Digital Signal Selective Circuit

1. Field of the Invention

The present invention relates to a digital signal selective or detecting circuit for providing an output signal in response to a periodic input signal which has a frequency within a specified range for a predetermined time. In particular, the present invention relates to a digital signal selective circuit suitable for use with a video tape recorder (VTR) and responsive to a cue signal recorded on a video tape used in the VTR.

2. Description of the Prior Art

Frequency detecting circuits have been proposed for detecting an indication or command signal which maintains a predetermined frequency for a predetermined length of time in order to initiate the operation of various associated systems. For instance, such frequency detecting circuits are commonly found in such widely ranging applications as remotely controlled automatic garage door openers and automatic telephone answering machines. However, prior art frequency detecting circuits employ analog rather than digital techniques. A typical analog frequency detecting circuit incorporates a resonance filter, such as a tank circuit, a reed filter, an electromechanical filter, or a time-constant circuit. In such analog circuits, detection errors often occur because of detector instability whenever an incoming waveform is distorted. In addition, it is difficult to adjust the alignment and bias of the various elements in an analog circuit. Further, in prior art analog frequency detecting circuits, it is difficult to change the response or detected frequency.

Accordingly, it is an object of the present invention to provide a digital signal selective or detecting circuit responsive to a periodic input signal which has a frequency within a specified range for at least a predetermined time, and which is free from the above-mentioned defects of the prior art.

More specifically, it is an object of the present invention to provide a digital frequency selective or detecting circuit, as aforesaid, which is of relatively simple construction, and suitable for use with a VTR for detecting cue signals recorded on a video tape used in the VTR.

According to an aspect of the invention, a digital signal selective or detecting circuit responsive to a periodic input signal having a frequency within a specified range for a predetermined time, includes a clock pulse generator providing reference clock pulses, first counting means counting the reference clock pulses whenever the periodic signal is present, a circuit for providing a counting pulse whenever a particular operating characteristic of the periodic signal, such as, a transition between portions of the input signal which are positive and negative in respect to a reference level, occurs at a time when the count in the first counting means falls between first and second specified counts, and second counting means counting each counting pulse and providing an output when the second counting means attains a predetermined count. Preferably, means are provided to reset the counting means, and thereby to prevent the same from attaining respective predetermined counts, whenever the frequency of the periodic input signal is outside the specified range of frequencies.

Because the second counting means operates only if the described transition or other operating characteristic of the periodic input signal occurs while the first counting means is between first and second predetermined counts, and because an output is produced only when the second counting means attains a predetermined count therein, the signal selective or detecting circuit accurately detects those signals which have a frequency in the predetermined frequency range and which exist for at least a predetermined length of time.

FIG. 1 is a block diagram showing an embodiment of a digital signal selective or detecting circuit according to the present invention;

FIGS. 2A-2P are waveform diagrams to which reference will be made in explaining the operation of the digital signal selective circuit of FIG. 1; and

FIG. 3 shows a diagrammatic view of a segment of a video tape on which cue signals are recorded.

Referring to the drawings in detail, and initially to FIG. 1 thereof, it will be seen that a digital signal selective circuit according to the present invention is intended to be responsive to an input signal S.sub.a of a predetermined frequency, for example, a 1 KHz tone or oscillation which may be a cue signal recorded on a video tape and suitably reproduced for application to an input terminal 10. The input terminal 10 is connected to a waveshaping circuit 12, which is preferably a Schmitt trigger. The output S.sub.b of the waveshaping circuit 12 is supplid to a J input terminal of a first JK flip-flop 14, and also through an inverter 16 to a K input terminal of JK flip-flop 14. A clock pulse generator 18 provides reference clock pulses P.sub.c, for example, at a frequency of 100 KHz and with a duty cycle of 50%. The reference clock pulses P.sub.c are supplied to a clock terminal CK of the first JK flip-flop 14. The first JK flip-flop 14 has an output terminal Q and an inverse output terminal Q, and the inverse output terminal Q is connected to a clock input terminal CK of a second JK flip-flop 20. The J and K terminals of the second JK flip-flop 20 are connected to a voltage source, for example, as represented by a reference potential V.sub.cc and to ground, respectively. The second JK flip-flop 20 has an inverse output terminal Q which is connected with a first pulse generator 22. The Q output terminal of first JK flip-flop 14 is connected to a second pulse generator 24.

