06.203GPPHalf rate speech transcodingTS
5.1 Functional description
The half rate speech codec as well as the DTX system and comfort noise generator are described in a bit exact arithmetic to allow for easy type approval as well as general testing purposes of the half rate speech codec.
The response of the codec to a predefined input sequence can only be foreseen if the internal state variables of the codec are in a predefined state at the beginning of the experiment. Therefore, the codec has to be put in a so called home state before a bit exact test can be performed. This is usually done by a reset.
To allow a reset of the codec in remote locations, special homing frames have been defined for the encoder and the decoder, thus enabling a codec homing by inband signalling.
The codec homing procedure is defined in such a way, that on either direction (encoder or decoder), the homing functions are called after processing the homing frame that is input. The output corresponding to the first homing frame is therefore dependent on the codec state when receiving that frame and hence usually not known. The response to any further homing frame in one direction is by definition a homing frame of the other direction. This procedure allows homing of both, the encoder and decoder from either side, if a loop back configuration is implemented, taking proper framing into account.
encoder homing frame: The encoder homing frame consists of 160 identical samples, each 13 bit long, with the least significant bit set to "one" and all other bits set to "zero". When written to 16 bit long words with left justifications, the samples have a value of 0008 hex. Test sequence SEQ05.INP described in GSM 06.07 (ETS 300 968)  defines the encoder homing frame. The speech decoder has to produce this frame as a response to the second and any further decoder homing frame if at least two decoder homing frames were input to the decoder consecutively.
decoder homing frame: The decoder homing frame has a fixed set of speech parameters as defined in test sequence SEQ05.INP described in GSM 06.07 (ETS 300 968) . It is the natural response of the speech encoder to the second and any further encoder homing frame if at least two encoder homing frames were input to the encoder consecutively.
5.3 Encoder homing
Whenever the half rate speech encoder receives at its input an encoder homing frame exactly aligned with its internal speech frame segmentation, the following events take place:
Step 1: The speech encoder performs its normal operation including VAD and DTX and produces a speech parameter frame at its output which is in general unknown. But if the speech encoder was in its home state at the beginning of that frame, then the resulting speech parameter frame is identical to the decoder homing frame (this is the way how the decoder homing frame was constructed).
Step 2: After successful termination of that operation, the speech encoder provokes the homing functions for all submodules including VAD and DTX and sets all state variables into their home state. On the reception of the next input frame, the speech encoder will start from its home state.
NOTE: Applying a sequence of N encoder homing frames will cause at least N‑1 decoder homing frames at the output of the speech encoder.
5.4 Decoder homing
Whenever the speech decoder receives at its input a decoder homing frame, then the following events take place:
Step 1: The speech decoder performs its normal operation including comfort noise generation and produces a speech frame at its output which is in general unknown. But if the speech decoder was in its home state at the beginning of that frame, then the resulting speech frame is replaced by the encoder homing frame. This would not naturally be the case but is forced by this definition here.
Step 2: After successful termination of that operation, the speech decoder provokes the homing functions for all submodules including the comfort noise generator and sets all state variables into their home state. On the reception of the next input frame, the speech decoder will start from its home state.
NOTE: Applying a sequence of N decoder homing frames will cause at least N‑1 encoder homing frames at the output of the speech decoder.
5.5 Encoder home state
In GSM 06.06 (ETS 300 967) , a listing of all the encoder state variables with their predefined values when in the home state is given.
5.6 Decoder home state
In GSM 06.06 (ETS 300 967) , a listing of all the decoder state variables with their predefined values when in the home state is given.
Annex A (normative): Codec parameter description