Abstract:
The drop-out detection circuit includes basically an oscillator arranged to generate a clock signal of a predetermined frequency; a first flip-flop arranged to have the output signal thereof inverted by the clock signal and to be reset by a binary signal to be detected; and a second flip-flop arranged to sample and hold the output signal of the first flip-flop according to the clock signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a drop-out detection circuit, and more particularly, to a circuit highly suited for detecting drop-out of angle modulated signals. 
     2. Description of the Related Art 
     In reproducing from a recording medium such signals as video signals with a reproducing apparatus, some parts of the signals often drop out due to various causes. The apparatus of this kind is, therefore, generally provided with some arrangement to compensate for such signal drop-out. 
     To have signal drop-out adequately compensated for, it is necessary to accurately detect the drop-out. There have been proposed various drop-out detecting methods. FIG. 1 of the accompanying drawings shows an example of the conventional drop-out detection circuit adapted for detecting the drop-out of frequency modulated signals in particular. Referring to FIG. 1, an input terminal 31 receives, for example, a frequency modulated luminance signal S-31 which is as shown at a part S-31 in FIG. 2. The signal S-31 is amplified by the amplifier 32 and then is changed into a signal of a given level by a limiter 33 as shown at a part S-32, in FIG. 2. The signal S-32 which is produced from the limiter 33, is applied to a fall synchronizing type retriggerable monostable multivibrator (hereinafter referred to as monostable multivibrator) 34. With the time constant T of the monostable multivibrator 34 set at a given value by a resistor 34a and a capacitor 34b, the output level of the monostable multivibrator 34 becomes low in the event of a long drop-out exceeding the time constant T, as shown at a part S-33 in FIG. 2. Then, the output S-33 of the monostable multivibrator 34 is obtained from an output terminal 35 as a drop-out detection signal. 
     However, in cases where a signal of a broad band is to be processed with the conventional device of the above-stated kind, by arranging to make the device capable of detecting also a very short drop-out such as 1 to 2 μs, for example, the operation of the device becomes extremely unstable. In that instance, the device often fails to perfectly follow the drop-out occurring for such a short period of time. In the case of the device arranged in the manner as mentioned above, the device may be enabled to detect such a short drop-out by arranging the monostable multivibrator 34 to have an extremely small time constant T. However, an unstable operation generally results from making the time constant of a monostable multivibrator extremely small and thus prevents the monostable multivibrator from accurately following a trigger signal. 
     Further, in the case of such a short period of time, it is also difficult to select and accurately adjust the time constant set by a capacitor and a resistor. Besides, since the device is provided with a capacitor, its operation tends to become unstable due to changes in temperature. It has been another problem that the conventional device does not readily permit use of an IC. 
     SUMMARY OF THE INVENTION 
     In view of the problems of the prior art mentioned above, it is a general object of this invention to provide a drop-out detection circuit which is capable of accurately operating. 
     It is a more specific object of this invention to provide a drop-out detection circuit which is capable of accurately carrying out drop-out detection even in the event of detecting a drop-out occurring only for a very short period of time. 
     Under this object, a drop-out detection circuit embodying this invention comprises: means for binary coding the level of a signal to be detected; means for generating a clock signal of a given frequency; means for detecting that the binary coded signal comes-to assume a predetermined state within each period of the clock signal; and means for generating a drop-out detection signal on the basis of a signal produced from the detecting means. 
     It is another specific object of this invention to provide a drop-out detection circuit which is suited for use of an IC. 
     Under that object, a drop-out detection circuit embodying this invention comprises: an oscillator arranged to generate a clock signal of a predetermined frequency; a first flip-flop arranged to have the output signal thereof inverted by the clock signal and to be reset by a binary signal to be detected; and a second flip-flop arranged to sample and hold the output signal of the first flip-flop according to the clock signal. 
     These and further objects and features of this invention will become apparent from the following detailed description of the preferred embodiment thereof taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing an example of the conventional drop-out detection circuit. 
     FIG. 2 is a timing chart showing the operation of the circuit shown in FIG. 1. 
     FIG. 3 is a circuit diagram showing the arrangement of a drop-out detection circuit embodying this invention. 
