Nelly Moser

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Speech codec found in older Flash Video files.

The Nellymoser Asao codec is a proprietary single-channel (mono) format optimized for low-bitrate transmission of audio. The format was 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 samplerate, and has a fixed ratio between the amount of input samples and output packet size (2 bits per input sample).

Use in Flash technology

Nellymoser Asao codec is a format owned by a Nellymoser and it licensed for use in Flash technology to Macromedia/Adobe. The Nellymoser Asao codec is an integral part of the Flash-plugin since Flash version 6. The codec is optimized for realtime and low-latency encoding of audio. Adobe Flash Player clients, when recording audio from a user's microphone, use the Nellymoser Asao codec and do not allow Flash programmers to select any other 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 the sent using Adobe's RTMP protocol to an RTMP server (Flash Media Server, Red5, Wowza).

References

Forum entry with some information about the codec: http://www.actionscript.org/forums/showthread.php3?t=20430

Blog that lists some information: http://www.progettosinergia.com/flashvideo/flashvideoblog.htm