question: Why do video streaming websites like YouTube choose to support 60 FPS only for 720p and above?
comment: The lower quality tiers are usually for people who have slow internet connection. 60fps would however need more data to be transferred so it'd be counter-intuitive to make the 480p and lower available with 60fps.
passages: BULLET::::- 50p/60p is a progressive format and is used in high-end HDTV systems. While it is not technically part of the ATSC or DVB broadcast standards yet, reports suggest that higher progressive frame rates will be a feature of the next-generation high-definition television broadcast standards. In Europe, the EBU considers 1080p50 the next step future proof system for TV broadcasts and is encouraging broadcasters to upgrade their equipment for the future. Many modern cameras can shoot video at 50p and 60p in various resolutions. YouTube allowed users to upload videos at 50 FPS and 60 FPS in June 2014. YouTube also allowed full HFR videos previously uploaded before 2014.
   PAL speed-up does not occur on native 25 fps video, such as British or European TV-series or movies that are shot on video instead of film.
   When support for 4K at 60Hz was added in DisplayPort 1.2, no DisplayPort timing controllers (TCONs) existed which were capable of processing the necessary amount of data from a single video stream. As a result, the first 4K monitors from 2013 and early 2014, such as the SHARP PN-K321, ASUS PQ321Q, and Dell UP2414Q and UP3214Q, were addressed internally as two 19202160 monitors side-by-side instead of a single display and made use of DisplayPort's Multi-Stream Transport (MST) feature to multiplex a separate signal for each half over the connection, splitting the data between two timing controllers. Newer timing controllers became available in 2014, and after mid-2014 new 4K monitors such as the Asus PB287Q no longer rely on MST tiling technique to achieve 4K at 60Hz, instead using the standard SST (Single-Stream Transport) approach.
   The number "720" stands for the 720 horizontal scan lines of image display resolution (also known as 720 pixels of vertical resolution). The "p" stands for progressive scan, i.e. non-interlaced. When broadcast at 60.00 frames/s frames per second, 720p features the highest temporal resolution possible under the ATSC and DVB standards. The term assumes a widescreen aspect ratio of , thus implying a resolution of 1280×720 px (0.9 megapixels).
   BULLET::::- For 525-line 60 Hz video (usually, and correctly, called "NTSC"), the active line duration is 52.856 µs, giving ≈713.5 pixels per line.
  
  In order to accommodate both formats within the same line length, and to avoid cutting off parts of the active picture if the timing of the analogue video was at or beyond the tolerances set in the relevant standards, a total digital line length of 720 pixels was chosen. Hence the picture will have thin black bars down each side.
   In October 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. The videos play back at a resolution of 720p or higher. 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 browser's user agent reports Windows XP, then H.264/MPEG-4 AVC video with stereo AAC audio is used instead.
   Section::::Video formats.:Progressive-scan video.
  
  Since the very beginning, the AVCHD specification had supported 720-line progressive recording mode at frame rates of 24 and 60 frames/s for 60 Hz models and 50 frames/s for 50 Hz models. Frame rates of 25 frames/s and 30 frames/s are not directly available in 720p mode, but can be simulated with frame repeating, when every frame is either repeated twice or a special flag in the video stream instructs a decoder to play every frame twice to adhere to output rate of 50 or 60 frames/s.
   Section::::See also.
  
  BULLET::::- UHDTV – digital television formats with resolutions of 4K / 2160p (3840×2160) and 8K / 4320p (7680×4320)
  
  BULLET::::- Rec. 2020 – ITU-R Recommendation for UHDTV with standard dynamic range
  
  BULLET::::- Rec. 2100 – ITU-R Recommendation for HDTV and UHDTV with high dynamic range
  
  BULLET::::- Image file formats based on HEVC
  
  BULLET::::- Better Portable Graphics – a file format for images based on HEVC
  
  BULLET::::- High Efficiency Image File Format – a file format for images and image sequences based on HEVC
  
  BULLET::::- Comparison of video codecs
  
  BULLET::::- List of open-source codecs
   The Rec. 2020 color space is supported by H.264/MPEG-4 AVC and H.265/High Efficiency Video Coding (HEVC). The Main 10 profile in HEVC was added based on proposal JCTVC-K0109 which proposed that a 10-bit profile be added to HEVC for consumer applications. The proposal stated that this was to allow for improved video quality and to support the Rec. 2020 color space that will be used by UHDTV.
  
