Variable Bitrate

Think about how you tell a friend about a fun trip. You describe the thrilling moment on the roller coaster at length, waving your hands around. But the bus ride? You wrap it up in a few words: "It took two hours." Good storytellers know when to speed up and when to slow down. Video compression needs the same kind of rhythm.

In the past, a fixed method that used the same amount of data every moment was common. But with that method, when the screen changes wildly, like fireworks or an action movie, there isn't enough data, and the picture breaks into ugly blocks, an effect called blocking. On the flip side, a motionless night sky scene gets far more data than it needs, which is a waste.

Variable bitrate appeared to stop this waste and quality loss. It flexibly changes the amount of data to fit the situation. It's a smart approach: focus data where it matters and go light where it doesn't.

To be a bit more precise, before compressing a video, the computer analyzes how visually complex each scene is. The more a frame is packed with particles of many colors, or the faster the camera spins, the more information the computer has to remember, and that amount explodes.

Variable bitrate technology gives these information-heavy scenes a generous, high bitrate (the amount of data handled in one second). On the other hand, a scene where the main character sits still and talks can be shown with little data anyway, so it gets a low bitrate. The compression trick of recording only what has changed is used in the fixed method too. The variable method simply adjusts how much of that data each scene receives.

Working this way produces excellent quality for the overall file size, compared with just throwing lots of data at everything. That's the secret behind a file that takes up less storage but still looks very clean to our eyes.

Variable bitrate shows its biggest strength when you watch a movie on Netflix or save videos on your smartphone. It lets you pack high-quality content efficiently into a limited amount of space.

But you need to be careful in settings like live internet broadcasts, where data has to be sent out in real time. If a flashy graphic effect suddenly pops up and the amount of data to send spikes for a moment, the internet connection can hit a bottleneck, and the stream may stutter.

That's why, for smooth live streaming, people either use a method that keeps the data flow steady, or mix in a method that puts a cap on the maximum so the data never spikes too high. Most variable methods used in real streaming services are capped like this.