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

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SFN (Single Frequency Network) THE SUM OF THE SIGNALS BETWEEN TWO TX SITES In the darker areas you have the phase sum: 6 dB increase Along these lines you can see the delay in micro-seconds (50 µS each line) µS µS TX1 TX2 40
This picture shows an example of an SFN network on a perfectly flat area, with no natural obstacles, and where there are two SFN transmitters. When exploring an area, you will find locations where signals are added and others where they are detracted, due to phase variations and depending on the distance between the transmitters. In areas, where the received fields from two sites are identical or very similar, variations are very strong. There are points where the signal goes to zero and points where it increases 6 dB, at a distance of about half a wavelength, because at every half wavelength there is a phase invertion: Wavelength λ = 300/frequency in MHz When you move away from a TX site, its signals become weaker in comparison to the other TX, making phase shift less strong and with a flatter spectrum, because the phase shifts depend on the frequency and distance. Single Frequency Networks work only while delays are kept within the guard interval. The diagram shows an area of about 100 km, where delay in µS is constant along the curves: this delay is shown for each curve. We must ensure that in the area, where the delay exceeds the guard interval, you receive signals from only one TX, using antennas with a good back to front antenna ratio, as already seen and shielding it from the other TX. 41
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 and 32: The reflections caused by a mismatc
Page 33 and 34: Since the system is immune to refle
Page 35 and 36: Operation is based on being able to
Page 37 and 38: Nearly every instrument has an SFN
Page 39: Here are a number of rules to impro
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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