| 摘要 |
PURPOSE:To reduce code regeneration erros when a reception level is lower than a specific level by decreasing a threshold value in case of the increase in noise level with the threshold value control signal of a threshold value control circuit which generates the threshold value control signal corresponding to the noise level. CONSTITUTION:When the reception level drops below the specific level, the substantial level of a demodulated signal S starts decreasing although an S+N level is held constant, and the noise level N increases relatively, so that a multilevel decision function does not operate normally. For the purpose, the level N of the output of a frequency discriminating circuit 12 is monitored and detected by a noise level detecting circuit 8 because the S+N level of the demodulation output of the frequency discriminating circuit 12 is held constant to the reception level by the amplitude limiting function of an intermediate frequency amplifying circuit 4, and the threshold value control circuit 9 corrects the threshold value of a threshold value varying circuit 10 according to the level N to maintain the multilevel decision making of a multilevel deciding circuit 6. |