The reference clock pulses P.sub.c from clock pulse generator 18 are further applied to an input of a first counter 26. In the illustrated example, counter 26 is capable of counting to a count of at least 106 before returning to 0. Counter 26 may consist of a pair of decade counters connected in series. Counter 26 has a first output connected with a third JK flip-flop 28 to apply a pulse P.sub.h to the clock input CK thereof when a count of 95 is present in counter 26. Another output of counter 26 provides a pulse P'.sub.h to the clear terminal CL of third JK flip-flop 28 whenever a count of 106 is present in counter 26. The J and K input terminals of third JK flip-flop 28 are connected to a reference voltage V.sub.cc and to ground, respectively. A logic AND gate 30 has two inputs which respectively receive pulses P.sub.g from second pulse generator 24 and an inverse output S.sub.i from the inverse output terminal Q of third JK flip-flop 28. An output P.sub.j of logic AND gate 30 is applied, as a counting signal, to a second counter 32, and to an OR gate 34. Second counter 32 is, in the example being described, capable of counting to 99 before returning to 0, and provides an output pulse P.sub.n when the counter 32 attains a predetermined count, for instance, a count of 99. The counter 32 may consist of a pair of decade counters connected in series.

OR gate 34 provides an output P.sub.k whenever it receives either a first pulse P.sub.f from first pulse generator 22 or a counting pulse P.sub.j from AND gate 30. The output P.sub.k of OR gate 34 triggers a monostable multivibrator 36 which, in turn, provides a gating signal S.sub.l to gate terminals of counters 26 and 32 so that the counters 26 and 32 may count the respective inputs during the existence of signal S.sub.l. The signal S.sub.l is further applied to a third pulse generator 38 which provides a reset pulse P.sub.m in response to the negative-going (trailing) edge of the gating signal S.sub.l provided by multivibrator 36. The reset pulse P.sub.m from third pulse generator 38 is applied through OR gates 40 and 42 to respective reset terminals of first and second counters 26 and 32. The reset pulse P.sub.m is also applied to respective clear terminals CL of first and second JK flip-flops 14 and 20. The output pulse P.sub.n from counter 32 is applied to a delay circuit 44 which provides a delayed signal S.sub.o 400 msec after counter 32 attains its predetermined count. The delayed signal S.sub.o is applied to inputs of OR gates 40 and 42 and to an output pulse generator 46. Further, OR gate 40 is connected with second pulse generator 24 to reset first counter 26 whenever a second pulse P.sub.g is generated.

