Overview of G.729a/b

 

 

 G.729 is an audio data compression algorithm for voice that compresses digital voice in packets of 10 milliseconds duration. It is officially described as Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear prediction (CS-ACELP).[1]

 

Because of its low bandwidth requirements, G.729 is mostly used in Voice over Internet Protocol (VoIP) applications (such as Skype) where bandwidth must be conserved. Standard G.729 operates at a bit rate of 8 kbit/s, but there are extensions, which provide rates of 6.4 kbit/s (Annex D, F, H, I, C+) and 11.8 kbit/s (Annex E, G, H, I, C+) for worse and better speech quality, respectively.
 

G.729 has been extended with various features, commonly designated as G.729a and G.729b.
 

Dual-tone multi-frequency signaling (DTMF), fax transmissions, and high-quality audio cannot be transported reliably with this codec. DTMF requires the use of the RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals as specified in RFC 2833.

G.729 Annex A

G.729a is a compatible extension of G.729, but requires less computational power. This lower complexity, however, bears the cost of marginally reduced speech quality.
 

G.729a was developed by a consortium of organizations: France Telecom, Mitsubishi Electric Corporation, Nippon Telegraph and Telephone Corporation (NTT), and Université de Sherbrooke.
 

The features of G.729a are:

• Sampling frequency 8 kHz/16-bit (80 samples for 10 ms frames)
• Fixed bit rate (8 kbit/s 10 ms frames)
• Fixed frame size (10 bytes for 10 ms frame)
• Algorithmic delay is 15 ms per frame, with 5 ms look-ahead delay
• G.729a is a hybrid speech coder which uses Algebraic Code Excited Linear Prediction (ACELP)
• The complexity of the algorithm is rated at 15, using a relative scale where G.711 is 1 and G.723.1 is 25.
• PSQM testing under ideal conditions yields Mean Opinion Scores of 4.04 for G.729a, compared to 4.45 for G.711 (u-law)
• PSQM testing under network stress yields Mean Opinion Scores of 3.51 for G.729a, compared to 4.13 for G.711 (u-law)
   

G.729 Annex B

G.729 has been extended in Annex B (G.729b) which provides a silence compression method that enables a voice activity detection (VAD) module. It is used to detect voice activity in the signal. It also includes a discontinuous transmission (DTX) module which decides on updating the background noise parameters for non speech (noisy frames). It uses 2-byte Silence Insertion Descriptor (SID) frames transmitted to initiate comfort noise generation (CNG). If transmission is stopped, and the link goes quiet because of no speech, the receiving side may assume that the link has been cut. By inserting comfort noise, analog hiss is simulated digitally during silence to assure the receiver that the link is active and operational.

 

Exerpt taken from Wikipedia