Topic: Telecommunication Engineering
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Broadcast Engineer's Handbook: Useful reference data for TV broadcasting engineers
The required protection ratio varies considerably depending on the frequency relationship between the wanted and the unwanted carriers and their frequency tolerance. The greatest protection is required when the frequency of one or both carriers is non-controlled.
Less interference is possible and therefore lower protection ratios are required for non precision offset (line frequency offset). Non-precision offset takes advantage of the line frequency structure of the video signal and, in particular, it is advantageous to offset the carriers by multiples of one-half or one-third of the line frequency. The long-term stability of these favourable protection ratios can only be guaranteed, however, if the frequencies of the wanted and unwanted signals are kept within ±500Hz.
Precision offset takes further advantage of the field frequency structure of the video spectrum. The least protection is required when both carriers are precision offset controlled within a tolerance of ±1 Hz for the wanted and unwanted carriers. In the following figure is shown the main characteristic of offset operation which plots in schematic form the protection ratio curves between 0/12 f line and 12/12 f line . These curves are cyclic and their extensions to the left and the right are symbolized by broken lines. These various conditions illustrated are similar within the luminance range up to about ±3 Mhz.
The upper and lower curves indicate, respectively, the protection ratio obtained with non-precision and precision offset. More precisely, these two curves trace the envelope of a series of fluctuations in the protection ratio which swings between the two curves at field frequency as represented by the thin line.
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