JPH0686225A - Reproduction mode discrimination circuit - Google Patents

Abstract

PURPOSE:To perform discrimination with universal constitution by comprising a discrimination circuit of an AGC circuit which supplies the demodulated output of a frequency demodulator, a signal sampling circuit which detects the DC level of the demodulated output by sampling envelope detection output in a synchronizing signal period from the AGC circuit, and a comparator which compares the detection level with a reference level. CONSTITUTION:The AGC circuit 8 is used in AGC processing in recording also in mode discrimination in reproduction, and the demodulated output of the frequency demodulator 2 is supplied in the reproduction, and the demodulated output is envelope- detected by a detector 8b via an amplifier 8a, and the detection output of an AGC detection voltage changing linearly for the frequency deviation of an FM luminance signal is supplied to the amplifier 8a and the signal sampling circuit 9. Thence, the detection output in the synchronizing signal period of the FM luminance signal is sampled and held by the synchronous pulse of an input terminal 6 at the circuit 9, and the hold output of DC level is supplied to the comparator 11 via an amplifier 10. After that, the processing characteristic of the reproduction circuit part of a low- pass filter 3, etc., is switched by the discrimination signal D of the comparator 11, and a reproduction mode can be automatically set.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reproduction mode discriminating circuit provided in a video processing device such as an 8 mm VTR.

[0002]

2. Description of the Related Art Conventionally, in a 8 mm VTR, particularly when a tape cassette suitable for a high band 8 mm (hereinafter referred to as Hi8) is used, recording in a mode (hereinafter referred to as normal mode) shown by frequency allocation in FIG. The recording in the Hi8 mode indicated by the frequency allocation in FIG. 4 can be freely performed by the user's selection. 3 and 4, a indicates a low-frequency converted chroma signal (low-frequency converted chroma signal), and a indicates an FM-modulated luminance signal (FM luminance signal).

The FM luminance signal in the normal mode has a sync signal tip of 4.2 MHz and a white peak of 5.4 MHz.
Is a signal with a frequency deviation of 1.2 MHz, and the Hi8 mode FM luminance signal is a signal with a frequency deviation of 2 MHz having a sync signal tip of 5.7 MHz and a white peak of 7.7 MHz. Since the luminance signal characteristics of the tape cassette recorded in the normal mode and the Hi8 mode differ depending on the frequency shift, the mode during reproduction is switched to the same mode as during recording and the processing characteristic during reproduction shifts in frequency. Need to change according to.

If the mode switching at the time of reproduction is carried out only by the judgment of the tape type based on the detection hole of the tape cassette, the judgment can be made especially for the Hi8 tape cassette capable of recording in both the normal mode and the Hi8 mode. I can't do it. Therefore, the conventional 8 mm VTR has a reproduction mode discriminating circuit having the FM demodulator configuration shown in FIG.

FIG. 5 shows a part of a reproduction circuit section for a luminance signal. In FIG. 5, reference numeral 1 denotes an FM which is separated and extracted from a reproduction signal of a video head through a preamplifier, a reproduction equalization circuit and the like.
Luminance signal input terminal, 2 is a signal reproducing FM demodulator connected to the input terminal 1, and 3 is a low-pass filter for extracting a necessary band provided in the next stage of the demodulator 2, and a de-emphasis circuit in the latter stage. The output signal is supplied to a processing circuit for the luminance signal such as.

Reference numeral 4 is a conventional reproduction mode discrimination circuit connected to the input terminal 1, and is formed by an FM demodulator 5 of model number CXA-2107 dedicated to mode discrimination. Reference numeral 6 denotes an input terminal for a sync pulse signal formed by synchronizing and separating the reproduction signal, and supplies the sync pulse signal to the discriminating circuit 4 as a timing control signal. Then, the FM luminance signal at the input terminal 1 is demodulated by the original reproduction processing route of the FM demodulator 2 and the low-pass filter 3 to extract the luminance signal in the required band, and this signal is a luminance signal processing circuit such as a de-emphasis circuit. Is supplied to a video output circuit or the like via.

