From: Mark Handley Subject: Z24/4C34/VIVE0011 Coursework Date: Mon, 07 Feb 2005 01:01:42 GMT To: gz24@cs.ucl.ac.uk, 4c34@cs.ucl.ac.uk, vive0011@cs.ucl.ac.uk cc: m.handley@cs.ucl.ac.uk, r.akester@cs.ucl.ac.uk ---------------------------------------------------------------------------- Z24/4C34/VIVE011 Multimedia Systems Coursework ---------------------------------------------- Skype (www.skype.com) uses at least two audio codecs. One is the iSAC wideband speech codec for higher bandwidth environments, and the other is the "Internet Low Bit Rate Codec" (iLBC) for lower bandwidth environments. This question is about iLBC. iLBC is specified in RFC 3951, and the way it is transported over RTP is specified in RFC 3952. These specifications are available at: http://www.ietf.org/rfc/rfc3951.txt http://www.ietf.org/rfc/rfc3952.txt Most audio codecs used over the Internet were originally designed for use over circuits, and only subsequently applied to packet networking. iLBC was designed from the outset for use over the Internet, which makes it more interesting for this course. Read both the two specifications above. RFC 3951 can be hard going in places, as it's very technical, but you don't need to understand everything. (a) What properties of iLBC and it's RTP packetization format make it especially suitable for Internet use? Explain what problems it solves, and how it solves them. [7 marks] (b) Explain how the features of the LPC algorithm are exploited to achieve the Packet Loss Concealment (PLC) example mechanism described in the iLBC RFC. How would the mechanism vary between the 20ms and 30ms frame cases? [4 marks] (c) The PLC mechanism in the RFCs only uses information from the packet received prior to a lost packet to mask the loss. Describe an alternative algorithm which might also use information from the packet following the lost packet, so as to more effectively conceal loss. [4 marks] [15 marks total] There is no hard limit for the length of your answer, but we're expecting something on the order of 1500 words (3 pages) in total plus any diagrams. If you use any material from sources other than the two RFCs above, you must identify the source of the material. If you quote any material directly, you must make it clear that it is a quote by enclosing it in quotation marks, or (for a lengthy quote) putting it in an indented italicised paragraph, together with a reference for where the material came from. Failure to do this will be treated as plagiarism. You should submit your answer, on paper, to the CS department reception no later than 12 noon on Monday 21st February.