Speex是一套免费开源，无专利保护的音频压缩格式，主要针对语音。Speex工程着力于通过提供一个可以替代高性 能语音编解码来降低应用中语音输入的障碍 。而且，Speex也是很适合网络应用，这方面，相对于其它编解码器Speex也有着自己独特的优势。Speex还是GNU工程的一部分，在改版的BSD 协议下得到了很好的支持。

Speex is based on CELPand is designed to compress voice at bitratesranging from 2 to 44kbps.Some of Speex's features include:

• Narrowband (8 kHz), wideband (16 kHz), and ultra-wideband (32 kHz) compression in the same bitstream
• Intensity stereo encoding
• Packet loss concealment
• Variable bitrate operation (VBR)
• Voice Activity Detection (VAD)
• Discontinuous Transmission (DTX)
• Fixed-point port
• Acoustic echo canceller
• Noise suppression

Codec :Speex
Requirement:Flash Player 10 or above. Recomemded flash player version (10,0,22,87 ) or above.
Type:Open Source

What is speex :Speex is a new audio codec introduced in flash player 10 and above. It overcomes many limitations of oldNellymoser codec. This new codec will provide better audio quality using less bandwidth. Speex can be used for both kind of communication , through Flash Media Server or P2P.Speex is opensource so it can be decoded or converted to any format unlike nellymoser .
Speex Description: Speex encoder and decoder are present in flash player 10 and above. Speex compression is controlled by settingencodeQuality. Encode quality can be set using 11 quality levels 0(Lowest) - 10(Highest).Default value is 6. Speex encoder for flash works in constant bit-rate (CBR). Speex is designed for Voice over IP (VoIP) which means it provides high quality speech at low bit rate.

Speex vs NellyMoser comparison

P2P and Speex: This codec can be used for P2P audio. For implementing P2P in application currently Adobe LabsStratus beta service can be used which allow developers to begin building applications . The new protocolRTMFP and UDP can be used withStratus only .Future version next to FMS 3.5 is likely to come with its support.

Code to use Speex in Flex AS3:
//Create a fms connection first
ns = new NetStream(your fms connection here );

//Give name to file and record
ns.publish("testingSpeex", "record");
activeMic = Microphone.getMicrophone();

//set codec to speex
activeMic.codec = "Speex"

//rate from 0 to 10 are supported in Speex. 0 is lowest quality.
activeMic.encodeQuality = 6;
ns.attachAudio(Microphone.getMicrophone());

Test and Results: The encodeQuality for speex codec becomes quite usable from 4 (12.8kbps) although 3 can also be used for very low bandwidth requirements.It was also noticed that almost no processor overhead was there.So this codec will not add any load to applications requiring heavy processor.

Bug:It was found that the sound seemed to be very choppy and noisy for users more than 2. This bug is fixed in Flash Player 10,0,22,87 and above. 

Speex vs Nellymoser

Nellymoser 是closed format codec， 而Speex 是opensource的. 
对Flash Player的要求: 
    Nellymoser 可在Flash player 6及以上版本使用， Speex 要求Flash Player 10.
音频质量: 
    Speex做了优化，音频质量优于Nellymoser.
带宽要求: 
    Speex在比Nellymoser音频好的情况下，带宽要求反而更低。 
编码质量: 
    Speex有11级 (0-10)  编码质量可选.Nellymoser有5级 (5,8,11,22,44)

License type: Nellymoser is closed format codec whereas Speex is opensource which means that files created using speex can be decoded or encoded without any licence requirement.

Flash Player Requirement: Nellymoser works from Flash player 6 onwards whereas Speex requires atleast Flash Player 10. Although speex works with flash player 10 but there is a audio disturbance bug on listener end which was fixed in version 10,0,22,87.So player 10,0,22,87 and above is recommended.

Quality: Speex is optimised for speech so better quality is expected from speex as compared to our old Nellymoser codec.

Bandwidth Requirement: Speex delivers better quality than Nellymoser using less bandwidth as compared to speex.our tests revealed that the quality with nellymoser becomes usable at 8(16kbps) where as in case of speex it is 3 (9.80 kbps).The highest quality in Speex uses 42.2 kbps thats half of the bandwidth being used by nellymoser which is 88.2


Encode Quality: Speex provides more flexibilityby giving 11 levels of quality to choose from (0-10).0 is lowest and 10 is highest audio quality. Nellymoser gives 5 settings(5,8,11,22,44) ,5 is lowest and 44 is highest quality.Remember more is the quality higher is the bandwidth requirement which may lead to choppy sound when sufficient bandwidth is not available.

Codec Feature Comparison

Codec Quality Comparison

These are the results of MUSHRA tests comparing CELT with AAC-LD, G.722.1C and MP3, with 10 listeners. We used version 0.3.2 of the CELT codec with 5.8 ms frames (256 samples) and 2.9 ms (128 samples) look-ahead,for a total of 8.7 ms at 44.1 kHz. As for AAC-LD, we used Apple's implementation, which has 11.6 ms frames and 23.2 ms look-ahead, for a total of 34.8 ms delay at 44.1 kHz. The MP3 implementation is LAME in CBR more and is only included for reference purposes only.

The first graph is for the 48 kbit/s listening test.

The second graph is for the 64 kbit/s listening test. Note that G.722.1C is still at 48 kbit/s in that test because it is the highest bit-rate it supports.

官方主页：

                http://www.speex.org/

                http://www.nellymoser.com/

参考：

                http://www.celt-codec.org/comparison/

                http://all-streaming-media.com/

                http://askmeflash.com/article/1/all-about-speex-for-flash