DEVICE FOR DEMODULATING A FREQUENCYMODULATED SIGNAL

Abstract: In order to reduce disturbances in a device for demodulating a frequency-modulated signal, which disturbances are cuased by head-switching in a video recorder, the low-pass filter which is normally arranged between the demodulator (10) and the hold circuit (12) is transferred. The low-pass filter (23, 24), i.e. at least the capacitive element (24) of this filter, is arranged in the hold circuit (12), at least the capacitive element (24) of the low-pass filter (23, 24) being arranged in the main signal path after the switching means (18).

Description: 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, comprising a demodulator circuit, for demodulating the frequency-modulated signal, having an input coupled to the input terminal, and an output, a low-pass filter comprising a capacitive element, a hold circuit having signal input coupled to the output of the demodulator circuit, a control-signal input, and an output which is coupled to the outer terminal, which hold circuit at least comprises 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 their input and output under the influence of said control signal for a first time interval T₁. 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, which device for this purpose further comprises two or more read heads arranged on a rotatable head drum, switching means having two or more inputs, an output and a control input, which switching means are 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 length of approximately 4.5 µs, are expanded to a length of approximately 8 to 10 µs 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 T₁, which is approximately equal to said 8 to 10 µs. 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 T₁ give rise to a very annoying contribution in the output signal of the device. It is the object of the invention to provide a device which substantially reduces spurious components in the signal on the output terminal. To this end the device in accordance with the invention is characterized in that at least the capacitive element of the low-pass filter is arranged in the hold circuit and is arranged in the main signal path after the first controllable switching means, in that a first impedance is arranged in the connection from the input of the hold circuit to the terminal of the capacitor which is not coupled to the point of constant potential, and in that the low-pass filter at least comprises said first impedance and the capacitive element and the first capacitor forms part of the capacitive element. The invention is based on the recognition of the fact that by arranging the capacitive element of the low-pass filter in the hold circuit the hold time (time interval T₁) can be reduced substantially. By arranging the capacitive element of the low-pass filter in the main signal path after the first switching means and making the first capacitor form part of the capacitive element of the low-pass filter it is achieved that the disturbances cannot be expanded in time in the main signal path before the first switching means, enabling the hold time (the time interval T₁) to be reduced substantially. In the above example a hold time of 4 to 5 µs is obtained, which is substantially shorter than the hold time of approximately 8 to 10 µs necessary in the known device. The shorter hold time results in a substantial reduction of the disturbance in the output signal of the device. Moreover, the number of components of the demodulator circuit can be reduced substantially. The first impedance may be realised, for example, by the parasitic resistance of the first switching means, so that no separate impedance is needed. It is to be noted that European Patent Application EP-A-0 180,717 describes a circuit which also aims at reducing disturbances in the output signal as a result of head switching. However, the operation of the circuit is based on a different principle. Moreover, the capacitive element of the low-pass filter is not arranged in the hold circuit. The device may be characterized further in that the first impedance is coupled between the output of the first swithing means and the terminal of the capacitor which is not coupled to the point of constant potential, and in that a second capacitor is arranged between the output of the first switching means and the point of constant potential. This may lead to a further reduction of residual disturbances caused by the switching means being open for said shorter hold time. Arranging a second impedance in the connection from the input of the hold circuit to the terminal of the second capacitor which is not coupled to the point of constant potential provides a further reduction of high-frequency noise and intermodulation components caused by the demodulator. This second impedance may be constituted by, for example, the parasitic resistance of the first switching means, so that no separate impedance is needed. The device may be characterized further in that an auxiliary signal path is arranged in parallel with at least the first impedance, which auxiliary signal path comprises second controllable switching means which have an input and an output and which are constructed to provide an internal coupling from this input to this output for at least a second time interval T₂ following the first time interval T₁. This enables an even further reduction of disturbances to be obtained. The device may be characterized further in that the two ends of the first resistor are respectively coupled to the inverting input and the non-inverting input of a differential amplifier having an output coupled to the output of the hold circuit via a series arrangement of second switching means and a voltage-to-current converter, and in that the junction point between the second switching means and the voltage-to-current converter is coupled to the point of constant potential via a second capacitor. In this way the device in accordance with the invention is provided with an extrapolator (known per se ) by means of which the disturbance can be reduced even further. Embodiments of the invention will now