  On September 11, 2013, ViXS Systems announced the XCode 6400 SoC which supports 4K resolution at 60 fps, the Main 10 profile of HEVC, and the Rec. 2020 color space.
  
  Section::::Implementations.:2014.
   Section::::Guidelines and recommendations.
  
  Section::::Guidelines and recommendations.:ITU-R Rec. 2100.
  
  Rec. 2100 is a technical recommendation by ITU-R for production and distribution of HDR content using 1080p or UHD resolution, 10-bit or 12-bit color, HLG or PQ transfer functions, and wide color gamut using the Rec. 2020 color space.
  
  Section::::Guidelines and recommendations.:UHD Phase A.
   While common multisync CRT computer monitors have been capable of running at even multiples of 24 Hz since the early 1990s, recent "120 Hz" LCDs have been produced for the purpose of having smoother, more fluid motion, depending upon the source material, and any subsequent processing done to the signal. In the case of material shot on video, improvements in smoothness just from having a higher refresh rate may be barely noticeable.
   BULLET::::- Motion estimation (ME) search strategy can also cause different visual quality for the same PSNR. So-called "true motion" search commonly will not reach minimum sum of absolute differences (SAD) values in codec ME, but may result in better visual quality. Such methods also require more compression time.
  
  BULLET::::- Rate control strategy. VBR commonly cause better visual quality marks than CBR for the same average PSNR values for sequences.
   In a PSNR based performance comparison released in April 2013 the Main 10 profile was compared to the Main profile using a set of 3840×2160 10-bit video sequences. The 10-bit video sequences were converted to 8-bits for the Main profile and remained at 10-bits for the Main 10 profile. The reference PSNR was based on the original 10-bit video sequences. In the performance comparison the Main 10 profile provided a 5% bit rate reduction for inter frame video coding compared to the Main profile. The performance comparison states that for the tested video sequences the Main 10 profile outperformed the Main profile. The Main 10 profile was added at the October 2012 HEVC meeting based on proposal JCTVC-K0109 which proposed that a 10-bit profile be added to HEVC for consumer applications. The proposal said this was to allow for improved video quality and to support the Rec. 2020 color space that has become widely used in UHDTV systems and to be able to deliver higher dynamic range and color fidelity avoiding the banding artifacts. A variety of companies supported the proposal which included ATEME, BBC, BSkyB, CISCO, DirecTV, Ericsson, Motorola Mobility, NGCodec, NHK, RAI, ST, SVT, Thomson Video Networks, Technicolor, and ViXS Systems.
   BULLET::::- Variable Refresh Rate (VRR) reduces or eliminates lag, stutter and frame tearing for more fluid motion in games
  
  BULLET::::- Quick Media Switching (QMS) for movies and video eliminates the delay that can result in blank screens before content is displayed
  
  BULLET::::- Quick Frame Transport (QFT) reduces latency
  
  BULLET::::- Auto Low Latency Mode (ALLM) automatic latency setting to the lowest ideal latency
   If the chosen bitrate is low, on the other hand, the video quality will be lower – thereby reducing the customer’s quality of experience.
  
  There are a number of ways of dealing with these challenges. One way is to take the YouTube approach. YouTube uses HTTP progressive download, and makes multiple versions of the video available at different resolutions and bitrates. Users themselves can then select the quality / bitrate that works best for them. If stalling or rebuffering occur, then can select the next lower resolution and continue viewing the video.
   DisplayPort1.3, finalized by VESA in late 2014, added support for 76804320 at 30Hz (or 60Hz with 4:2:0 subsampling). VESA's Display Stream Compression (DSC), which was part of early DisplayPort1.3 drafts and would have enabled 8K at 60Hz without subsampling, was cut from the specification prior to publication of the final draft.
  
  DSC support was reintroduced with the publication of DisplayPort1.4 in March 2016. Using DSC, a "visually lossless" form of compression, formats up to 76804320 (8K UHD) at 60Hz with HDR and 30bit/px color depth are possible without subsampling.
  
  Section::::Video Graphics Array.
  
  Section::::Video Graphics Array.:160120 (QQVGA).
   On September 9, 2016, Google announced Android TV 7.0, which supports Dolby Vision, HDR10, and HLG.
  
  On September 26, 2016, Roku announced that the Roku Premiere+ and Roku Ultra will support HDR using HDR10.
  
