As high pressure builds this week, TV and radio services may once again be affected by atmospheric conditions.
High pressure trapping a layer of low cloud, mist or fog is expected to create a temperature inversion across parts of the UK later in the week and through the weekend. This will result in an increased risk of disruption to Freeview TV, FM and DAB radio.
Under such conditions, the inversion creates a mirror-like effect on terrestrial TV and radio signals, reflecting signals around the curve of the earth. As a result, signals from distant, normally out-of-range transmitters, can begin to interfere with local signals.
Freeview says the North of England, the southwest and Wales are most prone to interference issues.
What are the symptoms of atmospheric interference?
During this time, TV pictures may break-up with audio glitches. Sometimes, the signal drops out completely. A ‘no signal’ message may then appear on TV screens. On FM radio, a distant station may be audible on top of a local station. On DAB, audio distortion or signal break-up occurs.
What action do I need to take?
Viewers and listeners are advised not to retune or adjust their Freeview or DAB devices. FM radio listeners may be able to find their desired station on an alternative frequency.
During this time, all or some services may be unavailable. There is nothing the broadcasters can do about atmospheric interference.
Cable, satellite and online streaming services are unaffected by atmospheric interference. So affected viewers may be able to switch to an alternative platform.
If you do retune, you may lose channels from your channel list. Equally, you may also see out-of-region services appear in the channel list. You’ll need to retune again when the conditions have returned to normal to restore all of your available services.
Why are only some channels affected?
Interference may only be present on some frequencies, and not others. This means only some channels are affected. Additionally, some Freeview channels are broadcast at lower power than others. This makes them more prone to be ‘drowned out’ by interfering signals.
Is this a new phenomenon?
No. Analogue TV used to be affected, but in different ways. This type of interference used to be commonly referred to as ‘co-channel interference’. The picture would gradually degrade as the interference got worse. Notably, horizontal lines began to appear. Sometimes, ghost-like images from the interfering channel were just about visible in the background.
In really bad instances of interference, the so-called ‘venetian blind’ effect got worse until the picture and sound were lost. Sometimes, the interfering television service would break through completely, resulting in a different channel appearing. Under these conditions, sometimes foreign channels were visible.
Television continuity announcers would routinely advise viewers to not adjust their set – something that still applies today.
Can broadcasters increase the signal to cancel out the interference?
Increasing the power of a transmitter would make interference worse for others. Transmitter networks are planned and co-ordinated across national boundaries to avoid undue interference.
Where can I get more information about this?
The Radio and Television Investigation Service, operated by the BBC, contains useful guides on reception problems. During severe instances of atmospheric interference, the Freeview website will post updates for viewers.