优于AAC的音频编码格式opus

http://www.opus-codec.org/

Opus is a totally open, royalty-free, highly versatile audio codec. Opus is unmatched for interactive speech and music transmission over the Internet, but is also intended for storage
and streaming applications. It is standardized by the Internet Engineering Task Force (IETF) as RFC 6716 which incorporated technology from Skype’s SILK codec and Xiph.Org’s CELT codec.

Technology

Opus can handle a wide range of audio applications, including Voice over IP, videoconferencing, in-game chat, and even remote live music performances. It can scale from low bitrate narrowband speech to very high quality stereo music. Supported features are:

Bitrates from 6 kb/s to 510 kb/s

Sampling rates from 8 kHz (narrowband) to 48 kHz (fullband)

Frame sizes from 2.5 ms to 60 ms

Support for both constant bitrate (CBR) and variable bitrate (VBR)

Audio bandwidth from narrowband to fullband

Support for speech and music

Support for mono and stereo

Support for up to 255 channels (multistream frames)

Dynamically adjustable bitrate, audio bandwidth, and frame size

Good loss robustness and packet loss concealment (PLC)

Floating point and fixed-point implementation

You can read the full specification, including the reference implementation, in RFC 6716. An up-to-date implementation of the Opus standard is also available from the downloads page.

为什么需要音频编（解）码（audio codec）
物理世界的音频信号（波）经过模数转换器（ADC）处理成 digital audio，digital audio 经数模转换器（DAC）变成音频信号，从而可以被耳朵感知。电脑的声卡就包含了支持音频输入的 ADC 和音频输出的 DAC。经 ADC 形成的音频信号为了减少存储空间或（网络）传输所需的带宽需要在 保证能够恢复语音信号可懂度和感知质量的前提下使用最少的比特表示高保真音频信号。音频编解码器能压缩（编码）与解压缩（解码）数字音频数据到音频文件或流媒体。
比特率（bit rate） : the number of bits per unit of time
使用 int16 数据类型存储单位时间（1 second）单通道 16k 采样率音频所需


通常我们使用以MP3格式存储的音乐文件，比特率要小于 256 kbit/s。
常见比特率[1]见下图


Opus:
高度灵活的语音和音频编码器


Opus 是为数不多完全免费且有官方开源实现的音频编码器，使用C语言开发，针对ARM、x86有特殊优化，fix-point实现，代码质量极高，是很好的多平台程序开发学习资料。


Opus 同时支持语音与音乐，自由的编码复杂度与 6k-510k 比特率。


Opus 融合了 SILK （源于Skype）和 CELT 编码方法，并且 1 +
1 > 2