iLBC (Internet Low Bitrate) Vocoder

VOCAL’s iLBC (internet Low Bitrate Codec) vocoder is ideal for robust voice communication over IP. iLBC speech coder is suitable for real time communications such as telephony and videoconferencing, streaming audio, archival, and messaging. Contact us to discuss your voice application requirements.

VOCAL’s iLBC voice compression algorithm is optimized for real-time multichannel applications on leading DSPs and RISC/CISC processors from TI, ADI, AMD, ARM, Intel and other vendors. iLBC voice codec is available for licensing as a standalone algorithm, as part of an embedded library suite, with a VoIP stack as well as a custom design.

iLBC Codec

iLBC vocoder is designed for narrow band speech and has two supported bit rates. The payload bit rate of 13.33 kbit/s has an encoding frame length of 30 milliseconds while the payload bit rate of  15.20 kbit/s has an encoding frame length of 20 milliseconds. iLBC uses a block-independent linear-predictive coding (LPC) algorithm. When the codec operates at block lengths of 20 ms, it produces 304 bits per block. Similarly, for block lengths of 30 ms it produces 400 bits per block. The two modes for the different frame sizes operate in a very similar way. Where they differ, it is usually explicitly noted by x/y, where x refers to the 20 ms mode and y refers to the 30 ms mode.

The iLBC codec smoothly handles the case of lost frames through graceful speech quality degradation; lost frames may often occur in connection with lost or delayed IP packets. Standard low bit rate codecs exploit dependencies between speech frames, which unfortunately result in error propagation when packets are lost or delayed. In contrast, iLBC encoded speech frames are independent and so this problem will not occur.

The iLBC algorithm responds to packet loss in a controlled manner similar to pulse code modulation (PCM) with packet loss concealment (PLC), such as the ITU-T G.711 standard which operates at a fixed bit rate of 64 kbit/s. At the same time, the algorithm enables fixed bit rate coding with a quality-versus-bit rate tradeoff close to state-of-the-art. For example, in comparison to G.729A, iLBC yields slightly better voice quality for about the same complexity of algorithms yet also has a higher robustness in dealing with packet loss.

Configurations

• DAA interface using linear codec at 8.0 kHz sample rate
• Direct interface to 8.0 kHz PCM data stream (A-law or μ-law)
• North American/International Telephony (including caller ID) support available
• Simultaneous DTMF detector operation available – (less than 10 talkoff hits on Bellcore test tape set)
• MF tone detectors, general purpose programmable tone detectors/generators available
• Data/Facsimile/Voice Distinction available
• Common compressed speech frame stream interface to support systems with multiple speech coders
• Dynamic speech coders selection if multiple speech codecs available
• Can be integrated with Acoustic Echo Canceller, G.168 Line Echo Canceller and Tone Detection/Regeneration modules
• Available with VoIP stack
• Multiple ports can be executed on a single DSP

Features

• Bitrate 13.33 kbps with frame size of 30 ms and 15.2 kbps with frame size of 20 ms
• Basic quality higher then G.729A with high robustness to packet loss
• Computational complexity in a range of G.729A
• Speech quality better than G.729A and G.723.1.
• Better packet loss handling compared to other low-bit rate codecs,including G.729A, G.729E,G.723.1 and G.728.
• iLBC supports multiple frame sizes to support the needs of different applications and/or VoIP equipment.
• Provide low delay and high packet loss robustness for low-bit rate codecs.
• iLBC delivers the same basic quality as G.729E and exceeds G.729E under packet loss conditions
• Gain-shape waveform matching forward in time
• Gain-shape waveform matching backward in time
• Start state encoding
• Pitch enhancement
• Packet loss concealment
• Optimized for high performance on leading DSP architectures
• Multichannel implementation

More Information

• Audio Examples
• MIPS/memory requirements
• Internet Low Bit Rate Codec (RFC 3951)
• RFC 3952 – RTP Packetization
• RTP Parameters