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What you should know about FREEVIEW and Digital Terrestrial Television (DTT)
Gareth Pont, Design Director, Connevans Limited.
DTT (Digital Terrestrial Television) is very different to ATV (Analogue Television). Currently, approximately 75% of the UK can receive DTT and this is unlikely to improve until about 2011 when the government intends to 'switch off' the ATV service. The transmission power of the DTT service may then be increased substantially. The reason that DTT coverage is poor is that the transmitted DTT power is limited to typically about 2% of that of ATV so that DTT does not interfere with ATV (where the effects of any interference are much more visible). Therefore the range from a DTT transmitter at which an acceptable service may be received is limited. Additionally, in many areas, the presence of interfering ATV transmissions will prevent DTT transmissions from being decoded in a DTT receiver.
Persons wishing to find out whether they are likely to be able to receive DTT (known as FREEVIEW in the UK) at their postcode should check on this website BEFORE purchasing a Freeview receiver or Integrated Digital Television. If the Freeview website suggests that DTT is not available at a particular postcode then the usual alternative options are to receive digital TV via cable or satellite. However it may still be possible to receive a partial DTT service in some areas (see below).
More Detailed Technical Information on DTT
The DTT service transmits on the same set of radio frequencies (channels) as the ATV service but the channels used by the two services are not the same in any one area. On the DTT service typically 6 to 12 TV and radio channels (programmes) are coded together to produce a “Multiplex” which is then transmitted on one ATV channel. In the UK there are 6 multiplexes and, just like the ATV service, each multiplex is transmitted on a different ATV channel from transmitters in different areas. The programmes on each multiplex are listed on this web page. “LCN” stands for the “Logical Channel Number” that is given to each programme.
Most of the programmes carried on the DTT service are free to watch (i.e. no viewing card required) and are collectively known as “FREEVIEW” programmes. As the transmitted power is low for the DTT service, it is always wise to check whether the predicted reception is likely to be good enough it the location where the receiver will be used in order to avoid disappointment and a wasted purchase of receiving equipment.
DTT receivers behave very differently to traditional ATV receivers when the received signal strength is poor. With ATV, the picture just gets noisy and “snowy” but may still be watchable. However, with DTT receivers the picture changes rapidly from good through blocky/breaking up/freezing to blank screen as the received signal level passes through the receiver “threshold”.
The Freeview website will give a conservative (safe) estimate of whether the full Freeview service can be received at any particular postcode. However, the site points out that an aerial upgrade may be required for satisfactory reception.
As with most things, the successful reception of DTT is not quite as clear cut as the Freeview website suggests. The Freeview website works on the basis of being able to receive all 6 multiplexes from one ‘local’ transmitter at a particular postcode. In many cases it is actually possible to receive some or all of the DTT multiplexes at postcodes where the Freeview site suggests otherwise, but not necessarily all from one transmitter. However, in these areas (those that are some distance from a transmitter or shielded by local terrain) it is almost certain that an aerial upgrade to a very high standard will be required. Typically this upgrade will be to a very high gain, directional antenna with a high gain low noise, high output amplifier and an interference filter. It may even be that the DTT aerial may need to point to a different transmitter to where the local ATV transmissions come from. The DTG website will give an estimate of which Freeview multiplexes may be available at a particular postcode and from which transmitter. The further the user lives from the transmitter, the less likely it is that a satisfactory DTT service will be possible and the more likely that, if a DTT service is possible, there will be occasional interference or temporary loss of one or more multiplexes. This means that some programmes may be OK (e.g. those on the BBC Multiplex 1, but perhaps not those on others e.g. the NG Transco Multiplex C).
Two things are required for successful decoding of DTT signals:
1) received signal strength of at least 46dBuV (for each useable multiplex)
2) carrier to noise + interference ratio of at least 26dB (or “quality” of 60%)
This website will give transmitter (name, transmitted power per multiplex, distance, compass bearing) and predicted reception information (aerial group, polarization, field strength) for a particular postcode. The entry in the column “Field dBuV/m” should be a minimum of 36 (assuming the use of the high gain aerial/amplifier/filter combination referred to earlier). This site does not take into account obstacles in the terrain between the transmitter and the receiving aerial such as buildings and trees which can make a significant difference to reception when the received signal levels are near the receiving threshold of the particular Freeview receiver proposed. Note that different makes of Freeview receiver have different sensitivities so some receivers may perform better than others at postcodes where the received signal level is low. In areas where the fitting of a high performance aerial system is necessary in order to receive Freeview, then the strength and quality of the received signal from each multiplex at the required postcode must be checked. A professional CAI aerial installer (see below) will check both strength and quality, but they can also be checked using a Freeview receiver. It is not enough just to have good signal ‘strength’ because this does not take account of any ATV or other interfering signals that might be present. The ‘quality’ indicator shows how well the DTT signal can be decoded and is a measure of the “carrier to noise + interference ratio” mentioned earlier. Typically, a quality value of 60% is required for decoding to take place and 80-100% for long term satisfactory reception. In low signal areas it will be common for the quality indicator to fluctuate as the DTT signal passes through moving trees (especially when wet) and as other interference is received.
If an aerial system upgrade is required (and this is a very common requirement over much of the UK) then the services of a CAI member should be used in order to make sure that the survey and installation is of satisfactory quality.
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