networking - Decoding G.729 using FFMPEG from RTP stream (PCAP)

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I have pcap file that contains RTP data which in turn is audio for G.729 codec. Is there a way to decode this stream by pushing it into FFMPEG? This would probably mean I have to extract the RTP payload data from the PCAP file and then somehow pass it to FFMPEG.

Any guidelines and pointers are highly appreciated!

this doesn't require ffmpeg, but I assume you dont really care how the audio is extracted... check out How to Decode G729 on the wireshark wiki...

In Wireshark, use menu "Statistics -> RTP -> Show All Streams". Select the desired stream and press "Analyze".

In the next dialog screen, press "Save Payload...". Save options are Format = .raw and Channel = forward. Name file sample.raw.

Convert the .raw file to .pcm format using the Open G.729 decoder. Syntax: va_g729_decoder.exe sample.raw sample.pcm. Or for Linux: wine va_g729_decoder.exe sample.raw sample.pcm.

The .pcm file contains 16-bit linear PCM samples at 8000 Hz. Note that each sample is in Little-Endian format. To convert to .au format, all you need to do is prepend the 24 byte au header, and convert each PCM sample to network byte order (or Big-Endian). The following Perl Script will do the trick.

USAGE: perl pcm2au.pl inputFile outputFile

$usage = "Usage: 'perl $0 <Source PCM File> <Destination AU File>' ";
$srcFile = shift or die $usage;
$dstFile = shift or die $usage;
open(SRCFILE, "$srcFile") or die "Unable to open file: $!\n";
binmode SRCFILE;
open(DSTFILE, "> $dstFile") or die "Unable to open file: $!\n";
binmode DSTFILE;
###################################
# Write the AU header
###################################
print DSTFILE  ".snd";
$foo = pack("CCCC", 0,0,0,24);
print DSTFILE  $foo;
$foo = pack("CCCC", 0xff,0xff,0xff,0xff);
print DSTFILE  $foo;
$foo = pack("CCCC", 0,0,0,3);
print DSTFILE  $foo;
$foo = pack("CCCC", 0,0,0x1f,0x40);
print DSTFILE  $foo;
$foo = pack("CCCC", 0,0,0,1);
print DSTFILE  $foo;
#############################
# swap the PCM samples
#############################
while (read(SRCFILE, $inWord, 2) == 2) {
    @bytes   = unpack('CC', $inWord);
    $outWord = pack('CC', $bytes[1], $bytes[0]);
    print DSTFILE  $outWord;
close(DSTFILE);
close(SRCFILE);
                Well this gives me an idea on how to go forward. Probably requires me to do some extra coding to make it into a program. Thanks indeed.
– user349026
                Oct 12, 2011 at 9:11
extract the RTP/RTCP packets: with a tool like rtpbreak: http://dallachiesa.com/code/rtpbreak/
generate (with and editor) a sdp file to be passed at ffmpeg or mplayer or vlan
launch ffmpeg (or mplayer or vlan) with SDP file
stream RTP/RTCP packets at ffmpeg with rtpplay: http://www.cs.columbia.edu/irt/software/rtptools/
But if your goal is "only" the pcap-audio decoding (with RTP G729 codec) then you can use videosnaf or xplico 
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