codec for MSN RTP Paload Type 111

posted in Audio Codecs

I have tried to decode an audio conversation from me and a collegue of mine done with MSN messenger audio connection. Starting from pcap, i removed all the headers to have only the RTP payload. I would like to decode this file. From SIP messagging, it results that the RTP Payload Type is 111 (rtpmap: 111 SIREN/16000). I have tried to use the vivog723.acm codec but it doesn’t work.
Someone can help me?

Thanks reply

hey! i am facing the same problem... i have voice conversation through MSN in rtpDump... I tried using JMStudio... but in vain...
if someone can assist me with some suggestions... can email me on

thnx in advance... reply


I also tried the same kind of exercice...

The difference is that the codec I see is rtpmap:114 x-msrtp

Anyone knows this codec ?

I use MSN 7.5.0324 reply


Made a mistake... Code is x-msrta reply


True... so far in my ordeal i have encountered 4 codec....

SIREN, G722.1, RED(97) and X-mstra... not a clue about any of them nor how to retrieve sound from them... reply


RED stands for “Redundant Audio Data”. It is used to send the actual and some former frames (timeslices) in one RTP packet. If packet loss is high, the encoder can send older frames as well, possibly with lower bitrate codec. Inside a RED packet, any codec can appear which is in the rtpmap.

RED is specified in RFC 2198:
It is quite easy to decode these packages. reply


True... how ever i have not been able to decode RED. Also i haven’t come across anyone who has made a decoder for RED reply


I have a problem about x-msrta.what is it?
Anyone knows this codec ?

I use MSN 7.5 too!
please email me on reply


I don’t think you will be able to decode these media stream.
They are encoded in Microsoft proprietary algorithms.

I don’t say it’s impossible.
I just say it’s not something you can decipher
using any lambda decoder.

Have fun reply


For Linux there is a tool named pcap2wav (a derivate project of xplico) able to decode RTP with msrta (x-msrta) codec.
I think it is open source:



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