G.729:
Multi-purpose ITU Standard
The G.729
speech compression standard, also referred
to as CS-ACELP® or Conjugate Structure
Algebraic Code Excited Linear Prediction,
was developed through the collaboration
of Université de Sherbrooke (a
shareholder of VoiceAge), France Telecom,
and Nippon Telegraph and Telephone Corporation
(NTT), and was adopted as an International
Telecommunication Union recommendation
in November 1995.
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Based on
the renowned Code Excited Linear Prediction
(CELP) coding model, G.729 delivers toll-quality speech, similar in quality to 32-kbps
ADPCM but at one-quarter the bit rate. With the low rate of 8 kbps, G.729 offers opportunities
for significant increases in bandwidth utilization
in existing telephony and wireless applications.
G.729 operates on 10-ms frames,
allowing moderate transmission delays, so applications
such as teleconferencing or visual telephony,
where quality, delay and bandwidth are important,
will benefit immediately from this state-of-the-art
standard.
Technical
Highlights
- At 8 kbps, the lowest bit rate ITU-T standard
with toll quality
- One
of the most tested standards for all applications,
including wireless
- Bit exactness ensures
that quality will be preserved regardless of implementation
- Quality less affected by
transmission errors relative to other low
bit rate vocoders
- A mainstream codec of the ITU with ongoing
development as demonstrated by its annexes:
- Annex
A: DSVD (Digital Simultaneous Voice
and Data) – Low complexity
-
Annex B: VAD/CNG/DTX (Voice Activity
Detection/Comfort Noise
Generator/Discontinuous Transmission)
-
Annex C: Floating point version of G.729
and its annexes
-
Annex D: Extension at 6.4 kb/s
-
Annex E: Extension at 11.8 kb/s
- G.729
Annex A offers the best complexity/quality
ratio in the industry
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Benefits of
Using G.729
The toll quality speech, low bit rate and moderate delay of the G.729 standard codec provide
benefits such as quality of service,
interoperability and increased bandwidth to its users.
Quality
of Service
G.729's low processing delay
(frame size of 10 ms) is well designed
to offer telephone quality voice over digital systems.
- Speech
quality is not sensitive to the distance
between telephones that are
communicating over the system or over satellite
links.
- G.729
is easily adaptable for communications over
packet communications systems such as frame
relay or asynchronous transfer mode (ATM)
that are designed to deliver the lowest
practicable transport delay.
- As
networks are becoming more efficient and
latency is diminishing, delay is increasingly
becoming a critical issue to compete with
traditional service providers and offer
toll quality speech over existing networks.
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Interoperability
Upgrades to add bandwidth to company networks are
costly investments, so
vendors are well advised to offer standardized
products that are interoperable with other manufacturers’ equipment to guarantee
investment protection for their customers.
Economy
of Bandwidth (Bit Rate)
Companies need increased
network bandwidth for voice and data communications,
while they also expect improved performance and want to contain costs. Service
providers and equipment manufacturers have to
meet these requirements while maintaining low
costs.
- G.729
defines a voice-quality equivalent to the
Public Switch Telephone Network (PSTN) while
consuming only 8 kbps, thus multiplying
by eight the actual network capacity while
maintaining telephone service providers
quality.
- With
Annexes D and E, G.729, allows an adaptive
variable bit rate to operate in circuit
multiplication equipment. Thus, during periods
of congestion, operation can continue at
6.4 kbps with minimal degradation of speech
quality. In contrast, when the bandwidth is
available, the bit rate can increase to
11.8 kbps to improve performance in
the presence of background noise and music.
- Cost
reductions can be obtained by maximizing
data networks by interleaving voice traffic
with data at little or no additional transport
cost and little or no impact on application
performance.
More
Information on G.729
Numerous articles
on “Standardization and Characterization
of G.729” are available in the September
1997, Vol. 35, No.9, issue of IEEE Communications
Magazine, which can be accessed online here
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