Question

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!

Answer 1

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);