Asao (also known as Nellymoser audio codec) is a proprietary single-channel (mono) codec and compression format optimized for low-bitrate transmission of audio, developed by Nellymoser Inc.

Sound data is grouped into frames of 256 samples. Each frame is converted into the frequency domain and the most significant (highest-amplitude) frequencies are identified. A number of frequency bands are selected for encoding; the rest are discarded. The bitstream for each frame then encodes which frequency bands are in use and what their amplitudes are. This codec does not take into consideration actual sample rate, and has fixed ratio between input samples amount and output packet size (2 bits per input sample).

On March 4, 2002 Nellymoser Inc. announced that Macromedia licensed Nellymoser's Asao speech and audio compression software to be part of FlashMX and Macromedia Flash Player 6. (Macromedia is now Adobe Systems.) The Nellymoser Asao codec is an integral part of the Flash-plugin since Flash version 6, released in 2003. The codec is optimized for real-time and low-latency encoding of audio. Flash Player clients, when recording audio from a user's microphone, can use the Nellymoser Asao codec. (Flash Player 10 released in 2008 also supports the open source Speex codec.) The sampling rate of the audio capture can be controlled by the Flash programmer to increase and decrease encoding bitrate and quality. Encoding is done on the client host, and compressed data is then sent using Adobe's RTMP protocol to an RTMP server (Flash Media Server, Wowza Media Server).

A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder". A codec (the program) should not be confused with a coding or compression format or standard – a format is a document (the standard), a way of storing data, while a codec is a program (an implementation) which can read or write such files. In practice, however, "codec" is sometimes used loosely to refer to formats.

A codec encodes a data stream or signal for transmission, storage or encryption, or decodes it for playback or editing. Codecs are used in videoconferencing, streaming media and video editing applications. A video camera's analog-to-digital converter (ADC) converts its analog signals into digital signals, which are then passed through a video compressor for digital transmission or storage. A receiving device then runs the signal through a video decompressor, then a digital-to-analog converter (DAC) for analog display. The term codec is also used as a generic name for a video conferencing unit.