YouTube originally offered videos at only one quality level, displayed at a resolution of 320×240 pixels using the Sorenson Spark codec (a variant of H.263),[78][79] with mono MP3 audio.[80] In June 2007, YouTube added an option to watch videos in 3GP format on mobile phones.[81] In March 2008, a high-quality mode was added, which increased the resolution to 480×360 pixels.[82] In November 2008, 720p HD support was added. At the time of the 720p launch, the YouTube player was changed from a 4:3 aspect ratio to a widescreen 16:9.[83] With this new feature, YouTube began a switchover to H.264/MPEG-4 AVC as its default video compression format. In November 2009, 1080p HD support was added. In July 2010, YouTube announced that it had launched a range of videos in 4K format, which allows a resolution of up to 4096×3072 pixels.[84][85] In June 2015, support for 8K resolution was added, with the videos playing at 7680×4320 pixels.[86]In November 2016, support for HDR video was added which can be encoded with Hybrid Log-Gamma (HLG) or Perceptual Quantizer (PQ).[87] HDR video can be encoded with the Rec. 2020 color space.[88]
In June 2014, YouTube introduced videos playing at 60 frames per second, in order to reproduce video games with a frame rate comparable to high-end graphics cards.[89][90] The videos play back at a resolution of 720p or higher.[91] YouTube videos are available in a range of quality levels. The former names of standard quality (SQ), high quality (HQ) and high definition (HD) have been replaced by numerical values representing the vertical resolution of the video. The default video stream is encoded in the VP9 format with stereo Opus audio; if VP9/WebM is not supported in the browser/device or the
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