In 1999, a discrete cosine transform (DCT) audio data compression algorithm called the modified discrete cosine transform (MDCT) was adopted for the Siren codec, used in the G.722.1 wideband audio coding standard.[74][75] The same year, the MDCT was adapted into the LD-MDCT speech coding algorithm, used for the AAC-LD format and intended for significantly improved audio quality in VoIP applications.[76] MDCT has since been widely used in VoIP applications, such as the G.729.1 wideband codec introduced in 2006,[77] Apple's Facetime (using AAC-LD) introduced in 2010,[78] the CELT codec introduced in 2011,[79] the Opus codec introduced in 2012,[80] and WhatsApp's voice calling feature introduced in 2015.[81]
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