JPS61144745A - Blank detecting device - Google Patents

Abstract

PURPOSE:To detect the end position of recording information accurately by counting a dropout detection signal by a counter circuit during fast reproduction and outputting a blank detection signal when a dropout of information extends for longer than a specific time. CONSTITUTION:A dropout detecting circuit 16 detects a state in which an FM signal level is lower than a threshold value and its detection signal is inputted as a count-up signal to the 1st counter 19 through an LPF17 and a waveform shaping circuit 18. A head switch pulse HSW is inputted to the reset terminal of the counter 19 through a frequency dividing circuit 21 and a pulse shaping circuit 22. This circuit 22 shapes the waveform of the input signal to vary the duty ratio to send an output equivalent to a dropout detection pulse to a pulse generating circuit 23 and a counter 24 as a reset pulse. Then the most- significant digit MSB outputs of the counters 19 and 24 are supplied to an exclusive OR circuit 26, whose output is sent to a CPU as a blank detection signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a blank detection device for detecting a tape blank portion and detecting a tape position for new recording in a magnetic recording/reproducing device using a rotary head.

[Prior art and its problems] In a magnetic recording/reproducing device using a rotary head, such as a VTR (video tape recorder), accurate positional information on a field-by-field basis is often required when searching for the beginning of a continuous video recording. When searching for the beginning of a continuous video as described above, conventionally there are four methods commonly used in radio cassettes, etc., in which a signal for position detection is recorded between the pieces of information, and a signal level for continuous information is recorded. It is detected depending on whether it is higher or lower than the threshold level. However, in the above-mentioned conventional method, an integrating circuit with a large time constant must be provided in the detection circuit in order to eliminate the effects of dropouts and the like. Further, by providing the above-mentioned integrating circuit, there is a problem that the length of the tape from when information is lost until the running system actually stops varies greatly due to variations in its time constant.

[Object of the Invention] The present invention has been made in view of the above points, and provides a method for reliably detecting the end or start position of recorded information on a magnetic tape during high-speed playback without recording a positioning signal even during high-speed running. The object of the present invention is to provide a blank detection device that can perform the following tasks.

[Summary of the Invention] The present invention takes advantage of the fact that the envelope signal is interrupted between tracks during high-speed playback, pulses it and counts it, and even when there is no position read signal, the relative position/relative time to recorded information can be determined. This allows accurate detection.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, reference numerals 11a and 11b are first and second heads that read video signals recorded on a video tape and input them to the reproduction amplifier 12. The reproduction amplifier 12 alternately selects the reproduction signals from the heads 11a and 11b for each field using the head switch pulse H8W, amplifies them to an appropriate level, and sends the signals to a color signal processing circuit (not shown) and a bypass filter 13. Output. This bypass filter 13 is a filter that passes only the FM signal, and its output is transmitted to the luminance signal processing circuit (not shown) and the AGC circuit 1.
Sent to 4. This AGC circuit 14 holds the FM signal level from the bypass filter 13 constant and inputs it to the dropout detection circuit 16 in the blank detection section 15 . This dropout detection circuit 16 detects a state in which the FM signal level is below a threshold and outputs a signal.The detection signal is sent to a dropout compensation circuit (not shown), and is also sent to the low buzz filter 11 and The signal is input to the first counter 19 via the waveform shaping circuit 18 as a count-up signal. Also, this first counter 1
The head switch pulse H8 is connected to the reset terminal 9.
W is input via a frequency dividing circuit 21 and a pulse shaping circuit 22. The frequency dividing circuit 21 has a head switch pulse H
When 8W is input, it is frequency-divided into a signal with a period of about 3 seconds, for example, and output to the pulse shaping circuit 22. This pulse shaping circuit 22 is a pulse generating circuit 2 that shapes the waveform of an input signal, changes the duty ratio for a reset pulse, and generates a continuous pulse equivalent to a dropout detection pulse from the output signal.
3 and the second counter 24 as a reset signal. Further, the first counter 19 and the second counter 24 are given a control signal from a count start/stop control circuit 25 to instruct them to start and stop counting. 1st above
The count numbers of the counter 19 and the second counter 24 are set to be the same and so that the most significant bit MSB is output at a slightly shorter cycle than the cycle of the reset pulse R8T output from the pulse shaping circuit 22. Then, the most significant bit MSB output of the first counter 19 and the second counter 24 is input to an exclusive OR circuit (hereinafter abbreviated as EX OR circuit) 26, and this EX OR circuit 2
The output of 6 is sent to CPtJ (not shown) as a blank detection signal.

