RE: Decoding G.729 using Ethereal, rtpdump and VoiceAgedecoder

I came across this article
(http://www.cisco.com/warp/public/788/pkt-voice-general/bwidth_consume.html)
that states there is a non-default version of g.729 which uses 30ms
packetization period, thus the RTP payload is 30bytes (I have just noticed
that now, I was under the impression that the payload was 20bytes).

Any idea what to do in this case?

 

  _____  

From: [email protected]
[mailto:[email protected]] On Behalf Of Alejandro Vaquero
Sent: Tuesday, July 25, 2006 4:41 AM
To: Ethereal development
Subject: Re: [Ethereal-dev] Decoding G.729 using Ethereal, rtpdump and
VoiceAgedecoder

 

Looks you are close because you get some voice patterns. If the RTP
packetization period is 20ms, then there are two 10byte samples of G729
encoded data per RTP packet, so make sure when you run the "rtpdump" the
samples are in the correct order when generating the bitstream.

Gabriel M. wrote: 

Hi,

 

I am trying to decode a VoIP conversation using G.729 encoding. I have read
older messages on this list (and others) but I am still not able to dump a
listenable .au file.

 

The steps that I made so far:

capture the traffic using Ethereal and dumping the RTP data (Statistics ->
RTP -> Show all streams -> SaveAs).

Use rtpdump (http://www.cs.columbia.edu/IRT/software/rtptools/) to dump only
the audio payload ("rtpdump -F payload -f in.rtp -o out.rtp") 

Use self-made code (see the code at the end of the e-mail) to convert the
rtpdump into the bitstream expected by the decoder

Decode the bitstream using the VoiceAge decoder (decoder out.rtp test2.au) 

Play the test2.au using GoldWave, (with all possible combinations between
16bit signed/unsigned 8000Hz)

 

Unfortunately all I can hear is some weird sound ( I can see voice patterns
in GoldWave, but the sound is metalic, almost alien-like). Am I doing
something wrong ?

 

References:

http://www.ethereal.com/lists/ethereal-dev/200505/msg00399.html

http://www.dsprelated.com/groups/speechcoding/show/666.php

 

//rtp_bitstream.c

 

[code]

#include <stdio.h>

 

int main(int argc, char **argv)

{

            char c;

            FILE *f=fopen("in.rtp", "r");

            FILE *fileout = fopen("out3.rtp","w");

 

            long lSize;

            char * buffer, * out;

            char qbyte, q;

 

            int i, j =0, k;

 

    fseek (f , 0 , SEEK_END);

    lSize = ftell (f);

    rewind (f);

 

            buffer = (char*) malloc (lSize);

            out = (char*) malloc (lSize * 16);

 

            fread (buffer,1,lSize,f);

 

            for (i = 0; i<lSize; i++)

            {

                        

                        qbyte = buffer[i];

                        for (k=0;k<8;k++)

                        {

 

                                    q = qbyte & 1;

                                    out[j] = 0x00;

                                    j++;

 

                                    if (q == 0)

                                                out[j] = 0x71;

                                    else

                                                out[j] = 0x81;

                                    j++;

 

                                    qbyte >>= 1;

                                    

                        }

            }

 

            fwrite (out , 1 , lSize * 16, fileout);

 

 

return 0;

}

[/code]

 

Best regards,

Gabriel.

 





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