G.723.1 — Delivering Highly Compressed Speech at Near-toll Quality
Adopted in November
1995 by the International Telecommunication Union
(ITU), the G.723.1 codec can be used
for compressing the speech or the audio component
of multimedia services at a very low bit rate
as part of the overall H.323 and H.324 families
of standards.
G.723.1 offers a dual rate of 6.3 kbps, based upon
an MP-MLQ2 codebook search, and 5.3 kbps,
based on the unique ACELP® technology platform developed at the Université de Sherbrooke. The higher
bit rate has greater quality. The lower bit rate
gives good quality and provides system designers
with additional flexibility. Both rates are mandatory
parts of the encoder and decoder. It is possible
to switch between the two rates at any 30-ms
frame boundary.
G.723.1 can perform full duplex compression and decompression functions for multimedia, visual telephony, and videoconferencing products. It was optimized to represent speech with high quality at low bit rates using a limited amount of complexity. Music and other audio signals are not represented as faithfully as speech but can be compressed and decompressed using this coder.
By delivering highly compressed speech, this codec is targeted
for communications applications where low bit-rates are
required without compromising the speech quality.
In fact, G.723.1 provides near-toll quality.
Technical Highlights
- Frame
size: 30 ms (240 samples)
- Lookahead: 7.5 ms (60 samples)
- Bit rates: 5.3 & 6.3 (MOS
at 6.3 is 3.9)
- Possibility of switching between
both rates during real-time operation
- Optional VAD/CNG (G.723 Annex
A)
All additional
delays in the implementation on top of the 30 ms
(totaling 7.5 ms) are due to:
- Actual
time spent processing the data in the encoder
and decoder
- Transmission
time on the communication link
- Additional
buffering delay for the multiplexing protocol
MP-MLQ2: Multi-Pulse Maximum-Likelihood Quantization
ACELP: Algebraic-Code-Excited Linear Prediction
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