In a particular example of a digital signal selective circuit according to this invention, an output pulse is provided from pulse generator 46 0.5 sec. after a cue signal of 1 KHz frequency is first applied to input terminal 10 for at least 0.1 seconds. More particularly, when the cue signal shown to have a sinusoidal waveform on FIG. 2A, is applied as the input signal S.sub.a through input terminal 10 to waveshaping circuit 14, the latter circuit provides a rectangular output waveform S.sub.b with a frequency (1 KHz) the same as that of the input signal S.sub.a. As shown on FIG. 2B, the rectangular output waveform S.sub.b has a transition from one level to another corresponding to an operating characteristic of the input signal S.sub.a, that is, corresponding to the transition of the signal S.sub.a from a positive portion to a negative portion thereof. The rectangular waveform S.sub.b is applied to the J terminal, and through inverter 16, to the K terminal of first flip-flop 14, while clocking pulses P.sub.c, shown on FIG. 2C, are applied from generator 18 to the clock terminal of flip-flop 14. As a result, first JK flip-flop 14 provides, at its Q output terminal a rectangular wave S.sub.d (FIG. 2D) which is generally similar to waveform S.sub.b but synchronized with clocking pulses P.sub.c. Flip-flop 14 provides, from its inverse output terminal Q, a signal which is the inverse of signal S.sub.d and which is applied to the clock input CK of second flip-flop 20. Flip-flop 20 provides from its Q output terminal a signal S.sub.e (FIG. 2E) with a negative-going leading edge at the commencement of rectangular wave S.sub.d. In response to signal S.sub.e, first pulse generator 22 provides a first pulse P.sub.f (FIG. 2F) at the commencement of the rectangular wave S.sub.d. Second pulse generator 24 provides second pulses P.sub.g (FIG. 2G) at 1 msec intervals in response to the positive-going leading edges of rectangular wave S.sub.d. OR gate 34 provides a triggering pulse P.sub.k (FIG. 2K) to multivibrator 36 at a time corresponding to the commencement of rectangular wave S.sub.d. Multivibrator 36 provides gating signal S.sub.l (FIG. 2L) to enable first and second counters 26 and 32 to count the clock pulses P.sub.c and counting pulses P.sub.j, respectively. Upon receiving the gating signal S.sub.l, first counter 26 commences registering a count in response to clock pulses P.sub.c. When first counter 26 attains a predetermined count of 95, a pulse P.sub.h, shown as a solid line in FIG. 2H, is applied to the clock terminal of third flip-flop 28, and when the counter 26 attains a second predetermined count of 106, a second pulse P'.sub.h, shown as a broken line in FIG. 2H, is applied to the clear terminal of third flip-flop 28. The output signal S.sub.i (FIG. 2I) from the inverse output terminal Q of third flip-flop 28 is shown to have a transition from a first or high level to a second or low level in response to pulse P.sub.h, and a transition from the second level back to the first level in response to pulse P.sub.h '. The logic AND gate 30 provides a counting pulse P.sub.j (FIG. 2J) for each second pulse P.sub.g from pulse generator 24 occurring while output signal S.sub.i is at its second or low level. Counting pulse P.sub.j is also supplied through OR gate 34 as a triggering pulse P.sub.k for multivibrator 36. Thus, gating signal S.sub.l (FIG. 2L) will be continuously provided to the gate terminals of first and second counters 26 and 32 so long as there is a continuous succession of counting pulses P.sub.j. Further, so long as a 1 KHz input signal S.sub.a is applied to input terminal 10, counting pulses P.sub.j will be applied to the input of second counter 32 to be counted by the latter during reception of the gating signal S.sub.l. When counter 32 attains its predetermined count, an output pulse P.sub.n (FIG. 2N) is provided therefrom to the delay circuit 44. Because counting pulses P.sub.j occur at 1 msec intervals and second counter 32 counts from 0 to 99, the output pulse P.sub.n occurs 100 msec (0.1 sec) after the commencement of the input signal S.sub.a. Delay circuit 44 provides delayed signal S.sub.o (FIG. 20) for 400 msec (0.4 sec) after receiving output pulse P.sub.n. The output pulse generator 46 provides a pulse P.sub.p (FIG. 2P) at the falling-down or trailing edge of signal S.sub.o, and which is, therefore, delayed 0.5 seconds in respect to the commencement of input signal S.sub.a. The falling down or trailing edge of delayed signal S.sub.o is also used to reset counters 26 and 32 at the end of the 0.5 second interval. In addition, first pulse P.sub.g from generator 24 is applied to reset the first counter 26, and thereby prevent signal frequencies which are harmonics of the 1 KHz signal S.sub.a, for instance, the second harmonic with a frequency of 2 KHz, from being mistakenly detected as the desired 1 KHz signal.

Pulse generator 38 provides the reset pulse P.sub.m (FIG. 2M) to reset terminals of first and second counters 26 and 32 in response to the trailing edge, or negative-going portion of gating signal S.sub.l. Multivibrator 36 should be selected to have a time constant greater than the normal period of the input signal S.sub.a which is to be detected. In the case where the signal S.sub.a to be detected has a frequency of 1 KHz, that is, a period of 1 msec., multivibrator 36 preferably has a time constant of 1.5 msec., that is, provides the gating signal S.sub.l for 1.5 msec. in response to each triggering pulse P.sub.k. Thus, reset pulse P.sub.m will be provided whenever counting pulses P.sub.j occur at intervals greater than the time constant of multivibrator 36. If the time constant of multivibrator 36 is selected to be greater than 1.0 msec., for example, 1.5 msec, as described above, reset pulse P.sub.m will be provided whenever there is a break in the succession of 1 msec. counting pulses P.sub.g.

