The invention relates to a digitally generated signal used in a stereo receiver for stereo decoding or for pilot tone suppression. The invention further relates to an integrable circuit array for generation of such a signal.
In integrated stereo receivers, digitally generated signals are required in many cases, for example as switching pulses for the stereo decoder or for pilot tone suppression. These digital signals are generally generated by oscillators and dual frequency divider stages in phase-locked loops that ensure a certain phase position of the generated rectangular signals in relation to the 19 kHz pilot tone of the tuned stereo transmitter. Rectangular oscillations have however the drawback that their frequency spectrum still contains in addition to the basic oscillation all the uneven harmonics. It is this harmonic content in particular that contributes, as a result of interference with signals in adjacent channels or with carrier signals in the sideband, to faults in the stereo reception that can be perceived in the audible range as chirping. The harmonics of the 38 kHz channel switching signal for example thus lead to interference with the adjacent channels, while the 19 kHz signal for pilot tone suppression mainly leads to interference with signals in the sideband, such as the RDS carrier.
A method is known from U.S. Pat. No. 3,962,551 for generation of a multi-stage switching signal used in stereo decoders. This switching signal is generated by the addition of two 60.degree.-phase-shifted 38 kHz rectangular oscillations, with the third harmonics of the individual oscillations cancelling each other out. The 38 kHz signal sum obtained is free of the third harmonic, but continues to contain uneven higher-order harmonics. The circuit array also provided in the above patent for generation of an oscillation of this type comprises an oscillator, a dual frequency divider stage, a shift register connected as a Johnson counter, and a large number of logic gates, and is hence very expensive to produce. In addition, the fifth harmonic in a digital switching signal generated in this manner is only reduced by 15 dB in relation to the basic oscillation, which still leads to considerable interference.