Codec iLBC

1. Introduction

This document describes how compressed iLBC speech, as produced by the iLBC codec [1], may be formatted for use as an RTP payload type. Methods are provided to packetize the codec data frames into RTP packets. The sender may send one or more codec data frames per packet depending on the application scenario or based on the transport network condition, bandwidth restriction, delay requirements and packet-loss tolerance.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [2].

2. Background

Global IP Sound (GIPS) has developed a speech compression algorithm for use in IP based communications [1]. The iLBC codec enables graceful speech quality degradation in the case of lost frames, which occurs in connection with lost or delayed IP packets.

This codec is suitable for real time communications such as, telephony and videoconferencing, streaming audio, archival and messaging.

The iLBC codec [1] is an algorithm that compresses each basic frame (20 ms or 30 ms) of 8000 Hz, 16-bit sampled input speech, into output frames with rate of 400 bits for 30 ms basic frame size and 304 bits for 20 ms basic frame size.

The codec supports two basic frame lengths: 30 ms at 13.33 kbit/s and 20 ms at 15.2 kbit/s, using a block independent linear-predictive coding (LPC) algorithm. When the codec operates at block lengths of 20 ms, it produces 304 bits per block which MUST be packetized in 38 bytes. Similarly, for block lengths of 30 ms it produces 400 bits per block which MUST be packetized in 50 bytes. This algorithm results in a speech coding system with a controlled response to packet losses similar to what is known from pulse code modulation (PCM) with a packet loss concealment (PLC), such as ITU-T G711 standard [7], which operates at a fixed bit rate of 64 kbit/s. At the same time, this algorithm enables fixed bit rate coding with a quality-versus-bit rate tradeoff close to what is known from code- excited linear prediction (CELP).