An IC includes an externally accessible terminal, a filter, and an output stage. The filter has an adjustable bandwidth and has an output node coupled to the externally accessible terminal, and the output stage has an input node coupled to the output node of the filter. By providing access to a node that is between the filter and an amplitude-limiting output stage, the IC allows one to adjust the bandwidth of the filter--or the bandwidth of a circuit, such as an amplifier, that includes the filter--by measuring the amplitude of a signal that can have a relatively high S/N ratio without being clipped by the output stage. Consequently, such access can eliminate the need for a high-precision bandwidth-adjust setup and its long bandwidth-adjust times.

The filter has a fixed Q and is coupled to a source of AC signals. A source of pulses is applied to a variable-voltage circuit, the circuit being responsive to each pulse to produce a threshold voltage which varies from a first value toward a second value. A comparator circuit has one input coupled to the filter and a second input coupled to the variable voltage circuit. The comparator produces a signal for a duration related to the time the amplitude of an AC signal from the filter is at least equal to the threshold voltage.

The threshold level for demodulating an FM signal, for instance a television picture signal, has been improved or lowered by adaptively restricting the bandwidth of the signal using an adaptive bandpass filter, and therefore, the picture quality and/or S/N ratio of the demodulated television signal has been improved. The adaptive bandpass filter has enough bandwidth when the signal level is sufficiently higher than the threshold level, but is narrow when the signal level is close to or lower than the threshold level. The bandwidth of the adaptive bandpass filter is adjusted according to the total power of the input FM signal and C/N (the carrier signal to noise) ratio. The improvement of the threshold level is almost 2 dB.

An infrared remote control signal is entered into a receiving circuit where it is converted into an electrical, modulated signal made up of a carrier and a command signal. The modulated input signal is sent to a control signal output circuit where it is filtered to send only the carrier of the remote input signal to a frequency-voltage conversion circuit. The frequency-voltage conversion circuit generates a control DC voltage according to the carrier frequency. The control DC voltage changes the pass band characteristic of a voltage-controlled filter in such a manner that the voltage-controlled filter selectively passes the desired fundamental frequency component and harmonic components of the carrier signal. The voltage-controlled filter also passes the command signal component. Thus, at the output of the voltage-controlled filter is obtained a modulated output signal whose carrier frequency is varied from that of the modulated input signal. With the carrier frequency changed, the resulting infrared remote control signal sent out from a transmitting circuit can be prevented from being affected or interfered with by external noise from fluorescent lamps.

The demodulator combines a conventional frequency discriminator with a bandpass filter for removing the noise which is present from the band occupied at a given instant by the modulated signal. The center frequency of the bandpass filter is controlled in dependence on the instantaneous frequency of the modulated signal and its bandwidth b.sub.u is controlled in dependence on the instantaneous width of the band occupied by the modulated signal.