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

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 answer 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...

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

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

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

4. 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 [my_inputFile] [my_outputFile]

$usage = "Usage: 'perl $0 [Source PCM File] [Destination AU File]' ";

$my_inputFile = shift or die $usage;
$my_outputFile = shift or die $usage;

open(INPUTFILE, "$my_inputFile") or die "Unable to open file: $!\n";
binmode INPUTFILE;

open(OUTPUTFILE, "> $my_outputFile") or die "Unable to open file: $!\n";
binmode OUTPUTFILE;

###################################
# Write the AU header
###################################

print OUTPUTFILE  ".snd";

$foo = pack("CCCC", 0,0,0,24);
print OUTPUTFILE  $foo;

$foo = pack("CCCC", 0xff,0xff,0xff,0xff);
print OUTPUTFILE  $foo;

$foo = pack("CCCC", 0,0,0,3);
print OUTPUTFILE  $foo;

$foo = pack("CCCC", 0,0,0x1f,0x40);
print OUTPUTFILE  $foo;

$foo = pack("CCCC", 0,0,0,1);
print OUTPUTFILE  $foo;

#############################
# swap the PCM samples
#############################

while (read(INPUTFILE, $inWord, 2) == 2) {

    @bytes   = unpack('CC', $inWord);
    $outWord = pack('CC', $bytes[1], $bytes[0]);
    print OUTPUTFILE  $outWord;
}

close(OUTPUTFILE);
close(INPUTFILE);