GSM Codec

The "Global System for Mobile communications" (GSM) is a digital mobile radio system. GSM employs a Regular Pulse Excited (RPE) coding technique, which is a hybrid coding. The speech signal is divided into 20-ms frames. A set of 8 short term predictor coefficients are found for each frame. Each frame is then further divided into four 5-ms sub-frames, and for each sub-frame the encoder finds a delay and a gain for the codec's long term predictor. Finally the residual signal after both short and long term filtering is quantized for each sub-frame. GSM full rate (GSM-FR) codecs give bit rates of 13 Kbps, with a subjective quality score of 3.6 on the 5-point ITU quality scale. The main advantage of GSM over other low rate codecs is it simplicity. A software encoder can operate in real time, while CELP needs a dedicated hardware to run in real time.