Recording circuits for colour video signals

Abstract: A recording circuit includes a frequency modulator (2) for frequency modulating an input signal, such as luminance signal of a colour video signal, a phase inverter (11) for phase-inverting a signal with a predetermined frequency which is included in an output signal of the frequency modulator (2) and which is to be cancelled therefrom, a limiter (12) for amplitude-limiting an output signal of the phase inverter (11), and an adder (3) in which the output signal of the limiter (12) and another signal, such as a low-frequency converted carrier chrominance signal of the colour video signal which has a carrier frequency equal to the predetermined frequency, are mixed prior to being recorded on a recording medium (6).

Description: This invention relates to recording circuits for colour video signals, and suitable for a video tape recorder (VTR), video disc apparatus or the like. When a colour video signal is recorded on a magnetic tape by a VTR which, for example, employs a carrier chrominance signal low frequency converting system, the colour video signal is separated into a luminance signal and a chrominance signal, the luminance signal is frequency modulated (FM) to provide an FM luminance signal, and the chrominance signal, originally having a carrier frequency of 3.58 MHz in the case of the NTSC system, is low frequency converted to a low frequency converted carrier chrominance signal having a carrier frequency of, for example, 668 kHz. Then, the FM luminance signal and the low frequency converted carrier chrominance signal are mixed or combined and the resulting combined video signal is recorded on the magnetic tape. When a video signal thus recorded on the magnetic tape is reproduced, a combined video signal reproduced from the magnetic tape by a magnetic head is separated into the FM luminance signal and the low frequency converted carrier chrominance signal. The FM luminance signal is demodulated to provide the luminance signal, and the low frequency converted carrier chrominance signal is frequency converted back to the original carrier frequency of 3.58 MHz. Then, the demodulated luminance signal and the reconverted carrier chrominance signal are mixed to provide a reproduced colour video signal. To obtain the FM luminance signal having a centre frequency f r from the combined video signal reproduced by the head, it has been proposed to remove the low frequency converted carrier chrominance signal having a carrier frequency f c by means of a trap circuit, or to pass only the FM luminance signal through a high-pass filter. Since the low frequency converted carrier chrominance signal has no frequency deviation, and the frequency band thereof is narrow, the low frequency converted carrier chrominance signal can be readily removed by the trap circuit. However, a problem arises when using a trap circuit. More specifically, a cross colour may occur when a recorded video signal enters the carrier chrominance signal to give colour to an uncoloured portion of the picture. In such a case, it is difficult to separate the carrier chrominance signal from the recorded colour video signal. To avoid this problem, it has been proposed to connect a trap circuit, having a trapping frequency corresponding to that of the low frequency converted carrier chrominance signal, to the output of the frequency modulator for the luminance signal, before the resulting FM luminance signal and the low frequency converted carrier chrominance signal are mixed and then recorded on a magnetic tape. Figure 1 shows an example of a circuit which embodies this arrangement, and in which a luminance signal is applied to an input terminal 1. The luminance signal is frequency-modulated by an FM modulator 2 and fed to one input of an adder 3, in which it is added or combined with a low frequency converted carrier chrominance signal supplied to the other input of the adder 3 from a terminal 4. The resulting combined or mixed signal is recorded on a magnetic tape 6 by a magnetic head 5. Further, a trap circuit 7 is interposed between the FM modulator 2 and the respective input of the adder 3. The frequency of the trap circuit 7 is selected to be substantially the same as that of the signal to be mixed with the FM luminance signal, that is, the low frequency converted carrier chrominance signal. Figure 2 schematically shows a frequency spectrum of a standard FM signal derived from the FM modulator 2, and from which it is apparent that the odd-numbered side band components of the lower side band (LSB) and the upper side band (USB) are opposite in phase, whereas the even-numbered side band components of the lower and upper side bands are the same in phase as each other and as the carrier. Since the FM luminance signal and low frequency converted carrier chrominance signal are mixed by the adder 3 and then recorded by the circuit shown in Figure 1, a frequency spectrum as shown in Figure 3A has to be considered as possible for the output from the FM modulator 2. In other words, an odd-numbered side band component having a frequency equivalent to the frequency f c of the low frequency converted carrier chrominance signal may occur in the LSB, and may interfere with the low frequency converted carrier chrominance signal when mixed with the latter in the adder 3, thereby causing a beat interference. In an effort to avoid such interference, there is provided the trap circuit 7 whose trap frequency corresponds to the frequency f c of the odd numbered side band components which is the same as the frequency f c of the low frequency converted carrier chrominance signal with which the FM luminance signal is to be mixed. The odd-numbered side band component of the frequency f c in the LSB is removed by the trap circuit 7, as shown in Figure 3B. However, if the recording/reproducing system that follows the adder 3, such as, the magnetic head 5 and the magnetic tape 6, has a limiter characteristic, the USB side of the FM luminance signal is folded in the course of the recording and reproducing operations, whereby the odd-numbered side band component which had been trapped in the LSB is restored therein, as shown in Figure 3C. The frequency f c of the restored odd-numbered side band component is close to that of the low frequency converted carrier chrominance signal, so that this odd-numbered side band component