JPS6113675B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention automatically keeps the frequency characteristics of the head and tape system of a video tape recorder constant at all times to prevent deterioration in image quality caused by head variations and differences in tape characteristics. It's something you try to get.

As shown in FIG. 2, the transmission frequency characteristics of the head/tape system of a video tape recorder decrease as the frequency range increases, so in order to transmit a frequency modulated signal, it is necessary to perform the opposite frequency correction. This is done through an equalizer having the characteristics of the curve shown in FIG. however,
The frequency characteristics of head tape systems vary depending on the type of tape and head, and change as the head wears out. It is nearly impossible to keep these characteristics constant at all times, so we had no choice but to set them at a suitable compromise. Additionally, devices with multiple heads required an equalizer for each head, making the circuit complex.

The present invention is a device that is particularly suitable for one-track, one-field recording employed in helical scan video tape recorders, etc., and is capable of automatically keeping the frequency characteristics of the head and tape system constant at all times. Examples of the present invention will be described below.

FIG. 1 is a frequency spectrum diagram of a signal recorded on a tape, hereinafter referred to as an RF signal, and shows a very general case. As shown in Figure 1, the color signal is low frequency converted and centered around the frequency indicated by C carrier.
The luminance signal is FM modulated, and the luminance signal is FM modulated around the frequency indicated by the Y carrier, each having a frequency band as shown in the figure. When this RF signal is reproduced, if the frequency characteristics of the tape head system are as shown in FIG. 2, the head output amplitudes of the C carrier and Y carrier will naturally differ. If the ratio of the signal current values of the C carrier and Y carrier during recording is constant, the ratio of the output currents of the C carrier and Y carrier depends on the curve shown in FIG. 2. In other words, by measuring the ratio of the amplitudes of the C and Y carriers of the head output during reproduction, the transmission characteristics of the tape head system can be determined to some extent. The present invention focuses on this point and changes the curve of the equalizer for correcting the frequency characteristics so that the frequency characteristics of the reproduced signal are always constant. This will be explained below with reference to the block diagram of FIG.

Reference numerals 1 and 2 are reproduction heads, which are usually also used for recording purposes. 3 and 4 are preamplifiers that amplify the RF signal that is the head reproduction output. In this example, a two-head helical scan system is used, in which one track is recorded and reproduced on a tape using one head, and one field is recorded on one track. 5 is a switch for alternately switching between the two heads; 6 is an equalizer amplifier whose gain frequency characteristics can be changed by an external control signal;
7 is an RF signal output terminal; 8 is a band filter that passes only the Y carrier frequency; 9 is a band filter that also passes only the C carrier frequency;
10 and 11 are detectors that detect the Y and C carriers respectively, 12 is a differential amplifier that detects and amplifies the difference between the two detection outputs, and 13 is the vertical one at the beginning of one field among the outputs of the differential amplifiers. This is a sample hold circuit that extracts only one horizontal scanning period (referred to as 1H) of a video blanking (referred to as V blanking) period and holds the value for one field. The equalizer amplifier 6 is controlled by the output signal of the sample and hold circuit 13.
The 1H period during the V blanking period, which is the sample and hold period, does not contain any video signal, and the black level is a constant value, so the
The FM RF signal always has a constant frequency,
This is because it is convenient to use for this purpose. FIG. 5 shows a specific configuration of the equalizer amplifier. 21 is an RF signal input terminal, 22 is a transistor forming an emitter follower, 23 is a resistor for impedance matching with the delay line 24, 24 is a delay line with a delay time τ, and the receiving end is received with high impedance. There is.
25 is an amplifier whose gain can be varied, 26 is a gain control signal input terminal, 27 is a differential amplifier, and 28 is an RF signal output terminal. Since the equalizer amplifier 6 is required to have a group delay characteristic that does not change even if the amplitude characteristic is changed, the sending end is matched with the configuration shown in FIG. When the sending end signal and the receiving end signal are combined by the differential amplifier 27, the output is 1-Kcosθ (K is a constant,
An amplitude characteristic expressed by a curve where θ is the product of angular frequencies ω and τ can be obtained.

The amplitude characteristic curve can be changed as shown in FIG. 6 by controlling the amplitude of one input signal of the differential amplifier 27. Therefore, the ratio of the reproduced outputs of the C carrier and the Y carrier is detected, and the correction curve of the equalizer amplifier 6 is changed so that the ratio is always constant.

According to the present invention, the curve of the equalizer amplifier 6 is controlled by comparing the playback output of the head for each field, so even if the head characteristics change during continuous playback or tapes with different characteristics are mixed. However, there is no need to readjust it each time. Furthermore, even when two heads alternately reproduce each track, the characteristics of the equalizer amplifier 6 are made equal in the first 1H of scanning, so there is an advantage that the equalizer amplifier 6 does not need to be provided for each head. Another advantage is that there is no need to insert a special control signal into the RF signal in advance, which is a great advantage in terms of circuit configuration.

As an application of the present invention, in the case of a video tape recorder in which one field is divided into four heads for recording and playback, the carrier of the first 1H video blanking period of the 1/4 field scanned by one head is used. It is obvious that if the configuration is configured to detect , it can be applied to the video tape recorder of the above type.

As described above, according to the present invention, it is possible to automatically control the frequency characteristics of the head and tape system to be constant, thereby eliminating deterioration in image quality caused by differences in tape characteristics and head variations. It is something that can be prevented.

[Brief explanation of the drawing]

Figure 1 is a frequency spectrum diagram of an FM-modulated video signal for recording on magnetic tape, Figure 2 is a characteristic diagram showing the transmission frequency characteristics of a general head tape system, and Figure 3 is a diagram of the transmission frequency characteristics of a reproduction equalizer amplifier. A characteristic diagram showing frequency characteristics, FIG. 4 is a block diagram of a frequency characteristic correction device in an embodiment of the present invention, FIG. 5 is a circuit diagram of an equalizer amplifier used in the same device, and FIG. 6 is a diagram of the equalizer amplifier of FIG. 5. FIG. 3 is a characteristic diagram showing changes in the correction curve of FIG. 1, 2...Head, 4, 3...Preamplifier, 5
...Selector switch, 6...Equalizer amplifier, 7
... Output terminal, 8, 9 ... Bandwidth filter, 10,
11...Detector, 12...Differential amplifier, 13...
Sample and hold circuit.

Claims (1)

[Claims] 1 A method for reproducing a plurality of video signals recorded by one or more magnetic heads on a single magnetic tape by adding the frequency-modulated carrier waves of different frequencies corresponding to each video signal. , among the reproduced signals, the reproduction output amplitudes of the plurality of carrier waves corresponding to the blanking period of the signal recorded on the first part of the track on the magnetic tape corresponding to the magnetic head are detected and compared with each other, and the difference is calculated. a detection means for holding a corresponding detection signal for a period when the magnetic head is on the track and outputting it as a control signal;
1. A frequency characteristic correction device comprising a frequency characteristic correction means capable of changing the value of K of a characteristic Kcosθ (K is a constant, and θ is a symbol representing frequency as an angle).