Wirebook › Bonus lesson
Why does a video go blurry, then sharp?
You press play and for a second or two the picture looks smudgy, like someone who's lost their glasses. Then, snap, it goes sharp. Nothing is wrong with your screen. The video app is being clever: it starts with a small, blurry version so you aren't kept waiting, measures how fast your connection really is, then switches to sharper versions as soon as it's sure they'll keep up. Here you can be that video player.
Why does it start blurry?
A streaming service doesn't keep one copy of a film. It keeps the same film at several sizes, from small and blurry (240p, a few hundred kilobits a second) up to sharp and big (1080p, around 5 megabits a second, and more for 4K). Each copy is chopped into short chunks, a few seconds each, lined up so the player can jump between sizes at any chunk. That set of sizes is called the quality ladder.
When you press play, the player has no idea how fast your connection is. If it grabbed a big sharp chunk first and the connection was slow, you'd stare at a spinning circle. So it starts low on the ladder, gets going in a moment, and times how long each chunk took to arrive. That's its measurement of your throughput. Once it's confident, it climbs a rung at a time. This is called adaptive bitrate streaming.
The snap to sharp comes a little later than the switch itself. The blurry chunks already in the buffer still have to play first, a bit like a queue of trains that left the station before the timetable changed.
The buffer is the video's packed lunch
The buffer is video that has already been downloaded but not shown yet. While the pipe is faster than the video needs, the buffer fills up. When the pipe gets thin, for a train tunnel or someone else starting a big download, the video keeps playing from the buffer. It only freezes if the buffer runs completely dry. That's why the smart player drops to a smaller rung as soon as the buffer runs low: a few blurry seconds are better than a frozen picture.
Why don't video calls do this as well?
They do change size, but they can't keep a big buffer. A film app can download the next half a minute ahead because the film already exists. On a live call the next second hasn't happened yet, so there's almost nothing to keep in hand. When the connection wobbles, a call has to go blocky or freeze straight away. You can see that in the Wi-Fi relay race and in how a video call squeezes a face into a thin pipe.
When a video stays blurry
- Give it a few seconds after pressing play or skipping ahead. Skipping empties the buffer, so it starts low again.
- Check the quality setting. "Auto" lets the player pick; a fixed low setting, or a phone's data saver, keeps it blurry on purpose.
- Someone else downloading a game or uploading photos takes a share of the pipe. See why the game lags when someone starts a download, and why the whole street slows down in the evening.
- Weak Wi-Fi is often the real bottleneck, not the internet plan. Move closer to the router or use 5 GHz.
- Big TVs and some plans only get top quality on certain devices or subscriptions, so it may never reach 4K.
Grown-up how-tos on Tooltern: why videos start blurry, then sharpen · video keeps buffering: causes and fixes · Mbps vs MB/s · streaming data calculator: will my data last the month?