JP2700389B2 - Video tape recorder - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording circuit of a video tape recorder (VTR), and more particularly, to a pre-emphasis circuit and / or a white / dark clip circuit in an oblique scanning VTR. About. [Conventional technology and its problems] Oblique scanning VTR such as VHS, Beta, U-Mactic system
As shown in FIG. 7, a magnetic tape 2 is wound around a drum 1 in an Ω shape, and is scanned obliquely by a pair of video heads 3. By the way, even if the video head 3 is designed and manufactured so that the contact pressure of the video head 3 with the tape 2 is uniform over the entire track, the video head 3 may be damaged due to abrasion of the head due to aging and damage to the edge of the tape. The contact pressure decreases at the point where the contact starts and the point away from the tape (both ends of the track). That is, the contact pressure decreases at the entrance side portion 4a and the exit side portion 4b of the oblique track 4 shown in FIG. Reproduction of a track on which a video signal (composite video signal) is FM-recorded with a non-uniform contact pressure
The level of the envelope signal of the RF signal is reduced at the head switching points t ₁ , t ₂ , t ₃ and in the vicinity thereof as shown in FIG. 6 (B). As a result, the image quality at the upper end and the lower end of the reproduced TV image is reduced. Also, when the level drop is remarkable, the reversal phenomenon (white becomes black,
(A phenomenon that black portions become white) occurs, and the reproduced image quality may be extremely deteriorated locally. Also, in the portion where the signal level changes abruptly at the boundary between the vertical synchronizing signal and the video signal, the sudden change of the signal level is strengthened by the action of pre-emphasis, and a spike-like portion as shown in FIG. 5 occurs. There is. If the signal level decrease due to the decrease in the adhesive pressure of the magnetic head at both ends of the track and the noise due to the spike-like portion on the dark level side shown in FIG. 5 occur simultaneously, it becomes difficult to accurately detect the synchronization signal, and the synchronization is disturbed. May occur. Accordingly, an object of the present invention is to provide a video tape recorder capable of reliably and easily preventing disturbance of vertical synchronization and image reversal phenomenon based on a decrease in contact pressure of a video head with a tape at both ends of the tape. Is to do. [Means for Solving the Problems] In order to solve the above problems and achieve the above object, the first invention of the present application is a method of obliquely scanning a magnetic tape with a pair of rotating magnetic heads to convert a video signal into an FM signal. In a video tape recorder for recording and reproducing a recorded video signal alternately by the pair of rotary magnetic heads, a pre-emphasis circuit for giving a pre-emphasis to the video signal, and a control circuit for controlling the pre-emphasis circuit Is provided, the pre-emphasis circuit is configured to selectively obtain a first pre-emphasis characteristic and a second pre-emphasis characteristic having a smaller pre-emphasis amount than the first pre-emphasis characteristic. Sufficiently narrower than the width of the head switching pulse in response to the leading and trailing edges of the head switching pulse A first pulse generation circuit for generating a first pulse having a constant time width (T1); A predetermined time width (T4) from a point shortly before to the leading edge point and trailing edge point of the head switching pulse
A second pulse generation circuit for forming a second pulse having the following formulas: the first pulse obtained from the first pulse generation circuit and the second pulse obtained from the second pulse generation circuit A logical sum circuit for forming a logical sum output pulse with the logical output circuit, wherein the logical sum output pulse corresponds to both end regions of a recording track which occurs obliquely on the magnetic tape in a video signal recorded on the magnetic tape. And a pulse width sufficiently including an image portion corresponding to an upper end portion and a lower end portion of a reproduced display screen obtained when the recorded video signal is reproduced, and the OR circuit outputs the OR output. Supplying a pulse to the pre-emphasis circuit, wherein the pre-emphasis circuit is responsive to the OR output pulse prior to a period of occurrence of the OR output pulse. A video tape recorder, wherein the second pre-emphasis characteristic is selected, and the first pre-emphasis characteristic is selected during a period in which the OR output pulse is not generated. is there. A second invention of the present application is a video tape recorder that obliquely scans a magnetic tape with a pair of rotating magnetic heads, performs FM recording of a video signal, and alternately reproduces the recorded video signal by the pair of rotating magnetic