G.723.1

G.723.1 is an audio codec for voice that compresses voice audio in 30 ms frames. An algorithmic look-ahead of 7.5 ms duration means that total algorithmic delay is 37.5 ms.

Note that this is a completely different codec from G.723.

There are two bit rates at which G.723.1 can operate:
* 6.3 kbit/s (using 24 byte frames) using a MPC-MLQ algorithm (MOS 3.9)
* 5.3 kbit/s (using 20 byte frames) using an ACELP algorithm (MOS 3.62)

Use

G.723.1 is mostly used in Voice over IP (VoIP) applications due to its low bandwidth requirement. Music or tones such as DTMF or fax tones cannot be transported reliably with this codec, and thus some other method such as G.711 or out-of-band methods should be used to transport these signals. The complexity of the algorithm is below 16 MIPS. 2.2 kilobytes of RAM is needed for codebooks.

Features

* Sampling frequency 8 kHz/16-bit (240 samples for 30 ms frames)
* Fixed bit rate (5.3 kbit/s with 20 byte 30 ms frames, 6.3 kbit/s with 24 byte 30 ms frames)
* Fixed frame size for each rate (5.3 kbit/s with 20 byte 30 ms frames, 6.3 kbit/s with 24 byte 30 ms frames)
* Algorithmic delay is 37.5 ms per frame, with 7,5 ms look-ahead delay
* G.723 is a hybrid speech coder, with high bit rate using Multi-Pulse Maximum Likelihood Quantization (MP-MLQ) and low bit rate using Algebraic Code Excited Linear Prediction (ACELP)
* The complexity of the algorithm is rated at 25, using a relative scale where G.711 is 1 and G.729a is 15.
* G.723.1 Annex A defines 4 byte Silence Insertion Descriptor (SID) frame for Comfort Noise Generation
* PSQM testing under ideal conditions yields Mean Opinion Scores of 4.08 for G.723.1 (6.3 kbit/s), compared to 4.45 for G.711 (μ-law)
*PSQM testing under network stress yields Mean Opinion Scores of 3.57 for G.723.1 (6.3 kbit/s), compared to 4.13 for G.711 (μ-law)

Licensing

Use of G.723.1 requires a license for the patents that cover the algorithm. The last patent will expire in 2014.

See also

* G.711
* G.718
* G.719
* G.722
* G.722.1
* G.722.2
* G.726
* G.728
* G.729
* List of codecs

External links

* [http://www.itu.int/ITU-T/ipr/index.html ITU Patent database]
* [http://www.sipro.com/g7231.php Sipro administers the patent pool for G.723.1 and G.729]