Operating Margins

Along with the level margins a further equally important quality margin is required to provided a reliable digital services. By the measurement of Post – Viterbi BER, the quality margin can be judged. To provide the level of this margin in relation with BER, knowledge of a parameter called MER is helpful. MER or Modulation Error Ratio is a single figure of merit for the in band quality that includes the noise, inter-modulation and other corruptions of the signal. MER is a very near equivalent of in band S/N. At the cliff edge or at the threshold C/N point, the MER equals this threshold value, being approximately 20dB for modulation of 64QAM with a FEC of 2/3 or 3/4. The Post – Viterbi BER at this level will be approximately 2E-4, with a consequential margin at the threshold of 0dB. With, for example, a BER of 2E-6, the MER maybe around 22dB, results in a 2dB quality margin. The relationship between BER and MER changes with the complexity of the signal, hence Fig. 3 can only be taken as a guide.

Figure 3: Signal Quality

Carrier to Noise Ratio (C/N)

The Carrier to Noise Ratio is defined as the ratio between the vision carrier signal level and the RMS noise level.
The carrier to noise ratio for any analogue channel shall not be less than 45dB at any point in the system.
The carrier to noise ratio or SNR (signal to noise) for any digital channel shall not be less than 32dB at any point in the system.

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