be described in more detail, by way of example, with reference to the accompanying drawings. Elements bearing the same reference numerals in different Figures are identical. In the drawings Figure 1 shows a first embodiment, Figure 2 shows a number of signals as a function of time, appearing on a number of points in the circuit of Figure 1, Figure 3 shows a second embodiment, Figure 4 shows a number of signals as a function of time, appearing on a number of points in the circuit of Figure 3, Figure 5 shows a third embodiment, Figure 6 shows a number of signals as a function of time, appearing on a number of points in the circuit of Figure 5, Figure 7 shows a fourth embodiment, and Figure 8 shows a number of signals as a function of time, appearing on a number of points in the circuit of Figure 7. Figure 1 shows an embodiment of the invention used in a video recorder. A number (in the present case two) of read heads 1 and 2 are arranged on a rotatable head drum (not shown) and are respectively coupled to inputs 3 and 4 of switching means 5. the output 6 of the switching means is coupled to the input terminal 7 of the device. The switching means 5 are constructed to couple one of the inputs 3, 4 to the output 6 under the influence of a control signal to be applied to a control input 8. This control signal may be supplied by a control unit (not shown), which can derive this control signal from an internal clock in the control unit, which clock can be synchronized in some way with the signal read from a magnetic record carrier (not shown) by the read head. The input terminal 7 of the device, to which the frequency-modulated signal read from the record carrier is applied, is coupled to an output terminal 9 via a main signal path. The main signal path in Figure 1 comprises a demodulator circuit 10 whose input 11 is coupled to the input terminal 7, a low-pass filter, to be described hereinafter, and a hold circuit 12 having a control signal input 22, an input 13 coupled to the output 14 of the demodulator circuit 10, and an output 15. The output 15 is coupled to the low-pass filter 17 via a buffer amplifier 16 which ensures that the circuits coupled in the main signal path after the buffer amplifier 16 do not load the first (or hold) capacitor 24. This low-pass filter 17 ensures that spurious components outside the audio band cannot reach the following circuitry. the output of the low-pass filter 17 is coupled to the output terminal 9. The hold circuit 12 comprises first controllable switching means 18, having an input 19 and an output 20, which are respectively coupled to the input 13 and the output 15 of the hold circuit. The switching means 18 further comprise a control input 21 coupled to the control signal input 22. The output 20 of the controllable switching means 18 is coupled to the output 15 of the hold circuit 12 via a first impedance 23 in the form of a resistor R₁. Further, the output 15 is coupled to a point of constant potential (earth) via a first capacitor 24. In the known devices for demodulating a frequency-modulated signal a low-pass filter is arranged between the output 14 of the demodulator circuit 10 and the input 13 of the hold circuit 12, and the impedance 23 is not present. This low-pass filter has a cut-off frequency of approximately 100 to 150 kHz and is intended to remove spurious signal components such as intermodulation products, residual carrier waves and noise above the audio band. The disadvantage of this low-pass filter is that it causes an expansion in time of the disturbances in the output signal of the demodulator circuit 10 which result from the change-over in the switching means 5 from reading with one read head, for example the read head 1, to reading with other read head, in the present case the read head 2. Consequently, the switching means 18 must be open for a comparatively long time. The cut-off frequency of 100 to 150 kHz of the low-pass filter cannot be selected (much) lower because this would result in the disturbances being spread over an even longer time. Therefore, it is proposed to arrange at least the capacitive section of the low-pass filter in the main signal path after the switching means 18. Thus, the capacitive element of the low-pass filter is in fact arranged in the hold circuit 12. In the embodiment shown in Figure 1 the low-pass filter comprises the first impedance (resistor R₁) 23 and the first capacitor 24 (C₁), which moreover functions as the hold capacitor for the hold circuit 12. Since at least the capacitive section of the low-pass filter 23, 24 is arranged after the switching means 18 in the main signal path the disturbances in the output signal of the demodulator circuit 10 are not expanded in time. This is because the energy content of the disturbance cannot reach the capacitive element of the low-pass filter on account of the switching means being open. Therefore, the switching means 18 need to be open for a shorter time (the time interval T₁, see Figure 2b), which means a smaller disturbance in the output signal on the output terminal 9. As a result of this, T₁ is reduced from approximately 8 to 10 µs to 4 to 5 µs, as already stated in the foregoing. The operation of the circuit shown in Figure 1 will now be explained by means of Figure 2. Figure 2a shows two signals A and B as a function of time, which signals appear on points A and B in Figure 1. As a result of the delay caused by the low-pass filter 23, 24 the signal on point B slightly lags the signal on point A. Figure 2b shows the control signal applied to the control signal input 21 of the switching means via the control signal input 22. A control signal which is logic "1" means that the switching means 18 are closed and a control signal which is logic "0" means that the switching means 18 are open. At the instant t=t₀ switching is effected from reading with one read head to reading with the other read head. The phase shift thus produced in the input signal of the demodulated circuit 10 ensures that in the output signal of the demolator circuit a disturbance occurs for a specific time after the instant t₀. This disturbance has a maximum length