各种各样的编解码在各种领域得到广泛的应用,下面就把各种codec的压缩率进行一下比较。
G.711, G.723, G.726 , G.729, ILBC, QCELP, EVRC, AMR, SMV
各种编解码都有其应用的重点领域。 TD-scdma和wcdma采用自适应多速AMR语音编码,而Cdma2000采用了qualcomm公司的QCELP或EVRC作为语音编码。 RFC 3267 RTP Payload Format for AMR and AMR-WB June 2002 Example of usage of AMR in a possible GSM gateway scenario: m=audio 49120 RTP/AVP 97 a=rtpmap:97 AMR/8000/1 a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change-neighbor=1 a=maxptime:20 Example of usage of AMR-WB in a possible VoIP scenario: m=audio 49120 RTP/AVP 98 a=rtpmap:98 AMR-WB/16000 a=fmtp:98 octet-align=1 Example of usage of AMR-WB in a possible streaming scenario (two channel stereo): m=audio 49120 RTP/AVP 99 a=rtpmap:99 AMR-WB/16000/2 a=fmtp:99 interleaving=30 a=maxptime:100 其中:m=<media> <port>/<number of ports> <transport> <fmt list> a=fmtp:<format> <format specific parameters> This attribute allows parameters that are specific to a particular format to be conveyed in a way that SDP doesn't have to understand them. The format must be one of the formats specified for the media. Format-specific parameters may be any set of parameters required to be conveyed by SDP and given unchanged to the media tool that will use this format. It is a media attribute, and is not dependent on charset. The general form of an rtpmap attribute is: a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding parameters>] For audio streams, <encoding parameters> may specify the number of audio channels. This parameter may be omitted if the number of channels is one provided no additional parameters are needed. For video streams, no encoding parameters are currently specified. Additional parameters may be defined in the future, but codecspecific parameters should not be added. Parameters added to an rtpmap attribute should only be those required for a session directory to make the choice of appropriate media too to participate in a session. Codec-specific parameters should be added in other attributes. Up to one rtpmap attribute can be defined for each media format specified. Thus we might have: m=audio 49230 RTP/AVP 96 97 98 a=rtpmap:96 L8/8000 a=rtpmap:97 L16/8000 a=rtpmap:98 L16/11025/2 RTP profiles that specify the use of dynamic payload types must define the set of valid encoding names and/or a means to register encoding names if that profile is to be used with SDP. Experimental encoding formats can also be specified using rtpmap. RTP formats that are not registered as standard format names must be preceded by "X-". Thus a new experimental redundant audio stream called GSMLPC using dynamic payload type 99 could be specified as: m=video 49232 RTP/AVP 99 a=rtpmap:99 X-GSMLPC/8000 Such an experimental encoding requires that any site wishing to receive the media stream has relevant configured state in its session directory to know which tools are appropriate. Note that RTP audio formats typically do not include information about the number of samples per packet. If a non-default (as defined in the RTP Audio/Video Profile) packetisation is required, the "ptime" attribute is used as given below. 双音多频 DTMF(Dual Tone Multi Frequency),双音多频,由高频群和低频群组成,高低频群各包含4个频率。一个高频信号和一个低频信号叠加组成一个组合信号,代表一个数字。DTMF信号有16个编码。利用DTMF信令可选择呼叫相应的对讲机
双音多频信号是贝尔实验室发明的,其目的是为了自动完成长途呼叫。
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来自: hginvent > 《linux network》