Friday, 13 January 2017

VoIP calls encoded with SILK: from RTP to WAV, update

Three and a half years ago (which really sounds like a lot of time!) I was working with a VoIP infrastructure using SILK. As it often happens to server-side developers/integrators, you have to prove whether the audio provided by a client or to a client is correctly encoded :-)

Wireshark is able to decode, and play, G.711 streams, but not SILK (or Opus - more on this later). So I thought of having my own tool handy, to generate a WAV file for a PCAP with RTP carrying SILK frames.

The first part requires extracting the SILK payload and writing it down into a bistream file. Then you have to decode the audio using the SILK SDK decoder, to get a raw audio file. From there to a WAV file it is very easy.

As I tried to describe in this previous post, I had to reverse engineer the test files contained in the SDK, to see what a SILK file looked like.

Since the SILK payload is not constant, all that was needed was to insert 2 Bytes with the length of the following SILK frame. At the beginning of the file you have to add a header containing "#!SILK_V3", and voilĂ .

This is accomplished by silk_rtp_to_bistream.c (from, a small program based on libpcap that extracts the SILK payload from a PCAP and writes it properly into a bistream file.

Build the binary with:

gcc silk_rtp_to_bitstream.c -lpcap -o silk_rtp_to_bitstream

(you'll need libpcap-dev installed)

Create the bistream with:

./silk_rtp_to_bitstream input.pcap silk.bit

Now you can decode, using the SILK SDK, from bitstream into raw audio with:

$SILK_SDK/decoder silk.bit silk.raw

Raw audio to WAV can be done with sox:

sox -V -t raw -b 16 -e signed-integer -r 24000 silk.raw silk.wav

This works fine with single channel SILK at 8000 Hz.

More to come: an update on how to accomplish the same but for Opus.

1 comment:

  1. Thanks. This looks pretty good to decode SILK codec and convert to wav file.

    Do you have any steps to extract bitstream of RTAudio codec of Microsoft, convert to raw to wav file ?