of T₁. During the time interval T₁ after t=t₀ the switching means 18 are open. In Figure 2b this is visible because the control signal is logic "0" during this time interval. During this time interval the voltage across the capacitor 24 remains substantially constant and decreases only slightly as a result of leakage, see Figure 2a, curve B. After opening of the switching means 18 the voltage on point A in Figure 1 decreases to the value of the voltage on point B. After closure of the switching means 18 at the instant t=t₀+T₁ the voltage on point A in the circuit of Figure 1 changes to the value of the voltage instantaneously appearing on the output 14 of the demodulator circuit 10. After the instant t=t₀+T₁ the signal on point B in Figure 1 will slowly change to the dashed prolongation of the curve B from t=t₀. The hatched area O₁ enclosed by the actual curve B from t=t₀ and the dashed extension of the curve B from t=t₀ is the disturbance ultimately occurring in the output signal on the output 15 of the hold circuit 12. However, this disturbance is smaller than the disturbance which would occur in the known device in which the low-pass filter is arranged in the main signal path before the switching means 18. If desired, the resistor 23 may be arranged before instead of after the switching means 18. This does not affect the operation of the circuit. The low-pass filter 17 will have a cut-off frequency of approximately 20 kHz. Depending on the degreee of attenuation to be attained for frequencies above 20 kHz a first, second or higher order filter characteristic may be selected for the filter 17. For a further reduction of disturbances as a result of opening of the first switching means it is known to provide an extrapolator. It is also possible to use such an extrapolator in the circuit of Figure 1. One example of an extrapolator will be described hereinafter. The two ends of the impedance 23 are respectively coupled to the inverting input and the non-inverting input of a differential amplifier (not shown). The output of this differential amplifier is coupled to the output 15 of the hold circuit via a series arrangement of second switching means and a voltage-to-current converter (not shown). Moreover, a capacitor (not shown) is arranged between the output of the second switching means and the point of constant potential. Figure 3 shows an embodiment which bears much resemblance to the embodiment shown in Figure 1. The difference is that in the embodiment shown in Figure 3 an auxiliary signal path 26 is arranged in parallel with at least the first impedance 23 (resistor R) and that the read heads 1, 2 and the switching means 5 are dispensed with. The auxiliary signal path 26 comprises second controllable switching means 27. Further, another (third) impedance (resistor) 28 is arranged in the auxiliary signal path 26 in series with the switching means 27. The switching means 27 are controlled by a control signal applied to a control input 29 of the switching means 27. When the control signal is logic "1" the switching means 27 are closed and when the control signal is logic "0" the switching means 27 are opened. The control signal input 22 of the hold circuit 12′ is coupled to the control input 29 via a block 30. The operation of the circuit shown in Figure 3, and in particular the operation of the block 30, will be described with reference to Figure 4. Figure 4a again shows the signals on point A and B in the circuit of Figure 3 as a function of time and Figures 4b and 4c show the control signals applied to the control inputs 21 and 29 of the switching means 18 and 27 respectively. The switching means 18 operate in the same way as described with reference to Figures 1 and 2. The signal A in Figure 4a and the control signal in Figure 4b are consequently identical to the relevant signals in Figures 2a and 2b respectively. If at the instant t=t₀+T₁ the first switching means 18 are closed again the block 30 derives the control signal, see Figure 4c, causing the second switching means 27 to be briefly closed for a time interval T₂. During this time interval the voltage on the junction point between the switching means 27 and the resistor 28 is equal to that on the point A. Consequently, the voltage on point B will increase and intersect the dashed prolongation of the curve B from t=t₀. After the switching means 27 have been opened the voltage on point B returns to the dashed prolongation of the curve B from t=t₀. The hatched areas O₂ and O₃ enclosed by the actual curve B respectively below and above the dashed prolongation and the dashed line itself again represent the disturbance ultimately appearing on the output 15. Since the disturbance now comprises to disturbance components on opposite sides of the prolongation this disturbance is less perceptible and is therefore less annoying. Preferably, the circuit of Figure 3 should be dimensioned in such a way that the areas O₂ and O₃ are of the same order of magnitude. For the operation of the circuit it is irrelevant whether the auxiliary signal path 26 is arranged in parallel with the resistor 23 only or is arranged in parallel both with the switching means 18 and the resistor 23. Moreover, the resistor 23 may again be arranged before the switching means 18. Figure 5 shows yet another embodiment. The difference with the embodiment of Figure 1 is that in the present case a second capacitor 33 (C₂) is arranged between the output 20 of the first switching means 18 and the point of constant potential (earth). In the circuit from the output 14 of the demodulator circuit 10 to point A a second impedance is arranged. This may be a separate resistor, for example a resistor R₂, but if desired the internal resistance of the switching means 18 may be utilized for this purpose. The operation of the circuit of Figure 5 is again illustrated by means of the signals shown in Figures 6a and b. Figure 6a again shows the signals on point A and B in Figure 5 and Figure 6b shows the control signal applied to the switching means 18. The hatched area O₄ between the curve B and the dashed prolongation from t=t₀ represents the disturbance relative