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M S Duhan
DDG (E)
Doordarshan

Radio and TV Signals Silhouette of RF Waves

Each and every TV viewer is different, behaves differently, and has his/her own preferences. Majority of viewers watch the same genre type regardless of the station/channel they are aired on. The programme choice of viewers depends upon content and not upon the scheduling. A cricket fan will watch a cricket match and a music lover will watch the music channel, irrespective of the station airing the same or the platform, for example radio, TV, terrestrial, DTH etc. Digitalization is expected to increase focus on quality of content and provide greater choice for viewers.

Growth of the broadcasting sector stimulates competition amongst the delivery platforms for more and more channels for diversity of content requirement, but quality of signal reception warrants for the minimum signal levels, the real task for broadcasters during system design for each platform is to know the silhouette of RF waves for targeted areas. Signal requirements for some prevailing platforms of radio and TV are discussed.

FM Radio

For frequency modulation, in the presence of interference from industrial and domestic equipment a satisfactory service requires a median field strength (measured at 10 m above ground level) not lower than the values are:

Analog Terrestrial TV

For an analogue terrestrial television service the recommendations of ITU-R BT.417-5 for the minimum signal strengths for which protection may be sought in planning is:

These values refer to the field strength at a height of 10 m above ground level. For other channels in bands IV and V, for systems using 8 MHz channel raster, the minimum field strength value should be derived as follows: Emin (dBµV/m) = 62 + 20 log (f/474), with f being the channel center frequency expressed in MHz.

Digital Platforms

The greatest advantage of the digital system for radio and TV is the effective use of the frequency spectrum and its lower radiated power in comparison with the analogue transmission, while the covered area remains the same.

In the digital broadcasting, we have to cope with a digital cliff effect. This means a sudden jump from a normal reception to a complete lack of content. Operation on the boundary is the reason that the picture appears to freeze up or show up like a mosaic. Typical values for DVB-T2 signals for various modulation parameters are as below:

Modulation error ratio (MER) and bit error rate (BER) are other important parameters in digital links. MER affects the ability of a digital receiver to recover data bits and is a useful metric which helps to gauge the end-to-end health of a network. If a constellation diagram is used to plot the landing points of a given symbol over time, the resulting display forms a small cloud of symbol landing points rather than a single point. The peak minimum MER is important, not mean MER. BER is the other parameter and is calculated from the number of bits received in error divided by the number of bits transmitted. BER is related to the distance between constellation points.

Minimum receiver MER for digital TV networks for stationary outdoor reception is as below:

  • DVB-T with 64QAM and code rate 2/3 > 18 dB
  • DVB-T with 16QAM and code rate 2/3 > 12 dB
  • DAB+ networks for mobile and stationary indoor reception: > 8 dB
  • DVB-T2: MER 256 QAM 2/3 FEC > 25 dB
  • DVB-T2: MER 256 QAM 3/4 FEC> 26.5 dB
  • DVB-T2: MER 256 QAM 5/6 FEC> 28.5 dB
  • DVB-S2: MER 8PSK, ¾ FEC >12 dB

The noise-limited sensitivity of receivers drives key considerations in designing the network and manufacturing of the receiving devices. So in order to ensure perfect system working and quality of service, minimum signal levels are essentials which need to be protected against interfering noises.

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