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ANTENNA INSTALLATION GUIDE for Digital Terrestrial Television (DTT)
What is the Digital Cliff, or Threshold?
With analog television, as the signal strength is reduced, so the pictures steadily become noisier. With digital transmission, the quality of the pictures remains unaffected until quite suddenly the pictures fail altogether. This point is known as the Digital Cliff, or Threshold.
It’s clearly very important to make sure that a receiver is normally operating well above the Threshold, so that variations in signal level (due for example to the antenna moving in the wind, or due to foliage growing on trees between the antenna and the transmitter) don’t tip the signal over the Cliff.
What is the ‘operating window’ of a receiver?
Two main factors limit the range of signal levels that should be present at the input to a DTT receiver. The first is the minimum level of the digital signal, which must not fall below the threshold (typically 35-40dBµV, but in some cases higher). The second is the maximum level of signal that can be applied before the receiver overloads. The highest signal is usually an analog signal, and this should not exceed 77dBµV.
All digital AND analog signals at the input to a digital receiver must fall within this ‘operating window’.
I’ve heard that it is important to measure signal levels, even in simple domestic installations. Why is this?
Simply to ensure that all signal levels are within the operating window of the receiver. Signal levels that are too high can impair or prevent reception. Signal levels that are too low can sometimes go over the Digital Cliff. For a reliable installation, both of these conditions must be avoided. Generally, installers who check signal levels have far fewer return visits.
Can I use my old analog signal level meter to measure digital signals?
NO! If you do, there will be an error in the measurement. It is important to only use a meter that is designed for use with DTT signals.
Why do I need to limit the use of masthead amplifiers?
Use of masthead amplifiers with too much gain can close a receiver’s operating window to zero, making reception impossible. Masthead amplifiers should normally only be used where the feeder losses from the antenna to the receiver are unavoidably large, and the gain of the amplifier should at most be no more than 5dB greater than the feeder loss it has to overcome. Only a masthead amplifier with an interstage gain control can be used.
In areas of very poor signal strength a masthead amplifier can improve reception, but it is still important to avoid excessive gain – ‘minimum gain for maximum benefit’.
DO use an antenna that will give reception on both channels 11 AND 12 where those channels are in use for digital transmissions
DO use an antenna with optimised coupling of the VHF/UHF sections for maximum signal transfer and best multiplex flatness
DO use double screened coax for the antenna download, use screened outlet plates and use double screened coax for the fly lead
DO use good quality screened connectors, only HEX crimp 360° concentric or compression connectors
DO ensure that all cable preparation is in accordance with the connector manufacturer’s specifications
DO put the DTT receiver (STB) as the first component in the receiving equipment connected at RF
DO use a signal level meter that is suitable for both digital and analog signals, and measure the signal levels on all channels
DO use the S video or component video connections rather than the RF connections to achieve better video quality
DO prove that normal signal levels are at least 6dB above threshold by attenuating the signal and checking that the receiver still works on all channels
DON’T use twist on or soldered connectors
DON’T rely on that old antenna giving adequate reception on channels 11 12 under all conditions
DON’T use a masthead amplifier unless absolutely necessary
DON’T use excessive masthead amplifier gain – there should be just enough gain to compensate for the feeder loss
DON’T leave an installation with digital signal levels just above threshold
DON’T let any signal level applied to the DTT receiver exceed 77dBµV
DON’T measure digital signals with a meter that is only suitable for analog
DON’T forget to check the analog services
(7 September 2001)
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