Definition
Constant bitrate (CBR) is a way of compressing and sending digital data, like video or music, so that the amount of data per second stays the same no matter what. It doesn't matter how busy the picture is or how loud the sound gets. The data per second stays almost perfectly even.
A faucet that's always turned to the same setting
Imagine a faucet that gives you exactly one liter of water every second. Whether you're rinsing a tiny cup or scrubbing a huge pot, the same amount keeps flowing.
Constant bitrate works just like that faucet. Bitrate is the amount of data sent in one second. With CBR, a quiet scene where nothing moves and a wild action scene with thousands of fireworks both get the exact same amount of data every second.
This makes managing a network much easier. The sender and the receiver can both plan ahead: "About this much data will arrive each second." That helps prevent sudden surprises, like a flood of data that overloads the internet connection or freezes a device.
Rock-solid stability, but some waste
The biggest strength of constant bitrate is predictable stability. Because the data size never changes, it's very easy to work out the total file size of a video. Just multiply the length of the video by the data per second, and you have the answer.
But there's a clear downside too. In a simple scene, like a plain black screen, CBR uses more data than it needs, so space gets wasted. It's like pouring a gushing stream of water into a tiny cup that hardly needs any.
On the flip side, a very complex, fast-moving scene can run short of data. The picture gets squeezed into a fixed limit, so you may see a drop in picture quality, like blurry patches or blocky, mosaic-style glitches.
A bit more precisely: the hidden workhorse of live streaming
To be a bit more precise, CBR shines less in saved files and more in real-time streaming, where the video has to keep flowing without breaks. When you're broadcasting live, your network can turn shaky at any moment.
If the amount of data jumps up and down, delays pile up and the stream can stutter. That's why, when sending a broadcast to a platform like YouTube Live or Twitch, people often accept a little wasted quality in exchange for CBR and its steady sending speed.
On the other hand, when you're storing or watching a movie that's already been made, like on Netflix, variable bitrate (VBR) works better. It adjusts the amount of data scene by scene. So if stability comes first, pick constant bitrate. If efficient file size and picture quality matter more, pick variable bitrate.
Common misconceptions
Since the data is fixed in constant bitrate, the video quality must always stay the same too.
Only the amount of data sent is constant. Picture quality still changes with how complex the scene is. Simple scenes look clean, but a busy, fast scene can fall apart because there isn't enough data.
Where you meet it
In the software people use to stream on YouTube or other platforms, you can turn on CBR for a smooth, steady live broadcast.
In early digital music files, CBR was used so the file size could be calculated exactly from the song length and played back evenly.
In one sentence
CBR sends the same amount of data every second, no matter how complex the scene is, so the stream stays steady and unbroken.