing feature:
+gsmfr_fulldec_good_frame() detects decoder homing frames and invokes
+gsmfr_fulldec_reset() when required, and also implements EHF output per the
+spec.