GSM adapts speech coding scheme based on RPE-LTP to establish a secret speech subliminal channel for speech secure communication over PSTN and VoIP by using a proposed ABS speech information hiding LPC-IH-FS algorithm and an extracting BD-IE-MES algorithm.

The RPE-LTP speech coding scheme has two advantages. On one hand, it has a relatively low coding bit rate of 13 kbps and leads to a good quality of reconstruction speech. On the other hand, some parameters of the coding bit have better robustness and thus a few changes make no sense for the reconstruction speech

Characteristics of GSM (RPE-LTP) Coding Scheme:

The GSM coding scheme has a relatively low bit rate (13 kbps),and the reconstruction of the synthetic speech retains good quality. Some parameters used here have a relatively strong robustness. Therefore, a small change to these parameters does not greatly influence the quality of reconstructed speech.

GSM Coding Scheme for RPE-LTP Speech Coder: The encoder of GSM RPE-LTP is essentially an ADPCM system, where a predictor is computed from the signal, with the prediction error found, and is subsequently quantized using an adaptive scheme. The predictor is implemented as a cascade connection of short-term and long-term predictors. Long-term predictors greatly increase the average prediction gain, thus elevating the overall performance.

Fig:Block diagram of the RPE-LTP encoder:

The above figure is block diagram of RPE-LTP encoder, where parameters of each frame/subframe are extracted and packed to form a bit-stream. Each frame has a length of 160 samples (20 ms) and is subdivided into four subframes of 40 samples each.