     FIG. 4 is a timing chart showing the operation of the circuit shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of this invention is arranged as shown in FIG. 3 and operates as shown in FIG. 4. In FIG. 3, component elements of the drop-out detection circuit which are similar to those shown in FIG. 1 are indicated by the same reference numerals and the details of them are omitted from the following description: 
     Referring to FIG. 3, the illustration includes a T type flip-flop 1 (hereinafter referred to as T-FF) which has a reset terminal; D type flip-flops (hereinafter referred to as D-FF&#39;s) 2 and 3; a D-FF 4 which has a reset input terminal; an inverter 5 arranged to receive a signal S-1 which is obtained with the waveform of a frequency modulated signal shaped through a limiter 33; a clock pulse generator 6 arranged to generate a clock signal S-2 which has a predetermined period; an output terminal 8; and a terminal 9 which is arranged to receive a power supply voltage Vcc. In this specific embodiment, all the flip-flops 1, 2, 3 and 4 are of the type arranged to be triggered in synchronism with the fall of a clock pulse signal and to be reset when a low level input is applied to the reset input terminal thereof. The inverter 5 receives the signal S-1 which remains at a low level during a drop-out period as shown in FIG. 4. 
     With the drop-out detection device arranged according to this invention as described above, it operates as follows: When a clock signal S-2 shown in FIG. 4 is supplied to a clock input terminal 1a of the T-FF 1 and a signal obtained by inverting an incoming signal S-1, which is as shown in FIG. 4, is supplied to a reset input terminal 1b of the T-FF 1, the T-FF 1 produces from its output terminal 1Q a signal S-3 as shown in FIG. 4. The output signal S-3 of the T-FF 1 is supplied to an input terminal 3D of the D-FF 3. The clock signal S-2 is supplied to a clock input terminal 3a of the D-FF 3. The D-FF 3 then produces from its output terminal 3Q, a signal S-4 as shown in FIG. 4. The D-FF 2 is arranged to have the incoming signal S-1, which is inverted by the inverter 5, supplied to an input terminal 2D thereof and to have also the clock signal S-2 supplied to a clock input terminal 2a thereof. The D-FF 2 produces from its output terminal 2Q, a signal S-5 as shown in FIG. 4. The D-FF 4 which is provided with a reset terminal, has an input terminal 4D connected to the power supply terminal 9 (+Vcc). The D-FF 4 is arranged to have the output signal S-4 of the D-FF 3 supplied to a clock input terminal 4a thereof and the output signal S-5 of the D-FF 2 to the reset input terminal 4b thereof. Upon receipt of these signals, the D-FF 4 produces from an output terminal 4Q, a signal S-6 a shown in FIG. 4. The signal S-6 is a drop-out detection signal. 
     In the embodiment described, the level of the output signal S-1 of the limiter 33 without fail becomes a high level within each period of the clock signal S-2 if the frequency of the clock signal S-2, which is generated by the clock pulse generator 6, is lower than the lowest frequency of the incoming frequency modulated signal. Therefore, if no drop-out exists in the frequency modulated signal, the T-FF 1 is reset without fail within one period of the clock signal S-2. When triggered by the clock signal, the T-FF 1 changes the level of its output 1Q from a low level to a high level. In other words, a change of the output of the T-FF 1 from a high level to a low level due to the clock signal indicates the occurrence of drop-out. This is detected by the D-FF 3 and then the D-FF 4 produces a drop-out detection signal indicating the start of a drop-out period. 
     Upon occurrence of drop-out, the input terminal 2D of the D-FF 2 receives a high level signal without fail. Then, at the end of the drop-out, the level of the output 2Q of the D-FF 2 changes to a low level to indicate the end of the drop-out. The end of the drop-out period is thus determined from the drop-out detection signal produced from the D-FF 4 according to the output 2Q of the D-FF 2. 
     In the embodiment described, each of the flip-flops are arranged to be of the fall synchronizing type and to be reset in a so-called &#34;low-active&#34; manner. However, a similar effect is also attainable with minor modification by replacing them with flip-flops which are of a rise synchronizing type and arranged to be reset in a so-called &#34;high-active&#34; manner. 
     Further, the output signal of the embodiment delays at least for one to two periods of the clock input signal from an actual drop-out period. However, the original signal is generally arranged to be processed through some frequency demodulation circuit, some deemphasis circuit, etc. and is thus delayed to a certain extent before it reaches a drop-out compensation circuit. Therefore, the delay caused by the drop-out detection circuit can be virtually cancelled out by selecting the frequency of the external clock pulses at a suitable value. The delay, therefore, does not present any substantial problem. 
     Further, although the invented system requires use of a clock signal of a fixed frequency. It requires no severe degree of precision nor much stability of the clock signal.