信息技术 音视频对象编码 第24部分：音频与系统交互

[中文适用范围]： 本文件描述了MPEG-4系统（MPEG-4文件格式）与MPEG-4音频编解码器之间期望的联合行为。期望MPEG-4音频编码器和解码器能够将有限长度的信号编码为文件（特别是MPEG-4文件），并再次解码以获得与原信号完全一致的结果（除编解码器造成的失真外）。这使得音频可以在系统实现（特别是MPEG-4系统）中与视频等其他媒体以可预测的方式共同使用。最重要的是，解码后的信号在开头没有多余的部分，在结尾也没有缺失的部分。这允许实现以下几点：a) 从原始音频到编码文件，再回到原始音频的精确“往返”过程（不包括编码失真）；b) 音频与其他媒体如视频之间的可预测同步；c) 随机访问以及从流的开头开始播放时的正确行为；d) 在原始域和编码域中进行编辑时行为的一致性（不包括编码失真）。此外，也期望编码器（以文件形式表示）与解码器之间具有可预测的互操作性。音频存在两种“偏移”（或“延迟”，在传输的上下文中）：一种来自编码过程，另一种来自解码过程。本文件主要关注后者。以下内容解决了这些问题：—— 明确规定了对音频组合单元的组成时间戳的处理方式。在处理压缩数据（如HE-AAC编码音频）时，特别注意其既可以以向后兼容方式解码，也可以以增强方式解码。—— 提供示例，说明有限长度的信号如何被编码为MPEG-4文件，并再次解码以获得与原始信号一致的结果（不包括编解码器失真）。最重要的是，解码后的信号在开头没有多余的内容，在结尾也没有缺失。 [外文原描述]： This document describes the desired joint behaviour of MPEG-4 Systems (MPEG-4 File Format) and MPEG-4 Audio codecs. It is desired that MPEG-4 Audio encoders and decoders permit finite length signals to be encoded to a file (particularly MPEG-4 files) and decoded again to obtain the identical signal, subject to codec distortions. This enables the use of audio in systems implementations (particularly MPEG-4 Systems), perhaps with other media such as video, in a deterministic fashion. Most importantly, the decoded signal has nothing “extra” at the beginning or “missing” at the end. This permits: a) an exact "round trip" from raw audio to encoded file back to raw audio (excepting encoding artefacts); b) predictable synchronization between audio and other media such as video; c) correct behaviour when performing random access as well as when starting at the beginning of a stream; d) identical behaviour when edits are applied in the raw domain and the encoded domain (excepting encoding artefacts). It is also expected that there be predictable interoperability between encoders (as represented by files) and decoders. There are two kinds of audio "offsets" (or "delay" in the context of transmission): those that are result from the encoding process, and those that are result from the decoding process. This document is primarily concerned with the latter. These issues are resolved by the following: — The handling of composition time stamps for audio composition units is specified. Special care is taken in the case of compressed data, like HE-AAC coded audio, that can be decoded in a backward compatible fashion as well as in an enhanced fashion. — Examples are given that show how a finite length signals can be encoded to an MPEG-4 file and decoded again to obtain the identical signal, excepting codec distortions. Most importantly, the decoded signal has nothing “extra” at the beginning or “missing” at the end.