The voice codec described in ITU Recommendation G.729 is the standard codec for private branch exchanges operating with the IP protocol and for trunk telephone networks.
The G.729/G.729A codec uses Conjugate Structure Algebraic Code Excited Linear Prediction( CS-ACELP) at a transmission rate of 8 kbit/s with a compression ratio of 8:1 and a delay time of 25 ms. The transmitted frequency range corresponds to that of telephony between 300 Hz and 3.4 kHz. The speech quality is good and has a Mean Opinion Score( MOS) of 4.2.
The voice codec G.728 is available in the versions G.729A and G.729AB. In G.729A, the input frames have a duration of 10 ms. The generated frames comprise 80 bits. The inputs and outputs contain samples of 16-bit pulse code modulation( PCM) compressed to a data rate of 8 kbit/s. The difference to the G.729AB codec is the speech pause detection. With this technique, the signal bandwidth is reduced when there is no voice signal at the input. So that the telephone subscribers do not think that the connection is interrupted in the case of voice pauses, pink noise is inserted by means of Comfort Noise Generation( CNG).