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ITU-T Study Group 16 (Study Period 2005-2008) |
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Question 9/16 - Variable Bit Rate Coding of Speech Signals |
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(Continuation of Question 9/16)
- Motivation
The ITU has noted a rapid increase in the use of IP for data and a growing commercial interest in deploying voice
over IP as an application. In addition, the worldwide usage of wireless communication systems is rapidly increasing,
which includes the possibility of deploying certain IMT-2000 and Beyond networks as IP-oriented systems. IP traffic
is evolving to become the dominant global communication system with both wired and wireless access. The transport of
IP voice traffic within and between wired and wireless networks is of special interest.
A key issue therefore is the design of new voice coding technology that can best exploit the properties of these
networks to provide efficient use of bandwidth and minimize the need for transcoding. As traditional circuit-switched
networks will stay with us for a long time, easy transcoding with them is also an important design aspect.
Hence, the work is to examine the applications, technical issues of IP and wireless networks, and investigate the
design of a suitable variable bit rate voice coder.
Examples of bearer networks for VoIP carriage which have been identified as benefiting from a variable bit rate voice coder are:
- Ethernet Networks;
- ATM Systems;
- Third and future Generation Wireless Systems, including CDMA and TDMA.
Additionally, multimedia applications may benefit from a variable bit rate voice coder.
For the activities of this Question, the term variable bit rate (VBR) has been defined as follows.
One possibility is a VBR encoder outputting a bit stream, which may have a variable number of bits in successive frames.
That is, each frame may contain a different number of bits relative to the last frame. Bit rate may vary, for example,
in large predefined increments/decrements or it may vary by as little as one bit resolution. Another possibility is an
embedded encoder always outputting a bit stream consisting of several layers of significances, and either the network
and/or the decoder can select the appropriate layers of coded signals considering the conditions of the transport network
or quality of the speech desired. In this case, effective bit rate may vary, resulting in the quality of speech the codec
offers. For example, higher quality wideband speech can be supported using all layers, and normal quality narrowband speech
can be supported only using core layer. The bit-rate may be dependent upon many factors such as network congestion, priority,
QoS, etc.
- Study Items
- Study and define which algorithm(s) should be specified for the coding of voice/audio, with specific attention being paid to the applications, and the consequent constraints imposed jointly by the networks and applications.
- Study and definition of applications and performance for variable bit rate voice coding;
- Consideration of network and terminal requirements and constraints;
- Definition of performance requirements and system constraints;
- Definition of the test conditions and evaluation procedures to be applied in selecting between candidate algorithms on the basis of subjective performance, as specified by SG 12, and non-voice performance;
- Study of encoding techniques and evaluation of the possibility of selecting appropriate VBR technique or techniques suitable for applications;
- Selection and specification of procedures to be used in verifying the implementation of selected algorithm or algorithms;
- Study of the support of text-phone devices in systems using variable rate coding of speech.
While studying these work items, specific efforts will be put on the key issues of reducing cost and complexity, minimizing transcoding, interworking with other systems, and quality of service.
- Tasks
Tasks include, but are not limited to:
- Definition of performance requirements and objectives;
- Approval of the Recommendations (basic versions);
- Approval of the Recommendation for wider bandwidth extension.
An up-to-date status of work under this Question is contained in the
SG 16 Work Programme.
- Relationships
Recommendations:
- H.300-series system Recommendations.
Questions:
- Q.23(E), 10/16 on speech coding.
- Q.1, 2, 3, 4, 11, 14/16 on interaction with Systems issues.
- Q.15, 16, 17, 18/16 on network signal processing.
Study Groups:
- ITU-T SG 2 to identify other potential user applications;
- ITU-T SG 11 on signalling requirements and protocols;
- ITU-T SG 12 on voice quality evaluation of specified algorithms;
- ITU-T SG 13 on the NGN Project;
- ITU-T SG 19 on speech coding in IMT2000 and beyond;
- ITU-R SG 8 to ensure compatibility with mobile transmission system constraints.
Other bodies:
- IMTC;
- ISO/IEC SC 29 WG11 (MPEG);
- ETSI (TISPAN);
- TIA;
- IETF;
- ATM Forum;
- 3GPP, 3GPP-2.
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