heads. A pre-emphasis circuit for providing a pre-emphasis to a video signal, a clip circuit for clipping the video signal to which the pre-emphasis is applied by the pre-emphasis circuit, and a control circuit for controlling the clip circuit. The clipping circuit is configured to selectively obtain a first clip level and a lower second clip level, the control circuit comprising: a leading edge of a head switching pulse; In response to the trailing edge, a predetermined time width (T1) sufficiently smaller than the width of the head switching pulse A first pulse generating circuit for generating a first pulse; and a head which is responsive to a leading edge and a trailing edge of the head switching pulse, from a time point slightly before a leading edge time and a trailing edge time of the head switching pulse. A second pulse generating circuit for forming a second pulse having a predetermined time width (T4) from a leading edge point to a trailing edge point of the switching pulse; and the first pulse obtained from the first pulse generating circuit. A video signal comprising a pulse and a logical sum circuit for forming a logical sum output pulse of the second pulse obtained from the second pulse generating circuit, wherein the logical sum output pulse is recorded on the magnetic tape. The upper end of a playback display screen obtained when a recorded video signal is reproduced while corresponding to both end regions of a recording track that occurs obliquely on the magnetic tape in a signal and A pulse width sufficient to include an image portion corresponding to an end, wherein the OR circuit is configured to supply the OR output pulse to the clip circuit, and the clip circuit further comprises: The second clip level is selected in a period in which the OR output pulse is generated in response to the OR output pulse, and the first clip level is selected in a period in which the OR output pulse is not generated. The present invention relates to a video tape recorder characterized in that Embodiment Next, a VTR according to an embodiment of the present invention will be described with reference to FIGS. In FIG. 1, an automatic gain control (AGC) circuit 12, a clamp circuit 13, a pre-emphasis circuit 4, a white / dark clip circuit 15, and a FM are provided at an output stage of an input terminal 11 of an NTSC video signal (composite video signal). A modulation circuit 16, a recording amplification circuit 17, and a video head 3 are sequentially connected. The video head 3 is supported by the rotating drum 1 as in the case of FIG. 7, and scans the tape 2 obliquely. First and second constant circuits 19 and 20 are connected to the pre-emphasis circuit 14 via a switch 18 in connection with the present invention. First and second constant circuits 22 and 23 are also connected to the white / dark clip circuit 15 via a switch 21. The control circuit 24 of the switches 18 and 21 receives a head switching pulse. The first constant circuit 19 for pre-emphasis is a circuit for providing a circuit constant for obtaining a normal pre-emphasis characteristic shown by a characteristic line A in FIG. 4, and the second constant circuit 20 lowers the relative gain of pre-emphasis to cause an inversion phenomenon. This is a circuit for providing a circuit constant for obtaining a pre-emphasis characteristic shown by a characteristic line B in FIG. First constant circuit of white / dark clip circuit 22
Is a circuit for providing a circuit constant to obtain a normal white the clip level a ₁ and the dark the clip level a ₂ shown in FIG. 5, the second constant circuit 23 is lowered level to prevent inversion White a circuit for providing a circuit constant for obtaining the Kuritsupureberu b ₁ and dark the clip level b _2. The control circuit 24 in FIG. 1 is configured as shown in FIG. A terminal for inputting a head switching pulse for alternately switching the output of a pair of video heads during playback
A trigger pulse generating circuit 26 is connected to 25, and a first monomultivibrator (MMV) 28 is connected to the trigger output line 27.
And a second mono-multivibrator (MMV) 29. A third MMV 30 is connected to an output stage of the second MMV 29, and the third MMV 30 is connected to a set terminal S of a flip-flop 32 via a trigger pulse forming circuit 31. The trigger pulse output line 27 is connected to the reset terminal R of the flip-flop 32. One input terminal of the OR gate 33 is connected to the first MMV 28, and the other input terminal is connected to the Q output terminal of the flip-flop 32.
The output line 34 of the OR gate 33 is connected to the switches 18 and 21 shown in FIG.
Is connected to the control terminal. FIG. 3 shows a state at points A to F in FIG. The head switching pulse shown in FIG. 3A is input to a trigger pulse forming circuit 26, and a trigger pulse indicating the leading edge and the trailing edge is obtained on a line 27. The first MMV28 is triggered by the trailing edge time t ₅ and the leading edge point t ₁ of when to quenching pulse of FIG. 3 (A), generates a pulse of width T ₁ shown in FIG. 3 (B).