In the circuit described above, some fluctuation in the frequency of the input signal S.sub.a can be tolerated, provided that such fluctuation remains within a predetermined range. The circuit in the above described embodiment is so designed that, when the input signal S.sub.a has a frequency of precisely 1 KHz, each pulse P.sub.g will occur at a time which is substantially midway between the 95th and 106th pulses registered on first counter 26. If the pulse P.sub.g occurs at a time before the 95th pulse, or after the 106th pulse is registered, the output of the third flip-flop 28 will then be at the first or high level, and a counting pulse P.sub.j will not be provided. However, if the pulse P.sub.g occurs at any time after counter 26 has registered the 95th pulse, but before it has registered the 106th pulse, output S.sub.i of flip-flop 28 will be at its second or low level and AND gate 30 will provide counting pulses P.sub.j to the second counter 32 in response to pulses P.sub.g. Thus, the output from the AND gate 30 is substantially that of a digital band pass filter with a frequency band extending from 950 Hz to 1050 Hz. The second counter 32 provides an output pulse P.sub.n only when it counts 100 pulses P.sub.j occurring without interruption and filtered, as if by a band pass filter having a pass band of 950-1050 Hz.

It should be noted that first and second counters 26 and 32 together with third flip-flop 28 and logic AND gate 30, constitute a counting means for providing an output P.sub.n upon counting 100 counting pulses occurring in response to the input signal S.sub.a being in the frequency band of 950-1050 Hz.

One particular application of the digital signal selective or detecting circuit of FIG. 1 is in conjunction with a cueing system for detecting a cue signal recorded on video tape, as mentioned above. In broadcasting, and particularly during news broadcasting, it is sometimes necessary to insert recorded program items, including prerecorded news, commentary, commercials, and so forth, between live program items. With reference to FIG. 3, it will be noted that a standard technique for inserting recorded program items involves the use of cue signals Q.sub.a, Q.sub.b, Q.sub.c, . . . etc. located on a cue track of the video tape so that, at the normal speed of advancement of the tape, the cue signals will occur five seconds before the commencement of the respective recorded program items, A, B, C, . . . etc. The five second interval between each of the cue signals Q.sub.a, Q.sub.b, Q.sub.c and the respective recorded program item, A, B, C provides a run-up time of five seconds to allow the various servo systems associated with the video tape recorded (VTR) to stabilize and lock before the VTR switches to the inserted program item A, B, C, . . . etc.

Initially, the video tape with the recorded items A, B, C recorded thereon is advanced in a VTR until cue signal Q.sub.a is detected by a cue reproducing head. The detected cue signal Q.sub.a is applied from the respective reproducing head as the input signal S.sub.a to the terminal 10 of the digital signal selective or detecting circuit of FIG. 1. Then, after 0.5 seconds, pulse generator 46 provides an output pulse P.sub.p to the tape advancement mechanism, such as, the capstan drive, of the VTR, so as to halt the tape at a position five seconds in advance of the commencement of the first recorded program item A. After the first recorded program item A has been inserted in the broadcast material, the tape continues to run until the next cue signal Q.sub.b is reproduced and causes the tape drive to halt with the tape disposed at a position five seconds in advance of the next recorded program item B. After the next recorded program item B is inserted, reproducing of cue signal Q.sub.c similarly prepares the VTR for subsequent insertion of recorded program item C.

The cueing signals may be recorded along the cue track according to a standard technique. For example, the VTR is first set into the playback mode and the video tape is advanced until the commencement of material desired to constitute a recorded program item. At that time, the VTR is changed-over to a pause mode, and the tape is rewound for 7.5 seconds. Thereafter, the tape is advanced for 2.0 seconds prior to the cue signal Q.sub.a being recorded on the cue track of the tape while the latter is further advanced for 1.0 second.

While the present invention is disclosed herein as being applied to a cueing system for use with a VTR, it is apparent that the digital signal selective or detecting circuit according to the present invention could be applied in various other environments where it is required to detect the occurrence, for a predetermined time, of a periodic input signal havng a frequency within a specified range.

Although an illustrative embodiment of the invention has been described in detail herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to that embodiment, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.