Next, the operation of the above embodiment will be explained with reference to the timing chart of FIG. When a cue operation is performed on the videotape, a control command is sent from the CPU to the count start/stop control circuit 25, and the count start/stop control circuit 25 commands the first counter 19 and the second counter 24 to start counting. is given. This causes the first counter 19 and the second counter 24 to become operational. As shown in FIG. 2, a head switch pulse H8W whose signal level is switched between high and low for each field is input to the reproduction amplifier 12. Above playback amplifier 1
2 alternately selects the video heads 11a and 11b for each field by the head switch pulse H3W, and amplifies the read signal to an appropriate level. From the output signal of the reproducing amplifier 12, only the FM signal is extracted by the bypass filter 13 and held at a constant level by the AGC circuit 14. Therefore, when searching for the beginning of the tape, the forward search cue is performed at a speed that is, for example, 11 times the normal playback speed. In this way, when the forward search CLJe is 11x speed, the video head 1
1a111b crosses 11 tracks as shown in FIG. 3, but the designated video heads 11a, 1 at that time
An image signal is output only from the track having the same azimuth as one of the tracks 1b (the shaded area in FIG. 3). Therefore, AGC
As shown in FIG. 2, the circuit 14 outputs five summer waveform signals (A) corresponding to tracks with matching azimuths, and the envelope signal is interrupted between the tracks. As a result, when the speed is 11x as shown above, there are 1
One by one, four noise bars appear in the center of the screen.

Thus, the output signal of the AGC circuit 14 is input to the dropout detection circuit 16 in the blank detection section 15. This dropout detection circuit 16 includes the AGC circuit 1
When the signal level of the output signal (A) of No. 4 becomes below the threshold level, that state is detected and a signal is output. The dropout detection signal output from this dropout detection circuit 1G is filtered by the low pass filter 1.
1, noise and small-scale dropout detection signal components are removed, and then the waveform is shaped by a waveform shaping circuit 18, and the output signal (C) of this waveform shaping circuit 18 is sent to a first counter 19 as a count-up signal. . On the other hand, the head switch pulse H8W is frequency-divided by the frequency dividing circuit 21 in the blank detection section 15 into a signal with a period of about 3 seconds, and further waveform-shaped by the pulse shaping circuit 22, and outputted as a reset pulse R8T. . Therefore, while the tape is being fast-forwarded by the forward search cue, the reset pulse R8T causes the first counter 1 to
9 and the second counter 24 are reset at regular intervals.

The first counter 19 counts the dropout detection signal C from the waveform shaping circuit 18, and the second counter 24 counts the output pulses D from the pulse generation circuit 23. In this case, the output signal of the pulse generation circuit 23 is a continuous pulse equivalent to that of the dropout detection circuit, so while the recording signal is being read from the video tape by the video heads 11a and 11b, the first counter 19 and The two counters 24 perform exactly the same counting operation as shown in FIG. 2, which shows the changing state of the least significant bit (SB), and the contents of the counts are the same.

The videotape is then run to an area where no video signal is recorded, and the output of the AGC circuit 14 becomes ``0''.
When the temperature reaches 0.degree., the output signals of the dropout detection circuit 16, the low-pass filter 11, and the waveform shaping circuit 18 become constant levels, and the counting operation of the first counter 19 stops.