The second of MMV29 is triggered by the trailing edge time t ₅ and the leading edge point t ₁ of when to quenching pulse, generates a pulse of width T ₂ shown in FIG. 3 (C). Third MMV30 generates a pulse of width T ₃ shown in FIG. 3 is triggered by the trailing edge of the output pulse of the second MMV29 (D). Flip flop 32 is triggered by the edge time t ₄ after the output pulse of the third MMV30, is reset at the leading edge time t ₁ and a trailing edge time t ₅ of the scan Germany quenching pulse, FIG. 3 to the Q output terminal of the (E) Generate an output pulse. OR gate 33
Is the width T ₅ of the sum of the output pulse of the first MMV 28 in FIG. 3B and the output pulse of the flip-flop 32 in FIG.
Is output as shown in FIG. 3 (F). The output pulse width T1 of the _first MMV 28 is sufficiently smaller than the width T (one field period) of the head switching pulse.
The output pulse width T ₂ of the second MMV29 sum T ₂ + T ₃ of the output pulse width T ₃ of the third MMV30 is slightly narrower than the head when to quenching pulse width T. Accordance connexion, the output pulse width T ₄ of flip-flops 32 is sufficiently narrower than T. The pulse shown in FIG. 3E is located immediately before the head switching times t ₁ and t ₄ , and the pulse shown in FIG.
Since the pulse is located immediately after the head switching time point t _1, t _4, the output pulse head switching time point t ₁ of the OR gate 33, t ₄
It is located straddling. The contacts b of the switches 18 and 21 of FIG. 1 are turned on in response to the OR gate output pulse of FIG. 3 (F), and the contact a is turned on in response to the low level period of FIG. 3 (F). Become. When the contact b of the switch 18 is turned on, the pre-emphasis characteristic becomes the characteristic line B in FIG. 4, so that the rate of increase in the high frequency range is suppressed, and an FM recording state in which the inversion phenomenon hardly occurs is obtained. Further, when the contact b of the switch 21 is turned on, white chestnut tree next b ₁ of flop level Figure 5, the dark the clip level b ₂ next to FIG. 5, inhibition of the over sheet Ute becomes larger than before, inversion Is prevented from occurring. If the gain of pre-emphasis is reduced and the clip level is also reduced in all tracks, the frequency characteristics and waveform reproducibility deteriorate. On the other hand, in this embodiment, the relative gain of pre-emphasis is reduced and the clip level is reduced only at the head switching point where the inversion phenomenon is likely to occur, that is, at both ends of the track, and the remaining portion is in the same state as the conventional state. , A normal image can be obtained in most of the display screen. The image quality may be deteriorated in a small area above and below the display screen, but this does not cause a serious problem visually. In this method, a vertical synchronization signal can be accurately obtained, so that it is possible to prevent disturbance of vertical synchronization during reproduction. (Modification) The present invention is not limited to the above-described embodiment, but can be modified. For example, only one of pre-emphasis and white or dark clip may be changed at both ends of the track. Also, in FIG. 1, the first and second constant circuits 19, 20, 22, and 23 are selectively connected to the pre-emphasis circuit 14 and the clip circuit 15, but usually, both ends of the track are connected to the pre-emphasis circuit and the clip circuit. The gain and clip level of pre-emphasis at both ends of the track may be changed only by adding or subtracting a constant circuit during the unit corresponding period. [Effects of the Invention] As is clear from the above description, in the first invention of the present application,
The pre-emphasis amount is reduced in both end areas of the recording track and sections corresponding to the upper end and the lower end of the reproduction display screen. More specifically, the predetermined amount after the leading edge point and the trailing edge point of the head switching pulse. In both the period of the first pulse of the time width (T1) and the period of the second pulse of the previous predetermined time width (T4), the pre-emphasis amount is reduced so that not only the vertical synchronizing signal section but also the upper end of the reproduction screen is displayed. Since the pre-emphasis amount is reduced in the video part corresponding to the part and the lower end part, not only the vertical synchronization disturbance is prevented, but also the video inversion phenomenon in the video part before and after the vertical synchronization signal section is prevented. Can also be reliably achieved. Also, since a pulse for selecting the first and second pre-emphasis characteristics is formed by a combination circuit of the first and second pulse generation circuits and the OR circuit responsive to the head switching pulse, this pulse is formed. Can be performed accurately and easily. In the second invention of the present application, the first and second clip levels are selected in place of the selection of the first and second pre-emphasis characteristics in the first invention, and the first and second clip levels are selected. Is the same as the selection of the first and second pre-emphasis characteristics of the first invention in that the excessive pre-emphasis amount is not generated. The same effect as described above can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a part of a VTR according to an embodiment of the present invention, FIG. 2 is a block diagram showing a switch control circuit of FIG. 1, and FIG. 3 is FIG. FIG. 4 is a diagram showing the characteristics of the pre-emphasis circuit of FIG. 1, FIG. 5 is a diagram showing the clip level of the white / dark clip circuit of FIG. 1, and FIG. 7 is a waveform diagram showing the head switching pulse and the reproduced RF signal, FIG. 7 is a plan view showing the relationship between the rotating drum of the VTR and the tape, and FIG. 8 is a diagram showing a track on the tape. 14 ... pre-emphasis circuit, 15 ... white / dark clip circuit, 18 ... switch, 19 ... first constant circuit, 20 ... second constant circuit, 21 ... switch, 22 ... first constant Circuit, 23... Second constant circuit, 24... Control circuit.