  On November 7, 2016, Google announced that YouTube would stream HDR videos that can be encoded with HLG or PQ.
   For the first years of its existence, YouTube offered only one, low-definition resolution of 320x240p at 30fps or less, only extending - first to widescreen 426×240, then to gradually higher resolutions - once the video service had become well established and had been acquired by Alphabet Inc. (Google), it had access to Alphabet's radically improved storage space and transmission bandwidth, and could rely on a good proportion of its users having high-speed internet connections. More recently, YouTube has also extended further into the LDTV realm by adding an even lower 256×144p resolution with a halved framerate - giving an overall effect reminiscent of early online video streaming attempts using RealVideo or similar, where 160×120p at single-figure fps was deemed acceptable - to cater to those whose network connections could not even sufficiently deliver 240p content.
   In April 2012, NHK (in collaboration with Panasonic) announced a display (7680×4320 at 60 fps), which has 33.2 million 0.417 mm square pixels.
  
  In April 2012, the four major Korean terrestrial broadcasters (KBS, MBC, SBS, and EBS) announced that in the future, they would begin test broadcasts of UHDTV on channel 66 in Seoul. At the time of the announcement, the UHDTV technical details had not yet been decided. LG Electronics and Samsung are also involved in UHDTV test broadcasts.
   At CES 2019, VESA announced that the new version would support 8K @ 60Hz without compression and was expected to be released in the first half of 2019.
   The human eye is most sensitive to flicker at the edges of the human field of view (peripheral vision) and least sensitive at the center of gaze (the area being focused on). As a result, the greater the portion of our field of view that is occupied by a display, the greater is the need for high refresh rates. This is why computer monitor CRTs usually run at 70 to 90 Hz, while TVs, which are viewed from further away, are seen as acceptable at 60 or 50 Hz (see Analog television Standards).
   Feature support will therefore vary from device to device, even within the same HDMI version.
  
  Section::::Versions.:Version comparison.:Refresh frequency limits for standard video.
   Action-safe or safe action is the area in which you can expect the customer to see action. However, the transmitted image may extend to the edges of the MPEG frame 720x576. This presents a requirement unique to television, where an image with reasonable quality is expected to exist where some customers won't see it. This is the same concept as used in widescreen cropping.
  
  TV-safe is a generic term for the above two, and could mean either one.
  
  Section::::Overscan amounts.:Analog to digital resolution issues.
  
  Section::::Overscan amounts.:Analog to digital resolution issues.:720 vs. 702 or 704.
   Today, video transmission standards in North America, Japan, and South Korea are still based on 60÷1.001 or ≈59.94 images per second. Two sizes of images are typically used: 1920x540 (1080i) and 1280x720 (720p); Confusingly, "interlaced" formats are customarily stated at 1/2 their image rate, 29.97 FPS, and "double" their image height, but these statements are purely custom; in each format, 60 images per second are produced. 1080i produces 59.94 1920x540 images, each squashed to half-height in the photographic process, and stretched back to fill the screen on playback in a television set. The 720p format produces 59.94 1280x720 images, not squeezed, so that no expansion or squeezing of the image is necessary. This confusion was industry-wide in the early days of digital video software, with much software being written incorrectly -- the coders believing that only 29.97 images were expected each second, which was incorrect. While it was true that each picture element was polled and sent only 29.97 times per second, the pixel location immediately below that one was polled 1/60th of a second later -- part of a completely separate image for the next 1/60 second frame. 
   BULLET::::- Tiles and WPP are allowed, but are optional. If tiles are present, they must be at least 64 pixels high and 256 pixels wide with a level specific limit on the number of tiles allowed.
  
presuppositions: Video streaming sites have no valid reason to disallow 60fps videos to be seen on lower quality videos.  Internet speeds under 720p have sufficient data to transfer 60FPS.
corrections: 60 FPS requires more data to be transferred and users watching lower quality videos likely don't have the capability to watch 60fps with their presumed connection speed.  60 FPS requires more data to be transferred making 60FPS counter-intuitive to internet connections lower than 720p.
raw_presuppositions: Video streaming sites have no valid reason to disallow 60fps videos to be seen on lower quality videos.  Internet speeds under 720p have sufficient data to transfer 60FPS. Video streaming sites have the capacity to support 60 FPS for below 720p.
raw_corrections: 60 FPS requires more data to be transferred and users watching lower quality videos likely don't have the capability to watch 60fps with their presumed connection speed.  60 FPS requires more data to be transferred making 60FPS counter-intuitive to internet connections lower than 720p. 60 FPS requires more data to be transferred than anything below 720p allows for.