Current Freeview changes make it a little less reliable

Please do not blame Freeview. They are pumping out all the usual signals if this happens to you. Photograph: UK Free TV

Brian Butterworth published on UK Free TV 8th November 2017 at 14:31

Freeview is reliable

If you enjoy watching TV using Freeview, this means that there is a wire going from the back of your TV set to an aerial on the roof of your home, flat or office.    In the air. the Freeview TV signals travel in straight lines, like light, so the aerial needs to be able to “see” the top of a Freeview transmitter tower. 

From the top of one of these towers is mounted a cylinder that is covered in transmission panels.   This allows the Freeview signals to be sent out in a controlled pattern, for several reasons:

• It is wasteful to send the signals out into space;
• It is unhelpful to send signals over a country borders;
• It is unnecessary to send signals into mountains where no-one can get them;
• To limit the possibility of interference in a very flat area;
• Sometimes to synchronise a signal in a single frequency network area.

Tropospheric Ducting

It is quite reasonable to think that all the above conditions would remain constant over time.

It was discovered that all TV signals sent horizontally do not always just zoom off into space.   From time to time, changes to air pressure cause “Inversion” and the signals can reflect down as if there were a mirror at cloud level.  Sometimes the signals can be found 800 miles beyond their expected locations.

The way the digital TV signals are encoded incorporates ways to reject these unplanned, unexpected interference.    There is a limit to what can be done in all instances.

Whilst it is possible to predict where the “mirrors” can occur in the atmosphere – see http://www.dxinfocentre.com/tropo_nwe.html - the loss of Freeview channels from your box can then depend upon:

• The Freeview multiplex you are watching (the ones in DVB-T2 mode are safer);
• The direction your aerial points because which TV transmitters are in a line projecting from your roof to your normal transmitter AND which transmitters are in line 180 degrees from the above one;
• If your aerial system has an amplifier (that might get overloaded and stop working);

This means that moving a few streets away can make the difference between no Freeview at these times and normal service during a time of Tropospheric Ducting.

People who watch satellite TV can have a similar, random problem caused by heavy rain and snowfall. 

Generally, these Tropospheric Ducting events happen in the UK about the times the clocks go forward and go back, and sometimes when there is a very still, cold winter. 

Why the 700MHz changes might be an extra problem

 In simple terms the 700MHz changes will reduce the number of transmission frequencies for Freeview from 40 (C21 to C37, C39 to C48) to just 30 (C21 to C48, C55 and C56).  This is being done in a way that keeps all the channels you can watch.

It does mean, however, that during those times of random Tropospheric Ducting it will be slightly more likely that an interfering signal might occur at any given location.

Do not adjust your set

So, please don’t blame Freeview.   They are pumping out all the usual signals if this happens to you.    

If it does happen, please don’t retune your equipment.    It is unlikely that the problem will continue for more than a few hours at the most, and you will most likely be able to watch most other Freeview channels.

Do we need an “Inversion Effect” predictor?

Would it be useful to develop a tool to work out how bad a potential Tropospheric Ducting event might be for a given location? 

See also https://ukfree.tv/article/1107052352/What_is_the_Inversion_Effect_and_why_does_it_effec