With the growth in TV channel numbers in the early 1970’s, channelised amplifiers for MATV head-ends first appeared. With the current massive expansion in TV channel availability, professional installers are once again looking at the concept and technology of the Single Channel Filter (SCF).

Effectively, each SCF consists of a broadband amplifier with pre-tuned input and output filters, allowing only the selected channel through. Filters are frequently factory-aligned, making them less flexible than a broadband amplifier. Despite this, the advantages can outweigh the disadvantages, even where signal levels of only four or five TV channels need equalising before launching into the network.

Additional Outlets
Signal equalisation not only benefits picture quality but also maximises the number of outlet points that can be provided across a network. Signals differing by as little 4dB could mean the difference between one or two trunk lines. This single trunk line may mean an extra 100 outlets! System growth can be accommodated by providing a separate filter amplifier module for each additional channel that is required.

Broadband Limitations
The TV and radio spectrum is becoming very full of transmissions, which means choosing and equalising individual signals is of growing importance. On larger networks with thirty or more outlets this is even more of an issue, as a broadband amplifier will amplify every inputted signal.

This means that an amplifier, rated for two channels, has to be de-rated to cope with more channels. Typically, this would be
-3dB every time the number of channels is doubled. A catalogue rating of 120dBµV for two channels at DIN 45004B would be reduced to maximum output of 117dBµV for four channels, and 114dBµV when for 8 channels.

De-rating can often be overlooked at design. At the writer’s home, signals from the Oxford, Sutton Coldfield, Sandy Heath and two fill-in transmitters can be received. A traditional broadband amplifier would have to be de-rated to accommodate twenty-one TV channels, leaving only 110dBµV of maximum output for launch afterwards.

New Technology
Application of new technology was needed to overcome these issues. This was the Single Channel Filter, and it brought two unique benefits. First, and since each amplifier only passes one channel, no de-rating is needed and maximum specified output is obtained. Secondly, the system is very clean after filtering because each amplifier rejects unwanted channels. (See Figure 1)

DTT Challenges
Added to the already full spectrum is now Digital Terrestrial Television, putting even greater requirements on the professional installer, particularly since DTT needs a very dependable Carrier/Noise ratio. Poor C/N ratio often means a Bit Error Ratio inadequate to support the data-stream, resulting in unreliable pictures or sound. Removing these unwanted signals- known as noise- can significantly improve the C/N ratio. A typical system improvement between using broadband and a SCF would be C/N ratios moving from 20dB to 40dB.

In an analogue system, high levels of interfering signals can cause a nuisance by patterning the picture. In DTT systems, this can cause complete loss of picture and sound. In many UK locations, private mobile transmissions can be experienced obliterating otherwise acceptable signals. Recently, a 460MHz signal was measured at 120dBìV from an antenna, overloading completely a broadband amplifier. A Single Channel Filter removed the problem by blocking this interfering signal. Typically, transmitters now have five analogue and six DTT channels. Although DTT signal strength has now been increased, it is still as much as -24dB lower than Analogue. Add to this normal signal propagation differences, and DTT transmissions are often too weak in comparison to strong analogue transmissions.

Current DTT Receivers are limited to maximum 80dBµV analogue inputs and 40dBµV minimum Digital inputs. However, this 40dBµV threshold refers to the Receiver only, with 50-55dBµV specified as the ideal level for a complete DTT system, leaving an input “window” of only 25-30dBµV. Less than this and it becomes very difficult to provide appropriate signal levels at every outlet.

Installing Single Channel Filters
Head-end Single Channel Filters allow the professional installer to increase gain on weaker digital channels, narrow the gap between DTT and analogue channels, and equalise digital and analogue levels. Recommendations are that DTT channels are set about 15dB lower than analogue levels to give a safety margin, as problems can occur if adjacent DTT and analogue channels are too close. Set at only 10dB apart there is a risk of the DTT signal distorting the analogue audio carrier. Many professional installers have found that Single Channel Filters are well suited to combining signals from different sources. Because each SCF selects only one channel, in a communal system like a block of flats, a fill-in station for analogue signals can be combined with DTT signals from the main transmitter. This means no TV set re-tuning when DTT is added as an upgrade! And unlike main transmitter analogue signals, which can ghost and be unusable, DTT signals are largely immune to ghosting and can be combined with other signals.
Modern Advances

With so many adjacent channels in use today, a Single Channel Filter must be selective enough to sharply pick out a single channel. Early models, designed for analogue TV, rarely saw this an issue, but one used in the era of DTT would mean that changing the gain on one Single Channel Filter would impact on the level of signal in an adjacent channel. (See Figure 2). Older models traded-off noise against Filter performance, using at most 2 cavities. More cavities meant a more selective Filter, but more noise. With modern transistor technology, Single Channel Filters commonly feature 8 or 9 cavities, but still generate very little noise.

Technical Choices
Most suppliers will specify and supply a Single Channel Filter suited to the standard CCIR channel width of 7.61MHz. Make sure that you are offered filters that are tighter than this, so that the filter mask following the sub-carrier pattern maximises channel separation and therefore selectivity.

Quality filters are aligned differently for analogue and DTT use. On adjacent channels, two standard CCIR filters provide only 18dB of selectivity. This is only a 3dB window of equalisation before one Single Channel Filter begins to alter the adjacent carrier. The best DTT and analogue filters have 27dB of selectivity providing 12dB of adjustment before starting to affect the adjacent channel.

Any SCF made outside the UK will normally be aligned for PAL-G. For UK use, special alignment is required since this setting will cause losses on the PAL-I signals used in domestic market, typically the NICAM sub-carrier or Teletext information.

The Case For SCF
Single Channel Filters are now very viable options for communal systems, offering significant benefits to the professional installer. Filter design, quality and alignment are paramount in the choice of which particular Single Channel Filter to choose. Increasing the attraction of SCF systems are fallings costs and better features, with variable gain now available at reasonable prices. A complete eleven channel digital and analogue head-end, driving thirty-five points, can work out at less than £10 per point.

Single Channel Filters are now capable of providing the best features, the best choice and the best value.