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When the instrument is in NICAM mode and the FM modulation is switched off (for example, by selecting the external modulation or by switching off the FM carrier), the RSSF bit is automatically set to LOW. The TEST key selects three special data sets. The TEST key has a stepping function and by repeatedly pressing the key, the signals DATA 1 - DATA 2 - DATA 3 are switched on. For DATA 3, both LEDs next the key are lit. The data sets do not comprise real NICAM data; they contain bit patterns to e