WEBVTT

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video compression is a very important aspect of how video streaming websites

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like youtube netflix and hulu work without it you wouldn't be able to

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stream such high quality high frame videos like this one first some math

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let's say you're watching a video with a resolution of

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pixels that's like 2 million pixels per frame and in this case you've got 30

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frames per second that's a whopping 62

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million pixels per second if each pixel needs 24 bits or three bytes of

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information you're looking at 178 megabytes of data per second completely

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uncompressed that would be 51 gigabytes of information for a single five minute

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video we've got pretty fast internet here at the office but even we can only

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download like two and a half gigabytes in five minutes

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that's a long ways away from 51. fortunately a five minute high

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definition youtube video is not 51 gigabytes it's in fact more like 72

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megabytes that's like 700 times smaller

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how did they do that that's the magic of video compression which works mostly by

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minimizing redundancy in the video data

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now there are all sorts of different ways to compress video and lots of

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different video containers and codecs but i won't go into detail about that

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because last already covered it in this episode and frankly i still don't really

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understand it all even though i am employed as a video editor so don't tell

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Linus so here's what you need to know there are two ways to compress video

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spatial compression and temporal compression also known as intra-frame

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and interframe respectively seriously who names this stuff

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anyway i'm going to describe each of these in terms of the jpeg image format

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and the mpeg video format spatial or

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intra-frame compression is applied only to individual video frames to compress a

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video frame you can use the same process that's used to compress a still image

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like a jpeg when a jpeg is created color

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information of the image is reduced in a process called chroma sub sampling and

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then the image is split into sections of 8x8 pixels called macroblocks then all

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of this fancy stuff is done to these blocks to further reduce the file size

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it's easy to see this effect on an image that has been saved with a high amount

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of compression all those ugly looking squares are known as compression

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artifacts some images like this red square can be compressed more than

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others like this complicated photograph because the red square is very simple

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with a lot more redundancy so that's how a jpeg uses spatial compression now for

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an mpeg sometimes called a motion jpeg

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things are a bit more complicated than just lining up a whole bunch of jpegs

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one after the other to be used as video frames doing it that way would not allow

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for inter frame or temporal compression

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temporal compression works again by reducing redundancy if you go through a

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video like this one frame by frame you'll notice that many frames are

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almost completely identical that's redundancy which can be easily

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compressed for example for all the parts of this video that don't need to change

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from one frame to the next like all this whiteness around me all we need is an

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instruction for those parts of the frame which says don't change anything like

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jpeg the mpeg standard breaks a video frame into 8x8 pixel macro blocks and

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each macro block receives instructions on what to do with the pixels they

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already have there are instructions for staying exactly the same for moving

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rotating changing color changing completely and so on video frames with

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instructions like this are called p frames and they use about half as much

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data as an iframe which is pretty much just a jpeg

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there are also b frames which are predictions or interpolations between i

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and p frames b frames use a quarter as much data as an iframe so they really

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save a lot of space this is all pretty easy to see on a video file that has

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been saved in an extremely low quality setting like what you're seeing right

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now again all these awful square thingies are called compression

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artifacts furthermore sometimes a video file will be corrupted or missing some

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data when that happens you get an effect like this one

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this particular effect is caused by a missing iframe which would have cleared

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away the old image to make room for a new one without the iframe the list of

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changes in the following b and p frames are applied to the wrong image so now

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you know why a video sometimes looks weird or gets all blocky those artifacts

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are a result of the video compression being either too high or else some kind

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of error in the coding or transmission of the video data now listen if none of

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that made any sense everything i've said can be broken down to one thing

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bit rate the bit rate is the amount of data or bits that are being used every

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second if a video has a low bid rate it will be low resolution and or low

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quality with lots of compression artifacts if a video has a high bitrate

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it will be high quality and or high resolution so the more bits you have to

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work with the better the video will look there's just no getting around that even

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with the best compression methods video compression is a balancing act between a

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good looking video or a small file size you can't have both but if compromise is

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not your style maybe you'll like fractal design now i'm sure you've all grown

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accustomed to embarrassing torturous events happening to line us in these

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spots let's look at them now

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behind in this manner

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speaking of mean fractal design is that buying up all of my advertising

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inventory so i was a little nervous about doing this ad i am however

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relieved to say that josh has granted me a reprieve i guess Linus is the one he

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wants to torture so we're using this spot to thank all of you the viewers who

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went to the survey page and responded so positively to our previous fractal

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design ads as fast as possible that they've decided to continue sponsoring

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this show so thanks to you guys and thanks to fractal design the company

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whose cases power supplies and fans bring all of the boys to all of the

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yards be sure to check back over the next few weeks to learn just what that

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reference might mean thanks for watching this episode of fastest possible give us

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a like or a dislike leave a comment subscribe do it subscribe and we'll see

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you next time