This invention relates to an FM stereophonic receiver having an ARC (automatic reception control) function.
FIG. 7 is a block diagram showing the arrangement of essential parts of a conventional FM stereophonic receiver.
In FIG. 7, reference numeral 1 designates an FM detector circuit which detects an intermediate frequency signal IF applied thereto, to output a main signal S.sub.M and a sub signal S.sub.S.
Reference numeral 2 designates a high cut circuit which operates as follows: The high cut circuit 2 controls the frequency characteristic of the main signal S.sub.M outputted by the FM detector circuit 1 in accordance with a high cut control signal S.sub.H provided by a high cut control circuit 8. The high cut control circuit 8 outputs control signal S.sub.H according to a received signal field strength level. The high cut circuit 2 outputs a high cut signal S.sub.H1 according to the main signal S.sub.M and the control signal S.sub.H.
Reference numeral 3 designates a variable attenuation circuit. The variable attenuation circuit 3 attenuates the sub signal S.sub.S outputted by the FM detector circuit 1 in accordance with a separation control signal S.sub.SP provided by a separation control circuit 3A. The separation circuit 3A outputs control signal S.sub.SP according to a received signal field strength level. The variable attenuation circuit 3 provides an attenuation signal S.sub.A in accordance with the sub signal S.sub.S and the control signal S.sub.SP.
Reference numeral 4 designates a stereophonic demodulation circuit which operates as follows: The stereophonic demodulation circuit 4 separates right and left channel signals S.sub.R and S.sub.LA according to the high cut signal S.sub.H1 outputted by the high cut circuit 2 and the attenuation signal S.sub.A outputted by the variable attenuation circuit 3, and outputs the right and left channels signals thus separated.
The FM stereophonic receiver thus organized operates as follows:
The separation control circuit 3A outputs the separation control signal S.sub.SP according to the received signal field strength level in such a manner that, when the received signal field strength level is high, the amount of attenuation of the variable attenuation circuit 3 is decreased, whereas when the received signal field strength level is low, the amount of attenuation is increased. Thus, the amount of attenuation of the variable attenuation circuit 3 continuously changes in accordance with the received signal field strength level.
Therefore, the sub signal S.sub.S outputted by the FM detector circuit 1 is attenuated according to the received signal field strength level, and the right and left channel signals outputted by the stereophonic demodulation circuit 4 is improved in S/N ratio.
The separation control is effected when the received signal field strength level is within the range of, for instance, from 45 dB/.mu.V to 25 dB/.mu.V, whereas the frequency control is carried out when the received signal field strength level is less than 35 dB/.mu.V.
The conventional FM stereophonic receiver designed as described above suffers from the following difficulty: Since the sub signal S.sub.S is attenuated according to the received signal field strength, when the received signal field strength level becomes relatively low, the sub signal S.sub.S is completely attenuated, with the result that the stereophonic broadcast turns into a monaural broadcast.