Claims (1)

(57) [Claims] To apply pre-emphasis to a video signal in a video tape recorder in which a magnetic tape is obliquely scanned by a pair of rotating magnetic heads, a video signal is FM-recorded, and the recorded video signal is alternately reproduced by the pair of rotating magnetic heads. Are provided, and a control circuit for controlling the pre-emphasis circuit is provided. The pre-emphasis circuit has a first pre-emphasis characteristic and a second pre-emphasis characteristic having a smaller pre-emphasis amount. The control circuit is responsive to a leading edge and a trailing edge of a head switching pulse, and has a predetermined time width (T1) that is sufficiently smaller than the width of the head switching pulse. A first pulse generating circuit for generating one pulse, and a leading edge of the head switching pulse And a second time width (T4) having a predetermined time width (T4) from a point slightly before the leading edge point and the trailing edge point of the head switching pulse to the leading edge point and the trailing edge point of the head switching pulse in response to the trailing edge. A second pulse generation circuit that forms the first pulse and a logical sum of the first pulse obtained from the first pulse generation circuit and the second pulse obtained from the second pulse generation circuit A logical sum circuit for forming an output pulse, wherein the logical sum output pulse corresponds to both end regions of a recording track which occurs obliquely on the magnetic tape in a video signal recorded on the magnetic tape, The logical sum circuit has a pulse width sufficiently including an image portion corresponding to an upper end portion and a lower end portion of a reproduced display screen obtained when a recorded video signal is reproduced. The pre-emphasis circuit is configured to supply an output pulse to the pre-emphasis circuit, and the pre-emphasis circuit selects the second pre-emphasis characteristic during a generation period of the OR output pulse in response to the OR output pulse. And a video tape recorder configured to select the first pre-emphasis characteristic during a period when the OR output pulse is not generated. 2. To apply pre-emphasis to a video signal in a video tape recorder in which a magnetic tape is obliquely scanned by a pair of rotating magnetic heads, a video signal is FM-recorded, and the recorded video signal is alternately reproduced by the pair of rotating magnetic heads. A pre-emphasis circuit, a clip circuit for clipping the video signal to which the pre-emphasis has been given by the pre-emphasis circuit, and a control circuit for controlling the clip circuit, wherein the clip circuit is a first circuit. The control circuit is configured to selectively obtain a clip level and a lower second clip level, wherein the control circuit is responsive to a leading edge and a trailing edge of a head switching pulse. Generate a first pulse of a predetermined time width (T1) sufficiently smaller than the pulse width A first pulse generation circuit, and in response to a leading edge and a trailing edge of the head switching pulse, a leading edge time of the head switching pulse and a leading edge time of the head switching pulse and A second pulse generating circuit for forming a second pulse having a predetermined time width (T4) up to a trailing edge point, the first pulse and the second pulse obtained from the first pulse generating circuit An OR circuit for forming an OR output pulse with the second pulse obtained from the generating circuit, wherein the OR output pulse is provided on the magnetic tape in a video signal recorded on the magnetic tape. Images corresponding to both end regions of the recording track that occur obliquely and corresponding to the upper end and lower end of the reproduction display screen obtained when the recorded video signal is reproduced. A pulse width sufficient to include the portion, wherein the OR circuit is configured to supply the OR output pulse to the clipping circuit, and wherein the clipping circuit is responsive to the OR output pulse. The second clip level is selected during a period during which an OR output pulse is generated, and the first clip level is selected during a period when the OR output pulse is not generated. Video tape recorder.