This means that the test is only valid for those parts of Dorset where people cannot currently receive Freeview. This is mainly the Poole area, the rest of the county can already receive Freeview from Rowridge.
In the document about the test, it says in the "small print" about the test:
"Although the test pattern is available all the time, we recommend that you only use it during the six monthsleading up to switchover in your area"
The transmitter that cover Dorset will switchover in mid 2012, this means the test is not valid until Christmas 2011 at the earliest.
There are also a few preconditions for the test too. "You can use the teletext check if:
you live in an area which will be covered by digital television through an aerial after switchover;
and you are not likely to need to make changes to your aerial as a result of changes at the
transmitter which covers you;
You may not be able to use the check if you use an aerial which is shared by several homes. These
systems might use components which mean that the result may not be correct. If you know that you are, or
suspect that you may be, connected to a shared aerial system then you should seek advice from the system
owner. This is probably your landlord."
There is another time the test will not work in those places where the current analogue signals are part of the Digital Divided Review.
Sadly, there is no BBC Research and Development White Paper yet, but Digital UK explain how the test works:
"Antenna systems which fail the test almost certainly need to be upgraded for digital. Antenna systems which pass the test are more likely to be OK, but other factors may mean that they are not. The test is intended to identify the very worst installations and so give consumers time to get them upgraded in advance of switchover.
The test itself is based on assessing the received analogue Carrier to Noise ratio (C/N) for a particular receiving installation using the onset of teletext decoding failure as a proxy, and relating this C/N to the level required for satisfactory DTT reception in the post-switchover situation, with an appropriate margin. The test assumes that the existing analogue services will be replaced by future digital PSB multiplexes operating at one fifth of the analogue power (-7dB) on channels within the analogue aerial group and the vast majority of transmitter sites will be converted in this way. The test pattern itself has been designed to be tolerant to multi-path interference and to permit errors to be clearly seen.
The just served level for existing analogue TV services equates to a picture grade of 2.5 which represents a future digital C/N margin of 10dB. The teletext test failure point equates to a picture grade of about 1.5 which represents a future digital C/N margin of between 3 and 4 dB. This margin is lower than the just served analogue margin and barely sufficient to reliably offset the normal day-to-day fluctuations in received signal level. The teletext test is therefore a useful guide for consumers in that failure means they do need to seek professional advice."