UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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SFN (Single Frequency Network) Urban area of coverage Extra-urban area of coverage 32 The reflected signal is delayed, but absolutely synchronous The DVB–T system may tolerate delayed reflections up to 224 µSeconds, according to the modulation parameters It is possible to run a network of synchronous transmitters all with the same frequency
Since the system is immune to reflections (within the limits), a network of completely synchronous transmitters can be put into action and already operating in some countries like Spain and Italy. All you have to do is transmit the same bit at the same time and at the same frequency. Achieving this goal is very complicated and expensive; completely synchronous digital distribution networks are required, many transposers will need to be converted into transmitters. Finally an accurate frequency reference at all sites must be guaranteed. It is sufficient to say, for example, that the required accuracy is about 1 Hz for the frequency and few nanoseconds for the timing. The following points clarify the topic: 1. A reflected signal, is still tolerable however far it is, but it must be at least 20 dB below the level of the desired signal, and does not reduce the margin too much (basically it is like any kind of noise). 2. If, however, the delay is below a certain limit, which depends on the transmission parameters, you could even tolerate a reflected signal at the same level as the one received. 3. It is a good idea to keep it about 6 dB lower than required, to make sure that it is safe from random deterioration and variations. 33
Page 1 and 2: UNDERSTANDING DIGITAL TV • Radio
Page 3 and 4: SUMMARY 6 Radio frequency spectrum
Page 5 and 6: The difference between the two sign
Page 7 and 8: The function that allows you to fin
Page 9 and 10: The power of the digital signal is
Page 11 and 12: The modulation is the same type for
Page 13 and 14: We know that signals can be “poll
Page 15 and 16: The DVB-T signal has this behavious
Page 17 and 18: Here you can see a schematic block
Page 19 and 20: When it is said that a DVB-T system
Page 21 and 22: Let us assume that the electromagne
Page 23 and 24: We will illustrate a laborious, but
Page 25 and 26: This chart is useful, even if it do
Page 27 and 28: Another situation where the antenna
Page 29 and 30: The back to front antenna ratio is
Page 31: The reflections caused by a mismatc
Page 35 and 36: Operation is based on being able to
Page 37 and 38: Nearly every instrument has an SFN
Page 39 and 40: Here are a number of rules to impro
Page 41 and 42: This picture shows an example of an
Page 43 and 44: As an example, we have superimposed
Page 45 and 46: The diagram clearly shows the trans
Page 47 and 48: To obtain minimum reception stabili
Page 49 and 50: These predictions are verified when
Page 51 and 52: The electromagnetic field is measur
Page 53 and 54: Even if the MER is a good indicator
Page 55 and 56: The diagram shows once again the th
Page 57 and 58: This table is calculated for system
Page 59 and 60: Micro-interruptions are not always
Page 61 and 62: So ripple (waves) in the spectrum p
Page 63 and 64: In Figure 3 and 4 you can see the s
Page 65 and 66: Figure 5 shows DVB-T signal ch.8 th
Page 67 and 68: Using this case study you can see h
Page 69 and 70: Directive 2004/108/EC requires that
Page 71 and 72: The diagram shows the real case of
Page 73 and 74: MPEG4 REAL TIME SPECTRUM 6-HOUR BA
Page 75 and 76: mod. HD COMPACT STC THE MOST ADVANC

UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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