and the low frequency converted carrier chrominance signal cause a beat interference to occur which deteriorates the quality of the reproduced picture. United States Patent US-A-3,893,168 discloses a video signal recording and playback system in which a sideband component is introduced into the signal prior to recording to cancel out an unwanted sideband introduced by saturation of the subsequent recording process. According to the present invention there is provided a recording circuit for recording a colour video signal including a luminance signal and a low frequency converted carrier chrominance signal comprising: frequency modulating means for frequency-modulating said luminance signal to produce a frequency modulated luminance signal; and means for recording an output signal on a recording medium; characterized by: phase inverting means for phase-inverting a lower sideband component of said frequency modulated luminance signal having a predetermined frequency substantially equal to the carrier frequency of said low frequency converted carrier chrominance signal to provide a frequency modulated luminance signal having said phase inverted lower sideband component; limiting means for amplitude-limiting said frequency modulated luminance signal having said phase inverted lower sideband component provided by said phase inverting means, said limiting means being operative to produce a further component having substantially the same frequency as said phase inverted lower side band component and a phase opposite thereto such that said lower sideband component is substantially cancelled by said further component; and adding means for mixing an output of said limiting means with a second input signal having a frequency substantially equal to said predetermined frequency to form said output signal. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like parts, and in which: Figure 1 is a block diagram showing an example of a previously proposed circuit including a trap circuit; Figures 2 and 3A to 3C are frequency spectrum diagrams to which reference is made in explaining a problem encountered in connection with the circuit shown in Figure 1; Figure 4 is a block diagram showing an embodiment of recording circuit according to the present invention; Figure 5 is a block diagram of a phase inverter in the circuit of Figure 4; Figure 6 shows the circuit of the phase inverter of Figure 5; and Figures 7A to 7C and 8A to 8B are schematic diagrams to which reference will be made in explaining the operation circuit of Figure 4. In the embodiment shown in Figure 4, a first limiter 10, a phase inverter 11 and a second limiter 12 are provided in sequence between the FM modulator 2 and the respective input of the adder 3. The limiter 10 is provided to limit the amplitude component of the FM luminance signal and thereby obtain an FM signal having no envelope component. The limiter 10 may be removed if an FM signal with no envelope component is already obtained from the FM modulator 2. The phase inverter 11 phase-inverts a signal of a particular frequency, for example, only a signal having a frequency corresponding to the frequency of a signal supplied to the adder 3 from the terminal 4, and which is preferably a low frequency converted carrier chrominance signal or AFM signal. The phase inverter 11 has a constant gain and may be formed as shown in Figure 5, by way of example. The phase inverter 11 of Figure 5 comprises a band-pass filter 14, an amplifier 15 and an adder 16. The band-pass centre frequency f₀ of the band-pass filter 14 is selected to be substantially equivalent to a frequency f₁ of the signal applied to the terminal 4. The signal including the frequency f₀ derived from the limiter 10 is supplied to one input of the adder 16, and is also supplied to the band-pass filter 14. The signal having the frequency f₀ passes through the band-pass filter 14, is doubled in amplitude and phase-inverted by the amplifier 15, and is then supplied to another input of the adder 16. Consequently, a signal having a frequency of -f₀ is supplied from the output of the adder 16. In other words, in the phase inverter 11 of Figure 5, a signal having a frequency f₀ substantially equal to the frequency f₁ of the signal applied to the terminal 4 is phase-inverted and supplied from the output of the adder 16 for application to the limiter 12 (Figure 4). In a practical circuit arrangement of the phase inverter 11, as shown in Figure 6, the signal including the frequency f₀ from the limiter 10 (Figure 4) is supplied to the base of the first transistor 17, and fed through the emitter of the transistor 17 and a resistor 18 to the base of a second transistor 19. This signal line corresponds to the signal path which extends directly from the limiter 10 to one input of the adder 16 in Figure 5. The signal applied to the base of the transistor 17 is derived, at the collector of the transistor 17, as a corresponding signal multiplied by minus two. A particular frequency component of such derived signal is extracted by a band-pass filter formed of a coil 20 and a capacitor 21, respectively, and is fed to the base of the transistor 19. In other words, the latter signal line including the coil 20 and capacitor 21 corresponds to the path extending to the other input of the adder 16 from the limiter 10 through the band-pass filter 14 and the amplifier 15 in Figure 5. The signals supplied through the resistor 18, and through the coil 20 and the capacitor 21, respectively to the base of the transistor 19 are added therein, and hence, a signal of -f₀ is developed at the output or emitter of the transistor 19. Turning back to Figure 4, it is to be noted that the limiter 12 is operated as a circuit for suppressing an amplitude modulated AM component contained in the phase-inverted signal from the phase inverter 11. Operation of the circuit of Figure 4 will now be explained with reference to Figures 7A to 7C and 8A and 8B. The input signal from the input terminal 1, for example, the luminance signal, is frequency-modulated by the FM modulator 2, and the AM component thereof is suppressed by the limiter 10 which provides an output signal as shown in Figure 7A. Figures 8A and 8B respectively illustrate examples of FM signals in which frequencies of the odd-numbered and even-numbered side band components become equivalent to the frequency f₁ of the signal applied to the