JPH0722939Y2 - Special reproduction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a special reproduction circuit of a video tape recorder for performing special reproduction without noise using a storage circuit such as a field memory.

(B) Prior Art A special reproduction circuit using a field memory is disclosed in Japanese Patent Laid-Open No. 60-37890. This special playback circuit stores the video signal in the noise-free period of the playback video signal in the filled memory during even-speed playback, and replaces the playback video signal with the field memory during the noise occurrence period when the level of the playback FM video signal decreases The read signal thus obtained is selectively derived, and the noise component on the reproduction screen due to the reduction of the reproduction output level is removed.

On the other hand, the dropout generation circuit tends to malfunction during special reproduction. Therefore, recently, the dropout compensation circuit is configured to detect the period during which the level of the reproduced FM video signal decreases due to the special reproduction, and prohibit the dropout compensation only during the detected period.

(C) Problems to be solved by the invention However, as described above, during special reproduction, an envelope detection circuit that detects long-period attenuation of the reproduced FM video signal is required for both signal switching and dropout compensation.

(D) Means for Solving the Problems Therefore, the present invention is characterized in that the envelope detection circuit for dropout compensation is also used as the envelope detection circuit for signal switching.

(E) Operation Therefore, according to the present invention, the envelope detection output for dropout compensation is input to the signal switching circuit to perform signal switching.

(F) Embodiment Hereinafter, the present invention will be described with reference to an embodiment.

This embodiment employs the present invention in a double speed reproduction circuit of a two-head helical scan VTR, and FIG. 1 shows a circuit block diagram of this embodiment. The reproduced signal obtained by alternately selecting the output of the video head is a low-pass filter (1).
Is input to the high-pass filter (2) and separated into a reproduction low-pass color signal and a reproduction FM luminance signal. The reproduced low-pass color signal is input to the color signal processing circuit and converted into the original high-pass color signal. On the other hand, the reproduced FM luminance signal includes a first envelope detection circuit (5) having a long time constant, a second envelope detection circuit (4) having a short time constant, and a first changeover switch (SW).
It is input to the first contact (T1) of 1). The output levels of both envelope detection outputs periodically change according to the double speed, but the second envelope detection circuit (4) responds to all the attenuation, and the first envelope detection circuit (5) attenuates for a short time. Does not respond to (dropout). Therefore, the gate circuit (6) controls the passage of the second envelope detection output by using the first envelope detection output as a control input. That is, the gate circuit (6) derives the dropout detection output generated when the reproduced FM video signal is at a high level and switches the first changeover switch to the second contact side (T ₂ ) Is acceptable. This first
The output of the switch (SW1) is also input to the 1H delay circuit (7), and the output of the 1H delay circuit (7) is supplied to the second contact (T2) to form a feedback type dropout compensation circuit. There is. The dropout compensation output obtained from the first switch (SW1) is input to the luminance signal circuit (8) and converted into the original luminance signal. The mixing circuit (9) mixes this luminance signal with the above-mentioned high frequency color signal to form a composite video signal. The configuration described above is a well-known configuration as a double speed reproduction circuit of a two-head helical scan VTR having no field memory.

The feature of this embodiment resides in that the output of the dropout detection circuit is partially used in a known configuration in which double speed reproduction is performed using a field memory. The first detection output (FIG. 3B) obtained by detecting the reproduction FM luminance signal (A in FIG. 3) is attenuated only when the video head straddles the recording track. Therefore, the clamp circuit (11) for inputting the first envelope detection output clamps the attenuated portion of the first envelope detection output to V ₁ . Fig. 2 shows the circuit after this clamp circuit concretely. The connection point voltage of the _first and second resistors (R1) and (R2) connected between the power supply grounds is set to V ₁ Diode (D) between connection point and signal line
Are connected in the forward direction. The clamp output is input to the comparator (12) and waveform-shaped. The comparison level of this comparator (12) is V ₂ (V ₂
> V ₁ ) is set as the positive input of the operational amplifier (12a), and the clamp output is set as the negative input. Therefore, the comparator output (C) becomes high level when the reproduced FM video signal level becomes small, and becomes low level when it becomes large. The field memory (10), to which the comparator output (C) is input, inputs the low level output to be in the writing state and the high level output to be in the reading state. On the other hand, the second selector switch (SW2) that inputs the comparator output (C) is also switched to the second contact (T2) on the field memory side by inputting the high level output, and the low level output is input to the mixing circuit side. Is switched to the first contact (T1). Therefore, when a mixed output having a small amount of noise is input, the mixed output is written into the field memory (10) and, at the same time, is output via the second changeover switch (SW2). On the other hand, when the mixed output contains noise, the video signal read from the field memory (10) is derived. This field memory (8) is equipped with an AD conversion circuit on the input side and a DA conversion circuit on the output side, and writes and reads data with a digital memory. The AD and DA conversion frequencies are 3 for color subcarriers. It is set to double.

Further, the reason for providing the clamp circuit (11) in the present embodiment is that it is better to set the comparison level based on the attenuation part because the envelope detection output levels in the standard recording mode and the long-time recording mode are different. This is because accurate operation can be expected.

According to the present invention, since the long-cycle first envelope detection output for dropout detection is also used for signal switching of the field memory and the second changeover switch, a separate envelope detection circuit is provided. The structure is simpler and more effective than the case where it is provided.

[Brief description of drawings]

1 shows an embodiment of the present invention, FIG. 1 is an overall circuit block diagram, FIG. 2 is a specific main circuit diagram, and FIG. 3 is a main signal waveform diagram. (5) ... First envelope detection circuit, (4) ... Second
Envelope detection circuit, (6) …… Gate circuit, (12)
……comparator.

Claims (1)

[Scope of utility model registration request] 1. A first envelope detection circuit having a long time constant for inputting a reproduction FM luminance signal and outputting a first envelope detection output during special reproduction; and a reproduction FM luminance signal inputting a second envelope detection output. A second envelope detection circuit having a short time constant, a gate circuit which inputs both the envelope detection outputs and outputs a dropout detection output corresponding to an exclusive OR output, and a first envelope detection output which inputs an attenuation part A clamp circuit that clamps, a comparator that inputs the clamp output, a field memory that controls writing and reading of a reproduction color video signal based on the comparator output, and the reproduction color video signal or the above based on the comparator output. A video switch consisting of a selection switch for selecting the field memory output Special playback circuit of the recorder.