Shrinking feeling: The tech that makes Netflix possible

If you have a Smart TV, you’ll know that when you switch on in the evening, it’s easy to feel paralysed by the sheer range of viewing options. Because of the explosion of streaming apps and services, there is almost an infinite number of viewing choices available. 

But have you ever wondered how it is that we can stream pretty much anything to our homes, in high definition, in an instant?  The secret to this technological miracle is one of the most important tricks in computing: compression. 

Compression has been a part of computing since the very beginning. The idea is simple: what if you could shrink down a piece of digital information to be the absolute smallest it needs to be? The smaller a file is, the more you can fit on a computer, and the faster it can be transferred over the internet. 

Imagine you have a row of 100 red pixels in a photograph. One way of storing that information would be to list “red” repeated one hundred times, but this would take up a lot of space! So a much better way would simply be to record “red x100”, and have the computer know that if it sees the “x100” instruction to repeat it 100 times. This, in essence, is how files are compressed. 

In practice, it gets a lot more complicated to really crunch files down to their smallest possible size, as modern computers use highly sophisticated algorithms based on complex maths to spot patterns in files. 

The difference in file size can be enormous. To test this, I just saved a photo of my cat, Hashtag, in two different formats. The uncompressed image (saved as a BMP file) was 36.6 megabytes, while saving it as a PNG file, which uses some clever compression algorithms, reduces it down to just 10.8 megabytes. That’s three times less storage space, and a three times faster download if I post it on the internet. 

There is, however, a way to make our files even smaller. And that’s essentially the digital equivalent of the Marie Kondo Method: throwing away what we don’t really need. 

This is another type of compression known as “lossy” compression, and is partially why streaming services don’t fall over, even though millions of people are logging on to watch at the same time. 

Take a video for example. Usually, video files are very large because they have to contain lots of information in order to digitally replicate the pictures and sounds. But it’s very easy to save space by stripping away the unnecessary details, like all of the super-quiet parts of the soundtrack that are barely perceptible if you’re not an audiophile wearing some expensive headphones. 

The picture can be cut down too—algorithms can subtly replace the sky in the background to use fewer colours, or reuse data from other frames in the video. Again, most of us won’t notice. Unless you’re obsessive about detail and watching your show on a massive TV, it’s probably more useful to you to have the video stream reliably, and download quickly, than it is to be absolutely pixel perfect. 

JPEG photos work based on the same lossy principles too. If I save my photo of Hashtag as a JPEG, it shrinks even further to just 1.3 megabytes. That means that if the photo is sent as a JPEG, it will arrive 28 times faster than the uncompressed version—and I’ll be able to store around 28 times as many photos on my computer. And the crazy part? Looking at it side by side with the original, even though the JPEG is significantly less detailed than the original, I can’t really tell the difference between the two at all. 

So, why not just compress everything down to be absolutely tiny? It isn’t always the answer. Sometimes you’ll need a more powerful computer to decompress a file, ie, to perform the complex maths in reverse. And other times you'll want to keep the originals at their maximum possible quality—especially if they are your own personal photos and videos—whereas lossy formats often reduce picture quality.

But for everything else, compression is a brilliant way to do more with less, whether you’re watching Netflix, streaming Spotify, or even sharing photos of your cat online. 

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