Further, the FM luminance signal of the input terminal 1 is also supplied to the discriminating circuit 4, and the demodulator 5 of the discriminating circuit 4 is based on the synchronizing pulse of the input terminal 6 and detects the period (sync tip period) of the leading edge of the synchronizing signal. The demodulated DC level of the FM luminance signal is detected.

Further, since the DC level differs depending on the frequency of the leading edge of the sync signal, the demodulator 5 compares the DC level with the reference level to determine the reproduction mode suitable for the reproduction signal of the video tape. A discrimination signal d for mode switching, which changes in two values depending on the Hi8 mode, is generated.

The discrimination signal d allows the processing characteristics such as the frequency characteristic and the phase characteristic of each portion of the reproducing circuit portion such as the preamplifier, the reproducing equalizing circuit, the low-pass filter 3 and the like to be the characteristics of the normal mode and the Hi8 mode alternatively. The playback mode is automatically switched to the same mode as the recorded mode.

[0010]

The conventional reproduction mode discriminating circuit shown in FIG. 5 requires a special FM demodulator 5 dedicated to mode discrimination and cannot be easily formed.

Even in video processing devices other than the 8 mm VTR, the same problems as described above occur when determining the reproduction mode according to the frequency deviation of the reproduced FM luminance signal. An object of the present invention is to enable discrimination by forming with a general-purpose circuit configuration without using a special FM demodulator dedicated to discrimination.

[0012]

In order to achieve the above object, in the reproduction mode discriminating circuit of the present invention, a demodulation output of an FM demodulator for demodulating an FM luminance signal is supplied.
A GC circuit, a signal sampling circuit having a sample and hold circuit configuration for sampling the envelope detection output of the AGC circuit during the synchronizing signal period to detect the DC level of the demodulation output, and the detection level and reference level of the signal sampling circuit. And a comparator which determines the reproduction mode and outputs a determination signal.

[0013]

In the reproducing mode discriminating circuit of the present invention constructed as described above, the demodulation output of the original FM demodulator used for reproduction processing is supplied to the AGC circuit to obtain the envelope detection output of the demodulation output. This detection output is supplied to the signal sampling circuit to sample and hold the envelope detection output in the sync signal period to sample and detect the DC level of the demodulated FM luminance signal in the sync signal period.

Further, the comparator compares the detection level of the signal sampling circuit with the reference level, the reproduction mode is discriminated from this comparison, and a discrimination signal for mode switching is generated. Further, it is formed by a general-purpose circuit configuration using an AGC circuit, a sample and hold circuit, and a comparator, and a discrimination signal for mode switching can be output without using a conventional special FM demodulator dedicated to discrimination.

[0015]

EXAMPLE One example will be described with reference to FIGS. 5 applied to the 8 mm VTR.
The same reference numerals denote the same items, and the difference from FIG. 5 is that a reproduction mode discrimination circuit 7 is provided instead of the conventional discrimination circuit 4.

The discrimination circuit 7 includes an AGC circuit 8, a signal sampling circuit 9 having a sample and hold circuit configuration, an amplifier 10,
It is formed by a general-purpose circuit of the comparator 11. And AG
The C circuit 8 has a variable gain type AGC amplifier 8a and a gain control detector 8b, and is originally used for recording processing. The FM demodulator 2 and the signal sampling circuit 9 are used only during reproduction by switching between recording and reproduction. Connected between.

That is, the AGC circuit 8 uses the AGC during recording.
It is used for both processing and mode discrimination during playback, and FM during playback.
The demodulation output of the demodulator 2 is supplied, the demodulation output from the amplifier 8a is subjected to envelope detection by the detector 8b, and the frequency shift of the FM luminance signal (FM frequency shift) is performed as shown in FIG.
The detection output of the AGC detection voltage that changes linearly with respect to is supplied to the amplifier 8a and the signal sampling circuit 9. Then, the signal sampling circuit 9 samples and holds the detection output of the FM luminance signal in the synchronizing signal period by the synchronizing pulse of the input terminal 6, and supplies the DC level hold output to the comparator 11 via the amplifier 10.

