ILBC

internet Low Bit Rate Codec (iLBC) is a royalty free^[1] narrowband speech codec, developed by Global IP Solutions (GIPS) formerly Global IP Sound. It is suitable for VoIP applications, streaming audio, archival and messaging. The algorithm is a version of block-independent linear predictive coding, with the choice of data frame lengths of 20 and 30 milliseconds. The encoded blocks have to be encapsulated in a suitable protocol for transport, eg. RTP.

iLBC handles the case of lost frames through graceful speech quality degradation. Lost frames 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.

iLBC is defined in RFC 3951. It is one of the codecs used by the Gizmo Project, Ekiga, OpenWengo, Google Talk, Skype and Yahoo! Messenger.

Parameters and features

• Sampling frequency 8 kHz/16 bit (160 samples for 20 ms frames, 240 samples for 30 ms frames)
• Controlled response to packet loss, delay and jitter
• Fixed bit rate (15.2 kbit/s for 20 ms frames, 13.33 kbit/s for 30 ms frames)
• Fixed frame size (304 bits per block for 20 ms frames, 400 bits per block for 30 ms frames)
• Robustness similar to PCM with packet loss concealment, like eg. the ITU-T G.711
• CPU load similar to G.729A, with higher basic quality and better response to packet loss
• Royalty free
• Commercial use of the source code supplied by GIPS requires a license [1]
• PSQM testing under ideal conditions yields Mean Opinion Scores of 4.14 for iLBC (15.2 kbit/s), compared to 4.45 for G.711 (u-law)

External links

• iLBC homepage
• RFC 3951
• Global IP Solutions homepage

References

1. ^ Global IP Solutions iLBC Freeware Public License (HTML)

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