introduced in 2006, Apple's FaceTime (using AAC-LD) introduced in 2010, and the CELT codec introduced in 2011. Opus is a free software audio coder. It combines both the MDCT (CELT) and LPC (SILK) audio compression algorithms, using the former for speech. It is widely used for VoIP calls in WhatsApp. The PlayStation 4 video game console also uses Opus for its PlayStation Network system party chat. A number of codecs with even lower bitrates have been demonstrated. Codec2, which operates at bit rates as low as 450 bps, sees use in amateur radio. NATO currently uses MELPe, offering legible speech 600 bps (with one nonstandard variant halving the number). Lyra by Google takes an unusual machine learning approach, giving "almost eerie" quality at 3 kbps. Microsoft’s Satin also uses machine learning, but uses a higher tunable bitrate and is wideband. Sub-fields Wideband audio coding Linear predictive coding (LPC) AMR-WB for WCDMA networks VMR-WB for CDMA2000 networks Speex, IP-MR, SILK and Opus for voice-over-IP (VoIP) and videoconferencing Modified discrete cosine transform (MDCT) AAC-LD, G.722.1, G.729.1, CELT and Opus for VoIP and videoconferencing Adaptive differential pulse-code modulation (ADPCM) G.722 for VoIP Narrowband audio coding LPC FNBDT for military applications SMV for CDMA networks Full Rate, Half Rate, EFR and AMR for GSM networks G.723.1, G.728, G.729, G.729.1 and iLBC for VoIP or videoconferencing ADPCM G.726 for VoIP Multi-Band Excitation (MBE) AMBE+ for digital mobile radio and satellite telephone Codec 2 See also Digital signal processing Speech interface guideline Speech processing Speech synthesis Vector quantization References External links ITU-T Test Signals for Telecommunication Systems Test Samples ITU-T Perceptual evaluation of speech quality (PESQ) tool Sources