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Thread: TV buying advice

  1. #21
    Diamond Member AndyD's Avatar
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    Quote Originally Posted by irneb View Post
    For UHD that speed's a bit on the low side. You'd most likely need lots of buffering to get something playing decently across that. Or more likely be able to download the file before you start watching.

    You're effectively on the limits for halfHD / interlaced fullHD (i.e. 720p / 1080i), 1080p should be fine over 5Mb/s or higher, though SD (DVD quality) should be quite decent across that (a 1Mb/s is around your bottom limit for such). But for UHD I'd say you require at least 10Mb/s, probably better at 15. Note, these are if the compression techniques used are "decent", e.g. using multi-pass H265 encoding (or something similar, I think NetFlix did say they're going with H265 these days). Else you either need much higher baud-rates or you're going to see block-artefacts, dropped frames, stuttering, etc.
    The terms HD and UHD are nasty and misleading. HD refers to anything that's 1080p resolution, the problem is even a movie that's been 'upscaled' to 1080p from a 720p source would still qualify as HD even though the quality will be poor. Likewise the term HD takes no account of the bitrate of encode so you will find mkv containered HD 1080p movies that range between 1.6 gigs and 50gigs in size and again the playback quality will be poorer for the small file even though both are technically 1080p resolution.

    I'm not sure what standard netflix encodes follow or what filesizes HD and UHD movies would be so I can't comment on ADSL speed requirements. They do have a guide on their website though.
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  2. #22
    Gold Member irneb's Avatar
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    Quote Originally Posted by AndyD View Post
    The terms HD and UHD are nasty and misleading. HD refers to anything that's 1080p resolution, the problem is even a movie that's been 'upscaled' to 1080p from a 720p source would still qualify as HD even though the quality will be poor. Likewise the term HD takes no account of the bitrate of encode so you will find mkv containered HD 1080p movies that range between 1.6 gigs and 50gigs in size and again the playback quality will be poorer for the small file even though both are technically 1080p resolution. But down scaling from UHD to FHD (or even lower) isn't a bad idea at all.

    I'm not sure what standard netflix encodes follow or what filesizes HD and UHD movies would be so I can't comment on ADSL speed requirements. They do have a guide on their website though.
    You're right about upscaled being a "silly" idea. You cannot make something better than it used to be by increasing its resolution. So if the movie wasn't recorded at FHD (usually they're recorded in RAW uncompressed format, these days at typical 4K resolutions at anywhere between 24 and 60 fps - meaning multi-TB files), then turning it into FHD is a futile exercise.

    And yes, the smaller file is probably worse than the larger. All other things being equal. However, a smaller file with a better compression codec might even be better than a larger file with a less optimized codec. It all comes down to the amount of loss in the lossy compression used in the codec. E.g. some would throw away certain colour shifts which human eyes cannot distinguish, others may combine similar pixels, yet more may combine portions of consecutive frames which they deem not to change, and some of the later versions also try to figure out movements from frame to frame. It's usually a bad idea to push these to their limits.

    Personally, I've done some re-encodings myself. And what I've found is best results (both minimum file size and best quality) tend to come from using the better variants of compression codecs (used to be xvid, though these days VP8/H265 give even better results), using a multi-pass encoding (checking how much loss will happen and reducing the loss to acceptable levels in final pass), and never use a constant bitrate encoding scheme (rather allow increased bitrate for scenes where it's needed and reduced where it's not). Unfortunately, this means a 2h movie of 1080p@24fps could be fine at 2GB, while another of similar length, resolution and framerate may start to show artefacts at 5GB. It is usually a factor of how much movement and complex shapes are changing between frames. Increasing the resolution tends to mean a multiplicative increase in size as well, e.g. 1080 to 2160 is not just twice as large, but rather 4 times (i.e. the pixel count is the effect, not the height) - though that's also an inexact measurement due to the actual compression algorithm. It's possible to derive the statistics of how much an encoded file deviates from the original, and this is where multi-pass encodings comes in, it uses such statistics to figure out which parts of the movie requires higher bitrates and which can be made even lower. My own investigations showed that best results is obtained by using a constant quality limit with multiple pass encoding and a variable bitrate, as soon as a constant bitrate is forced the quality suffers.

    I'm not exactly sure just how NetFlix encodes their FHD/UHD movies, or others for that matter. They tend to keep the exact "science" behind these a secret (from each other at least). The trouble is bitrate on its own (i.e. the size of the file delivered) is only a part of the story. The compression algorithm and how it's applied tends to have a bigger effect on the quality than the file size, and the actual raw material (i.e. the original recording) has the greatest effect. Some reading material:
    http://www.imore.com/netflix-increas...why-isnt-apple
    http://mdp-labs.co/netflix-leads-wit...on-algorithms/
    https://donmelton.com/2015/12/21/wha...le-and-amazon/
    http://variety.com/2015/digital/news...ty-1201661116/
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