However, since the second counter 24 is counting the output pulses D from the pulse generation circuit 23, the second counter 24 continues the counting operation. At this point, the output signals (E) and (F) of the first counter 19 and the second counter 24 are both zero, and the output of the EX OR circuit 26 is held at "0". Thereafter, the second counter 24 counts up sequentially, and when the most significant bit MSB reaches ``1'' after a little less than 3 seconds have passed since the start of the counting operation, that is, the output of the first counter 19 When the output (F) of the second counter 24 becomes 1'' while (E) is in the state of “O”, a “1” signal is output from the EX OR circuit 26. Then, the output signal of this EX OR circuit 26 ( G) is blank detection section 1
5 is sent to the CPU as a blank detection signal. CPU
When a blank detection signal is sent from the EX-OR circuit 26, the videotape stops running, and if necessary, fast-forwards the videotape in the reverse direction for about 2 to 3 seconds.
Return to the recording end and stop. The beginning of the videotape is located as described above.

In the above embodiment, the first counter 19 and the second counter 24 are provided, and the blank detection signal is obtained by comparing the outputs of both counters 19.24.
Alternatively, the blank detection signal may be obtained as shown in FIGS. 4(a) and 4(b). That is, in FIG. 4(a), the dropout detection signal is counted by the sampling counter 31, and the count output is inputted to the count terminal of the counter 32. The sampling counter 31 has a length of 1.5 m for each track, for example.
Sampling and counting the input signal 16 times within 5ec,
A No blank signal NBL (L) is output when the number of low level signals is a certain number or more, and a blank signal BL (H) is output when the number of low level signals is less than a certain value. The counter 32 receives a blank signal 8 output from the sampling counter 31.
The count-up operation is performed by L. Further, this counter 32 is reset by the output of the counter 33. This counter 33 is a counter that determines the search period, counts the low clock pulses ck, and provides a reset signal to the counter 32 at regular intervals. Therefore, if the counter 32 overflows before being reset by the counter 33, a blank detection signal is output from the counter 32,
Sent to CPU.

In FIG. 4(b), the counter 33 is omitted in the circuit of FIG.
2 is being reset. That is, in FIG. 4(b), when the sampling counter 31 outputs the blank signal B for a predetermined number of times, the counter 32
A blank detection signal is output from the sampling counter 31 and sent to the CPLJ.If the No blank signal NBL is output even once while the blank signal 81 is being output from the sampling counter 31, the counter 32 is reset. , no blank detection signal is output.

In each of the above embodiments, forward search at 11 times speed has been described, but the present invention can also be applied to backward search, and the search speed is not limited.

[Effects of the Invention] As detailed above, according to the present invention, the discontinuation of the envelope signal between tracks is utilized, the signal is converted into pulses and counted, and the relative position/time with respect to recorded information is detected. Therefore, there is no need to record a position readout signal, and even if the videotape is being fed at high speed, the end or start position of the recorded information can be accurately detected. Furthermore, since the position information is detected digitally, the detection operation can be performed very stably, and the same operation can be performed even with different devices and tapes. It is not affected by the winding position.

[Brief explanation of drawings]

The drawings show an embodiment of the present invention, and FIG. 1 is a block diagram showing the circuit configuration, FIG. 2 is a timing chart to explain the operation, and FIG. 3 is a track diagram for forward search at 11x speed. A diagram showing the relationship between and video head scanning,
FIGS. 4(a) and 4(b) are block diagrams showing other embodiments of blank detection. 11a, 11b... Video head, 12... Playback amplifier, 13... Bypass filter, 14... AGC
Circuit, 15... Blank detection section, 16... Dropout detection circuit, 17... Low pass filter, 18.
... Waveform shaping circuit, 19... First counter, 21...
- Frequency division circuit, 22... Pulse shaping circuit, 23... Pulse generation circuit, 24... Second counter, 25... Count start/stop division circuit, 26... Exclusive OR circuit (EX OR circuit), 31...Sampling counter, 32.33...Counter.

Claims (1)

[Claims] A blank detection device for a magnetic recording and reproducing device using a rotary head includes a dropout detection circuit that detects dropout of tape read information during high-speed playback, and a counter that counts dropout detection signals output from the dropout detection circuit. From the contents of this counter,
A blank detection device comprising means for detecting a state in which the dropout detection signal is interrupted for a set time or more and outputting a blank detection signal.