Reduced Complexity Tree Coding of Speech in the Presence of Background Impairments

在过去的几年里，语音编码取得了非凡的进步，国际电信联盟和数字蜂窝产业为电话和视频会议制定了语音编码标准，其性能在十年前可能是无法想象的。然而，在低于12kbit/S的速率下，这些编码器有时会因其实现复杂性和在存在外部背景损伤的情况下对语音进行编码而受到批评。复杂性问题无处不在，但在移动通信环境中变得越来越重要，无论是在数字蜂窝通信系统中还是在新兴的个人通信系统中，其中电池功率是一个主要问题。此外，对于各种速率的分组交换视频会议，有许多正在开发和建议的应用，其中希望语音编码器或者完全在标准平台(MAC、PC、SPAC)上的软件中实现，或者其实现不会篡夺DSP芯片的大部分能力。能够在存在背景损伤的情况下对语音进行编码一直是人们感兴趣的问题，但最近由于数字蜂窝移动无线电和不断发展的个人通信系统中遇到的移动环境而变得突出起来。桌面应用程序也对具有背景损伤的编码提出了挑战，因为免提交互是关键的，因此即使是一些相对安静的办公环境也没有手持设备来帮助抑制背景噪声。因此，挑战是在不将背景损伤改变为烦人的伪像的情况下，对于各种实现方法以中低复杂度实现8kbit/S的通信质量语音。该研究从一种称为树形编码器的低延迟波形跟踪编码器入手，去除了低延迟要求，并采用了基于频域的可变速率技术、前向自适应思想、码树自适应创新和改进的感知加权以在保持重建语音质量的同时降低实现复杂度。

Extraordinary advances in speech coding have been achieved in the past few years, with the ITU and digital cellular industries producing voice coding standards for telephony and videoconferencing with performance perhaps unimaginable even ten years ago. At rates below 12 kbits/s, however, these coders are sometimes criticized for their implementation complexity and for their difficulty in coding speech in the presence of extraneous background impairments. The complexity issue is omnipresent, but becomes increasingly important in the mobile communications environment, both in digital cellular and emerging personal communications systems, where battery power is a dominating issue. Furthermore, there are many applications being developed and proposed for packet-switched videoconferencing at various rates, where it is desirable to have voice coders implemented either wholly in software on standard platforms (Macs, PCs, Sparcs) or whose implementations do not usurp large portions of the capabilities of a DSP chip. Being able to code speech in the presence of background impairments has always been of interest, but has recently moved to the fore because of the mobile environments encountered in digital cellular mobile radio and evolving personal communications systems. Desktop applications also present a challenge for coding with background impairments, since hands-free interaction is critical and thus even some relatively quiet office environments do not have the handset to aid in rejecting the background noise. The challenge is therefore to achieve communications quality speech at 8 kbits/s with medium-to-low complexity for a variety of implementation approaches without changing background impairments into annoying artifacts. This research approaches this problem by starting with a low delay waveform following coder, called a tree coder, removing the low delay requirement and employing frequency domain based variable rate techniques, forward adaptation ideas, code tr ee adaptivity innovations, and improved perceptual weighting to reduce implementation complexity while retaining reconstructed speech quality.