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

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SFN (Single Frequency Network) REDUCING ECHOES & MAX distance • The signals travel at the speed of light • They go at 300.000 km per second, 300 metres in a microsecond • <strong>Ro</strong>ughly 3.3 microseconds per kilometer • Map in hand, you know immediately if you are receiving a TX in or out of the guard interval distance 38
Here are a number of rules to improve management of SFN reception: 1. It is inevitable that when covering vast territory some areas will be in the shade; with digital DVB-T is more a problem of coexistence of SFN signals. 2. Even when the arrival times of signals are contained within the guard interval, it is dangerous to operate systems with SFN signals at the same level. To obtain an adequate MER and BER margin echoes should be at least 10 dB below the main signal. 3. There could, in theory, be a situation in which two antenna signals arrive at the same level and with the same delay, resulting in no pictures or what is received is extremely unstable. 4. Use directional antennas and take a close look at the SFN ECHOES screen, to check the best position of the antenna (for example, try to shield in the direction of unwanted incoming ECHOES). 5. Be very careful to combine 2 or more antennas: an antenna pointed in one direction will inevitably pick up signals from another direction, causing, unwanted reflections (ECHOES). 6. Contributions outside the guard interval are considered interferent and must be reduced by at least 25 dB. IMPORTANT: Orientation of the receiving antenna must be optimized, to look mainly where you can make one signal stronger than the others and reduce the others that fall outside the guard interval. It is worth remembering that it is better to reduce the ECHOES, especially those outside the GI, rather than just searching for the maximum power of the signal received. 39
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: Nearly every instrument has an SFN
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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