terminal 4 in figure 4. More specifically, Figure 8A shows odd-numbered side band components having the frequency f₁ which appear when the modulation degree is small, whereas Figure 8B shows even-numbered side band components having the frequency f₁ which appear when the modulation degree is large. When the signal shown in Figure 7A is emitted from the output of the limiter 10, the odd-numbered side band components are as shown in the left-hand side portion of Figure 8A, and the even-numbered side band components are as shown in the left-hand side portion of Figure 8B. It will be understood from Figure 8A that the odd-numbered side band components in the lower and upper side bands LSB and USB are in opposite phase relationship to each other, whereas, it will be understood from Figure 8B, that the even-numbered side band components in the lower and upper side band are in an equi-phase relationship with the carrier. When the signal from the limiter 10 is supplied to the phase inverter 11 in which the signal having the frequency f₁ is phase inverted, the result thereof is to provide the envelope of the signal f₁ with an AM component, as shown in Figure 7B. From the central portions of Figures 8A and 8B, it will be seen that the signal of the frequency f₁ in the LSB is phase-inverted by the phase inverter 11. Thus, the AM component of the envelope of the signal having the frequency f₁ is caused by the phase inversion. The signal having the frequency f₁ is merely phase-inverted by the phase inverter 11, and then is supplied to the limiter 12, in which the described AM component of the envelope is suppressed, so that the limiter 12 provides at its output a signal as shown in Figure 7C. In other words, AM components which cancel the AM components of the output from the phase inverter 11, and which are shown within parentheses at the central portions of Figures 8A and 8B, are added in the limiter 12 to the output from the phase inverter 11. It is clear from the right-band side portions of Figures 8A and 8B that the odd-numbered side band components are cancelled in both the USB and LSB (Figure 8A) and that the even-numbered side band components are cancelled only in LSB (Figure 8B). The signal developed at the output side of the limiter 12 is supplied to the adder 3, in which it is added to the other signal applied from the terminal 4, and the resulting combined signal is recorded on the magnetic tape 6 by the magnetic head 5. The envelope of the signal from the limiter 12 contains no AM component so that, even when it is processed by a recording system having limiter characteristics, the restoration of the side band components at the frequency f₁ can be prevented. By way of summary, it is to be noted that the phase inverter 11 acts on the output of the FM modulator 2 to phase-invert only the signal having the particular frequency f₁ which it is desired to cancel before the FM signal is mixed with another signal, such as the low-frequency converted chrominance signal, whereupon, the output of the phase inverter is amplitude-limited. Thus, when the FM luminance signal is added or combined with the low-frequency converted carrier chrominance signal and is recorded on the recording medium, regardless of the limiter characteristic of the recording system, interference of the FM luminance signal with the low frequency converted carrier chrominance signal can be prevented for avoiding a beat interference.

Claims: A recording circuit for recording a colour video signal including a luminance signal and a low frequency converted carrier chrominance signal comprising: frequency modulating means (2) for frequency-modulating said luminance signal to produce a frequency modulated luminance signal; and means (5) for recording an output signal on a recording medium (6); characterized by: phase inverting means (11) for phase-inverting a lower sideband component (f₁) of said frequency modulated luminance signal having a predetermined frequency substantially equal to the carrier frequency of said low frequency converted carrier chrominance signal to provide a frequency modulated luminance signal having said phase inverted lower sideband component; limiting means (12) for amplitude-limiting said frequency modulated luminance signal having said phase inverted lower sideband component provided by said phase inverting means (11), said limiting means being operative to produce a further component having substantially the same frequency as said phase inverted lower side band component and a phase opposite thereto such that said lower sideband component is substantially cancelled by said further component; and adding means (3) for mixing an output of said limiting means (12) with a second input signal having a frequency substantially equal to said predetermined frequency to form said output signal. A circuit according to claim 1 wherein said recording medium (6) is a magnetic tape (6). A circuit according to claim 1 wherein said phase inverting means (11) includes a band-pass filter (14), an amplifier (15), and an adder (16) for adding said frequency-modulated output of the frequency modulating means (2), after being passed through said band-pass filter (14) and amplified by said amplifier (15), to said frequency-modulated output in an opposite phase. A circuit according to claim 3 wherein said phase inverting means (11) includes first and second transistors (17, 19) each having base, emitter and collector electrodes, means for applying said frequency-modulated output from the frequency modulating means (2) to said base electrode of the first transistor (17), means connecting said emitter electrode of said first transistor (17) to said base electrode of said second transistor (19), means for deriving said frequency-modulated output from said collector electrode of said first transistor (17) as a corresponding signal multiplied by minus two, a coil (20) and a capacitor (21) connected between said collector electrode of said first transistor (17) and said base electrode of said second transistor (19) and forming said band-pass filter (14), and means for deriving said output of the phase inverting means (11) from said emitter electrode of said second transistor (19).

Inventors: GANSE SHIGERU C O PATENTS DIVI, GANSE, SHIGERU C/O PATENTS DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1989
Main classification: H04N9
IPC codes: H04N9, H04N5