I noticed that a certain video had several ‘versions’ and all of them were the same 1080p resolution, but they all differed in sizes. Some versions differed by a few MB, but it could range from around 300 MB to more than 1.5 GB! All of them have the same length and resolution and are exactly the same video. Also, a 720p version of that video was twice the size of some of the 1080p versions despite it being the same video.
I noticed something similar in some other videos as well. They all have the same length and resolution, and are even the same video, but their sizes differ by several times. Why is that the case?
In: Technology
Video (and audio!) is compressed in a way that causes quality loss. The objective is that the loss is minor and your eye/ear shouldn’t notice it, but it’s there.
How much loss happens, and how effective the compression is, are settings for when you encode and save the video. Discarding more image quality will make the file size shrink dramatically, but also cause the image to look more and more distorted when you play it. In the most extreme it can look like everything is blurry and out of focus, but the file size is tiny compared to what you’ve been looking at.
Sometimes you can tell the software to spend more effort on compression, and save a few megabytes on a long video. The downside is saving takes longer. Having a 30 minute video take anywhere from 5 to 60 minutes to encode (compress) is completely normal depending on if you have the quality turned way up or way down. Graphics cards can assist, but they don’t offer as much quality flexibility.
Tbh im not sure about the technical details (someone else can answer that) is but it boils down to bitrate. It refers to amount of data processed per second by the video playback system. Download the tool media info or examine the properties of the different video files. You’ll likely see that the largest files have the highest bitrate. When videos are uploaded, they are often compressed to fit within size restrictions or enhance streaming speed. This compression often causes loss of data. The actual pixel count (ie 1920×1080 for 1080p) will be maintained but some of the details are lost so theres more artifacts. Netflix or example will limit the bitrate of their 4k streams to 10-20mbps. But a 4k blu ray of the same movie might be upwards of 75mbps. For most people it isn’t noticeable FWIW.
u/DeHackEd is correct here. (I’m a video editor animator for 20+ years now). There are different kinds of video compression and they’re usually variable. Most video editing software has some default rendering outputs, to create your final version; but you can usually go deeper and customize the settings, if there’s a target file size you’re shooting for. There are compression schemes for editing (smaller file sizes but minimal compression) and for delivery (a lot of compression, and compressed in a way that makes editing difficult).
I’ve done things like video banner ads for web sites that aren’t really standardized (like a small local news and events website for a small community), and they will tell you the physical size of the video (how big it appears on screen, the pixel dimensions that fit their page) and a maximum file size (how much storage it takes). I’ve done them where their specs were like “maximum 20k” for a 10 second spot, and when I compressed the edit to that point, it *really* looked like ass; where 60 or 80k file size would have looked pretty good, by 20k it was a hot mess. Same “size” and length of video.
Just like saving a JPEG from Photoshop – set it for 90% and it looks great, 60 and you may not see much difference, but 10 or 20? It’ll look like you need to clean your glasses.
Raw video takes up crazy amounts of space. Let’s take an example of 1080p60:
1920 x 1080 pixels x 3 subpixels x 8 bits per subpixel x 60 fps = 3 gigabits per second ≈ 370 MB/s. Quadruple that for 4k60, and add another 25% for 10-bit video.
Hence, we need compression to get manageable file sizes. The other responses address that nicely.
They would be the same if they were both uncompressed (and the same codec & color space). But that’s a gigantic file size and YouTube and Netflix don’t want to be streaming those huge bitrates. So most videos are compressed, where they do their best at reducing the bitrate without a huge noticeable change to the quality.
Tom Scott has 2 videos on this (as of course he does):
Imagine a blindfold person trying to draw a picture based on your verbal instructions alone.
Resolution is merely the size of the canvas that they’re drawing the picture on at the end. What they draw on it with (a paint roller or a tiny paintbrush), and how much care you take to draw it correctly is determined by how much information you’re giving them (“draw a cat”, versus “draw a cat, facing right, it’s tail tucked down, the colour is between orange and brown but alternating every 20 pixels between dark and light, starting with dark….” etc,)
The way the video is stored is highly compressed – those are the “instructions” that you are giving the painting (I can explain how it’s done, but I’ve done that in a previous ELI5 – look up Fourier and Discrete Cosine Transforms… basically it finds regular patterns of regular patterns of regular patterns and then just reproduces those NEARLY perfectly by cleverly choosing only a small part of the pattern information). How it was compressed is determined by who did the compression. Sometimes that’s a broadcaster (e.g. iPlayer), sometimes that’s a guy on the Internet.
Depending on the compression settings used, even though the picture is still recreated on the same size canvas at the end it might have been painted carefully and accurately with a tiny fine brush, or a huge blob of a paint roller in a rough fashion. It all depends what settings the person compressing it used, how many instructions they decided to give the painter. High compression = lower image quality but smaller file size, low compression = higher image quality but larger file size.
It also matters what codec they used as well. A codec is the way that you find those regular patterns. Modern codecs do a better job than older codecs but take far more processing power to do so. So if you’re targeting old or slow devices, you’d compress with an older compression (e.g. MPEG, H264, etc.) but if you’re targeting powerful new phones, desktop GPUs, etc. and are going for perfect quality you’d compress with a newer compression (H265+, etc.). This is like being able to only use a few basic instructions and so painting a complicated image is rather tedious and long-winded and takes up a lot of paper, or only a few instructions that are more complex but which the painter has to be able to understand and spend a long time interpreting and getting exactly right. One takes up less paper (storage) to describe than the others, but the more complex one contains more detail even though it will take longer to “decode”.
So you can easily have a 4K movie that’s compressed badly that’s actually smaller than a 720p movie that’s been compressed really well, and yet the latter will look far better. And you can have a 720p movie that’s been compressed with slow devices in mind that makes a far larger movie movie than anything you have in 4K. Video editing software lets you literally tweak those settings, even on a frame-by-frame basis.
It’s all about trade-offs… in processing power required, in bandwidth/storage required, in where you need detail or won’t notice it among the fast-paced action, in the time it takes to compress the movie, and what the individual settings and standards available were when that movie was made.
I know if I download an episode from BBC iPlayer, the quality is amazing but the file size is often huge. I can also go online, or record from my TV card, that same video in the same “resolution” and get smaller file sizes for the same quality (e.g. by using a better codec than iPlayer uses), larger filesizes with worst quality (e.g. when quickly encoding it on my laptop from recording the TV), and any combination thereof.
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