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rd technicalsolutions
Digital Doctor takes an in-depth look at various technical issues within the field of Digital TV installation. A well-respected expert on the subject, Peter Chester is Senior Digital TV NVQ Assessor/Verifier at Matthew Boulton College. In the first series, Peter looks at how to determine which measurement values are the most important when installing a Digital terrestrial aerial system
Part 1: The Digital Cliff
To have an appreciation of the importance of signal measurement a review of some basic digital transmission theory is necessary. A traditional data transmission system transmits the binary data bits serially (one after the other) on a single radio frequency carrier. To guard against interference an error correction method is usually applied in an attempt to ensure that the data can be reconstructed perfectly if corrupted by noise or interference. If the interference is excessive, then the data stream can slow down, become unreliable even with error correction or may stop altogether in extreme cases.
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digitaldoctor Broadcast signals are prone to interference, particularly due to multipath reflections which cause ghosting on analogue pictures and data corruption of digital signals. They are also limited to the 8MHz PAL I bandwidth with a further complication being that most UHF channels are already in use. The advantage of digital signals is that they can be broadcast at much lower power (typically 1/1000th) than analogue and can therefore use channels that would otherwise not be available.
Coded Orthogonal Frequency Division Multiplex – COFDM The chosen system for Digital Terrestrial TV (DTT) broadcasting is a very robust modulation method called “Coded Orthogonal Frequency Division Multiplex”, or COFDM. COFDM is different because it transmits the data in parallel across a very large number of closely spaced radio frequency carriers (typically 1705 in the UK soon to become 8K once switchover is completed across the country), rather than as serial data – see fig 1. This means that if noise or interference corrupts several of the carriers, the data should still get through on the remaining interference free carriers. An error correction method is applied to the data before transmission that enables any lost data to be recreated at the receiver from the data that does get through. This is found to be extremely effective in practice and most errors can be corrected up to a certain point. Because each carrier contains very low data rates, it is much
more immune to the effects of multipath interference and can actually use echoes to add to the energy of the direct transmission path. In addition, the energy is more evenly spread over the channel bandwidth and the spectrum can be modified to notch out those carriers that could cause interference to other services. In practice, several programmes can be accommodated in each 8MHz PAL I channel, the same frequency being used many times over throughout the UK without causing co-channel interference because it is broadcast at lower power. Such interference will occur only if the two “co-channel” power levels are less than 20dB apart at a particular location.
8MHz
1705 carriers
fig 1 DTT uses an error correction system called Forward Error Correction (FEC). FEC allows the digital MPEG-2 signal multiplex to be received correctly even when moderate levels of interference and noise are present. The combination of COFDM and FEC is very powerful and produces a system that is very resistant to the type of interference that usually ruins analogue reception. Effects