to the desired signal, which disturbance appears on the output 15 of the hold circuit 12″. If the time constant of the RC filter comprising the elements R₂, C₂ is smaller than the time constant R₁C₁ of the filter of Figure 1, the area O₄ in Figure 6 may be smaller than the area O₁ in Figure 2. this means that the disturbance in the circuit arrangement shown in Figure 5 is smaller than the disturbance in the arrangement of Figure 1. Figure 7 shows still another embodiment. In fact, it shows the embodiment of Figure 5 extended by the auxiliary signal path 26 of Figure 3. Figure 8a again shows the signals on points A and B in Figure 7 as a function of time and Figures 8b and c again show the control signals for the switching means 18 and 27 respectively. In Figure 8a the curve B illustrates a behaviour which bears much resemblance to that of the curve B in Figure 4a. A disturbance occurs which results in excursions of the desired signal towards a smaller signal amplitude (the area O₅) and towards a larger signal amplitude (the area O₆). As already stated with reference to Figure 4, this disturbance in Figure 8 is also less perceptible and consequently less annoying than the disturbance obtained when the circuit of Figure 5 is used. Moreover, the circuit of Figure 7 produces less disturbance than the circuit of Figure 3, which will become apparent when the areas O₂ and O₃ are compared with O₅ and O₆ respectively. It is to be noted that the invention is not limited to the embodiment shown. The invention also relates to those devices which differ from the embodiments shown in respects which are irrelevant to the invention. For example, it is possible to split the first impedance 23 (R₁) in Figures 3 to 7 into two parts, to arrange one part between the output 20 of the switching means 18 and point A, and to arrange the second part between points A and B. In the circuit arrangement of Figure 7 the auxiliary signal path 26 may then be arranged in parallel with only the second part of R₁.

Claims: A device for demodulating a frequency-modulated signal, having an input terminal (7) for receiving the frequency-modulated signal and an output terminal (9) for supplying a demodulated signal, a main signal path being provided between the input terminal and the output terminal, comprising a demodulator circuit (10), for demodulating the frequency-modulated signal, having an input coupled to the input terminal, and an output, a low-pass filter (23,24) comprising a capacitive element, a hold circuit (12, 12,' 12", 12'") having a signal input coupled to the output of the demodulator circuit (10), a control-signal input, and an output coupled to the output terminal, which hold circuit at least comprises first controllable switching means (18), having an input and an output coupled respectively 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 (24) 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 (18) being constructed to interrupt the internal connection between their input and their output under the influence of said control signal for a first time interval T₁, characterized in that at least the capacitive element of the low-pass filter (23,24) is arranged in the hold circuit and is arranged in the main signal path after the first controllable switching means (18), in that a first impedance (23) is arranged in the connection from the input of the hold circuit to the terminal of the capacitor (24) which is not coupled to the point of constant potential and in that the low-pass filter at least comprises said first impedance and the capacitive element and in that the first capacitor (24) forms part of the capacitive element. A device as claimed in Claim 1, characterized in that the first impedance (23) is coupled between the output of the first switching means (18) and the terminal of the capacitor (24) which is not coupled to the point of constant potential, and in that a second capacitor (33) is arranged between the output of the first switching means and the point of constant potential. A device as claimed in Claim 2, characterized in that a second impedance (R2) is arranged in the connection from the input of the hold circuit to the terminal of the second capacitor (33) which is not coupled to the point of constant potential. A device as claimed in any one of the preceding Claims, characterized in that an auxiliary signal path is arranged in parallel with at least the first impedance, which auxiliary signal path comprises second controllable switching means (27) which have an input and an output and which are constructed to provide an internal coupling from this input to this output for at least for a second time interval T₂ following the first time interval T₁. A device as claimed in Claim 4, characterized in that a series arrangement of the second controllable switching means and a third impedance (28) is arranged in the auxiliary signal path. A device as claimed in Claim 1, characterized in that the two ends of the first impedance (23) are coupled respectively to the inverting and the non-inverting input of a differential amplifier having an output coupled to the output of the hold circuit via a series arrangement of second switching means and a voltage-to-current converter, and in that the junction point between the second switching means and the voltage-to-current converter is coupled to the point of constant potential via a second capacitor. A device as claimed in any one of the preceding Claims, characterized in that the device is incorporated in a video recorder for demodulating, after it has been read from a record carrier, a frequency-modulated (audio) signal recorded on the record carrier, which device for this purpose further comprises two or more read heads (1,2) arranged on a rotatable head drum, switching means (5) having two or more inputs, an output and a control input, which switching means are 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 (10).

Inventors: PIJNENBURG, JOSEPHUS ADRIANUS, VAN DE PAVOORDT, FRANCISCUS GIJSBERTUS JOHANNES
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H03D3
IPC codes: G11B20, H03D3, H03G3, H04N5, H03H7, H03D1