At this time, the detection output during the synchronizing signal period is F
Based on the difference in frequency shift of the M luminance signal becomes V ₁ of the Figure 2 during regeneration of the normal mode recording, the time of reproducing the record Hi8 mode V ₂ of FIG. (> V _1). further,
The reference level of the intermediate value between V ₁ and V ₂ is set in the comparator 11, and the level of the input hold output (detection level)
Is compared with the reference level, and a discrimination signal D for mode switching similar to the discrimination signal d in FIG. 5 for setting the normal mode to the low level and the Hi8 mode to the high level is generated.

Then, the discrimination signal D switches the processing characteristics of each part of the reproduction circuit such as the preamplifier, the reproduction equalization circuit, and the low-pass filter 3, and the reproduction mode is automatically switched. Therefore, the conventional special FM demodulator 5
General purpose AGC circuit 8, sample-and-hold circuit (signal sampling circuit 9), amplifier 10, comparator 1 without using
A discriminating circuit 7 is formed by 1 to discriminate the reproduction mode as in the conventional case.

Moreover, in the case of this embodiment, the AGC circuit 8
Since it is also used for recording, there is an advantage that it can be formed at low cost. Further, in the above embodiment, the present invention is applied to the discrimination of the reproduction mode of 8 mm VTR, but it is applied to the discrimination of the reproduction mode of various video processing devices such as the discrimination of VHS and S-VHS in the reproduction mode of the VHS system VTR. Of course you can. At this time, the number of reproduction modes may be three or more, and the recording medium may be other than the magnetic tape. Also,
The amplifier 10 of the above embodiment may be omitted.

[0021]

Since the present invention is configured as described above, it has the following effects. A demodulation output of the FM demodulator 2 for reproduction processing for demodulating the FM luminance signal is
The demodulated FM is demodulated by supplying it to the GC circuit 8 to obtain the envelope detection output of the demodulation output, and also supplying this detection output to the signal sampling circuit 9 to sample and hold the envelope detection output of the synchronization signal period. The DC level of the luminance signal in the synchronizing signal period is detected, the comparator 11 compares the detection level of the signal sampling circuit 9 with the reference level, and the reproduction mode is discriminated from this comparison to generate a discrimination signal for mode switching. Therefore, it can be formed by a general-purpose circuit configuration using an AGC circuit, a sample and hold circuit, and a comparator, and outputs a discrimination signal for mode switching without using a conventional special FM demodulator dedicated to discrimination. The playback mode can be switched automatically.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a characteristic diagram of a detection output of the AGC circuit of FIG.

FIG. 3 is an explanatory diagram of frequency allocation in a normal mode of 8 mm VTR.

FIG. 4 is an explanatory diagram of frequency allocation in high band 8 mm (Hi8) mode of 8 mm VTR.

FIG. 5 is a block diagram of a conventional circuit.

[Explanation of symbols]

 2 FM demodulator 7 Playback mode discrimination circuit 8 AGC circuit 9 Signal sampling circuit 11 Comparator

Claims (1)

[Claims] 1. A video process for switching to a reproduction mode having a processing characteristic according to a frequency deviation of an FM luminance signal included in a reproduced signal of a recording medium, demodulating the FM luminance signal by an FM demodulator, and reproducing the image. A reproduction mode discriminating circuit provided in the device for discriminating a reproduction mode according to the frequency shift and generating a discriminating signal for mode switching, an AGC circuit to which a demodulation output of the FM demodulator is supplied, and the AGC circuit. A signal sampling circuit having a sample-and-hold circuit configuration for sampling the envelope detection output during the synchronizing signal period of the circuit to detect the DC level of the demodulation output, and a reproduction mode by comparing the detection level of the signal sampling circuit with a reference level. And a comparator for outputting the discrimination signal.