| 摘要 |
A tuner includes circuitry for conditioning an audio signal including (i) a pilot signal canceller, (ii) a noise limiter, (iii) a notch filter, and (iv) a de-emphasizer; as well as an attenuator for RF signals. In an exemplary embodiment, an AM stereo pilot signal canceller utilizes a 25 Hz signal which is added or subtracted from the left and right channels to eliminate the pilot signal. The noise limiter includes a capacitor which is switched into the feedback circuit of an operational amplifier when the input is at a selected level. The diverting of the feedback signal to the capacitor causes the amplifier to saturate thereby removing the noise from the signal. The notch filter includes first and second band pass filters which are centered at a selected frequency, e.g. 10 kHz, for removing a 10 kHz beat note. The passed band is phase shifted so that it is 180 degrees out of phase with the original signal and then added back with the original signal to remove the selected band. The de-emphasizer performs a shelving function to attenuate an audio signal which has been compressed prior to broadcast transmission. The RF attenuator includes a light dependent resistor which forms a part of a voltage divider and which is remotely switched at the tuner to eliminate feeding the RF signal to a front panel switch.
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