The invention relates to a device for demodulating a frequency-modulated signal, having an input terminal for receiving the frequency-modulated signal and an output terminal for supplying a demodulated signal, a main signal path being provided between the input terminal and the output terminal, the device comprising:
a demodulator circuit, for demodulating the frequency-modulated signal, having an input coupled to the input terminal, and an output, PA1 a low-pass filter comprising a capacitive element; and PA1 a hold circuit having signal input, a control-signal input, and an output which is coupled to the output terminal, the hold circuit at least comprising first controllable switching means, having an input and an output respectively coupled to the input and the output of the hold circuit, and a control input coupled to the control signal input of the hold circuit, and a first capacitor coupled between the output of the hold circuit and a first point of constant potential, the hold circuit being constructed to at least substantially hold the signal applied to its input under the influence of a control signal applied to the control signal input, and the first switching means being constructed to interrupt the internal connection between its input and output under the influence of said control signal for a first tine interval T.sub.1. Such a device may be incorporated in, for example, a video recorder for demodulating, after it has been read from a record carrier, a frequency-modulated audio signal recorded in the tracks on the record carrier, the device for this purpose further comprising two or more read heads arranged on a rotatable head drum, switching means having two or more inputs, an output and a control input, the switching means being constructed to couple one of the inputs to the output under the influence of a control signal to be applied to the control input, an output of each of the two or more read heads being coupled to an associated input of the two or more inputs of the switching means, and the output of the switching means being coupled to the input of the demodulator circuit.
The device is intended for precluding momentary disturbances in the input signal of the demodulator circuit. Momentary disturbances are to be understood to mean those disturbances which affect the regularity in said signal. Examples of these are drop-outs or drop-ins in the input signal of the demodulator circuit or phase shifts in this input signal which arise a result of switching over from reading the record carrier with one read head to reading with another read head (referred to as the "head-switch" operation). These momentary disturbances result in the operation of the demodulator circuit being disturbed, causing (audible) disturbances in the signal demodulated by the demodulator circuit.
The low-pass filter which is arranged directly after the demodulator circuit in the known device serves to remove spurious signal components produced in the demodulator circuit, such as intermodulation products, residual carrier wave components and noise above the audio band. A typical value for the cut-off frequency of such a filter is, for example, 100 to 150 kHz. The disadvantage of this low-pass filter is that the disturbances on the output of the demodulator circuit are expanded in time. For example, disturbances as a result of a head-switch operation, which may have lengths of approximately 4.5 .mu.s, are expanded to lengths of approximately 8 to 10 .mu.s by a low-pass filter having a cut-off frequency of the order of magnitude of 100 kHz. This means that the first switching means are (must be) open during the time interval T.sub.1, which is approximately equal to said 8 to 10 .mu.s. The purely periodic character of the actuation of the hold circuit (opening of the switching means) at the head-switching frequency in conjunction with the substantial length of the time interval T.sub.1 give rise to a very annoying contribution in the output signal of the device.
USPTO Backgrounds
USPTO Backgrounds