It always happens on the same evening, on the channel everybody wanted to watch. The picture breaks into squares, the sound stutters, and the speed test comes back fine. Both things are true at once, and that is the clue.
Why the television breaks when everything else works
A web page and a live channel do not travel the same way. Anything you browse is sent to you alone, and when a packet goes missing your device quietly asks for it again.
Live television from a set-top box is sent once and copied close to the subscribers who want it, which saves bandwidth and removes the polite retry. A packet lost on the way to your television is never replaced, and the screen shows you a block where the gap was.
Then there is timing. A live channel from a box runs under a second behind the event, and that speed is bought with a very short buffer, a handful of frames in reserve. A streaming app, ten to forty-five seconds behind, absorbs a wobble the box cannot. What breaks your picture is rarely the average speed of the line; it is the regularity of the last few metres.
What a television stream actually asks for
The numbers are smaller than people expect: about 2.4 Mbps for standard definition, 5 Mbps for high definition, 10 Mbps for full HD. That last figure is why installations that behaved for years started stuttering, since the move to full HD doubled what one channel asks for on home networks that never changed.
That is the point of the sum. Take the number from a proper broadband speed test and set it against what your screens ask for. If the line has room and the picture still breaks up, stop blaming the provider and look at the twenty metres between the router and the telly.
Powerline adapters: the fix everyone tries first
They are cheap, need no drilling, and the box promises a four-digit number. Measured in a real house, an older powerline pair moves around 35 Mbps and a recent one around 80 Mbps. Enough for a channel in the abstract, but the throughput swings about, and swings are what a set-top box cannot cope with.
Four things in a British house pull that down hard.
- Plugging the adapter into an extension strip: the other appliances on it dump interference onto the pair, and most of the signal never arrives.
- RCDs and RCBOs in the consumer unit, sitting between the two sockets and weakening the signal on the way through.
- Two sockets on different ring finals, upstairs and downstairs say: the signal climbs back to the consumer unit and down again, multiplying both the cable length and the branches it has to survive.
- Old wiring with plenty of junctions, where every spur is another place for signal to leak away.
Providers have quietly stopped recommending powerline for a television box, and the symptoms they list are the ones in this article: dropouts, pixellation, sound going first. The advice now points at a current-generation booster. None of that condemns powerline everywhere: a games console or a printer at the end of the hall will not care.
The four options, ranked by how well they hold
Before spending anything, the free moves, in this order.
- Look for a network socket you already have: plenty of newer homes were wired at build, and a faceplate behind the sofa beats everything in the table.
- Lift the router off the floor and out of the cupboard: shut inside a media unit it loses much of its reach before it starts.
- Pick the band against your instinct: 5 GHz in line of sight, 2.4 GHz as soon as walls are involved. Behind two of them the slower, tougher band gives the steadier picture, and 10 Mbps is all you need.
- Ask your provider for a Wi-Fi check: several will send a booster off the back of one. Where a Wi-Fi extender is placed matters more than what it cost.
A flat Cat 6 run tucked along the skirting costs about £15 and ends the conversation. One concrete wall halves a Wi-Fi signal, and no adapter argues with that.
The option that removes the weak link entirely
Some versions of this problem disappear rather than get solved. Most televisions sold since 2018 run apps of their own, so the set-top box drops out of the chain. The television is usually better placed and better equipped than the box in the cabinet underneath it, and it costs nothing to test this evening.
An app streams like any other internet content, outside the optimised path the box uses, so it loses the priority the box has when the line is busy, and channel line-ups can differ. In exchange you keep live, catch-up, restart and recording, signed in automatically on the home network.
If the apps hold up on the same television, in the same spot, on the same Wi-Fi, the box was the problem and the twenty metres never were.