Video signal processing circuit

Abstract: A video signal processing circuit for reproducing an FM video signal. A video reproduction signal output from a magnetic head (11) is supplied to a low-pass filter (13), which extracts a chroma signal. The video reproduction signal is also supplied to a chroma signal removing circuit (18) through a high-pass filter (15) of low degree and sharpness. The chroma signal removing circuit (18) removes a chroma signal component. The output of the chroma signal removing circuit (18) is amplitude-limited to a predetermined degree by a limiter (16), and supplied to a demodulator (17) which extracts a luminance signal.

Description: This invention relates to a circuit for use in a video tape recorder or the like, and more particularly to a video signal processing circuit comprising a first low-pass filter for extracting a chroma signal component from a video reproduction signal including the chroma signal component and a luminance signal component; a first high-pass filter supplied with the video reproduction signal; and a chroma signal component removing circuit for extracting a chroma signal component from the first high-pass filter and for removing the chroma signal component from the output of the first high-pass filter. Such a video signal processing circuit is disclosed in EP-A-0311117. In a video tape recorder (hereinafter abbreviated to as VTR), an FM luminance signal and a low-pass filtered chroma signal are recorded on magnetic tape. As shown in Fig. 1, in the FM converted luminance signal (Y), the syncchip carrier and the white peak have frequencies of 3.4 MHz and 4.4 MHz, respectively. The frequency of the low-pass filtered chroma signal (C) is 625 KHz. Thus, the FM luminance signal and the low-pass filtered chroma signal have different frequency bands. In general, filters are used to separate and extract the above-mentioned signal components from the video signal obtained by playing back the magnetic tape. More specifically, a low-pass filter (LPF) extracts a chroma signal component, and a high-pass filter (HPF) extracts a luminance signal component. Fig. 2 is a circuit diagram showing the conventional video signal processing circuit for separating and extracting those signal components from the FM video reproduction signal. The signal reproduced by magnetic head 11 is amplified by preamplifier 12. The output of preamplifier 12 is supplied to LPF 13, in which a chroma signal (C) is extracted from the video reproduction signal. The output of preamplifier 12 is also supplied through FM-AGC (FM-automatic gain control) circuit 14 to HPF 15. The output of HPF 15 is amplitude-limited to a predetermined degree by limiter 16, and then supplied to demodulator 17, in which the luminance signal (Y) is extracted. In the conventional video signal processing circuit, HPF 15 must be of high degree and sharpness (Q) to obtain sufficient separation characteristics. Since a filter circuit of high degree and sharpness requires a large number of elements and is therefore expensive, the cost of manufacturing the circuit is high. Moreover, a great phase shift may be made in such a filter circuit, and it is difficult to compensate the phase shift. Further, when a filter circuit of a high sharpness is used as a HPF, since the low-pass component is removed from the FM signal, a high-pass component is excluded from the demodulated luminance signal, resulting in an image having poor frequency characteristics (low shade gradient). On the other hand, when a filter circuit of low degree and sharpness is used as HPF 15, a low-pass converted chroma signal component is regarded as a lower band wave of the FM signal, and included in a luminance signal component. The present invention has been made in consideration of the above-described circumstances, and its object is to provide a video signal processing circuit in which a high-pass component is maintained in the demodulated luminance signal, and a chroma signal component is excluded from the luminance signal component. According to the present invention, there is provided a video signal processing circuit comprising: a first low-pass filter for extracting a chroma signal component from a video reproduction signal including the chroma signal component and a luminance signal component; a first high-pass filter supplied with the video reproduction signal; and a chroma signal component removing circuit for extracting a chroma signal component from an output of the first high-pass filter and for removing the chroma signal component from the output of the first high-pass filter, characterised by a demodulator for receiving the output of said chroma signal component removing circuit and reproducing a luminance signal. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a diagram showing a frequency distribution state of an FM video reproduction signal; Fig. 2 is a circuit diagram showing a conventional video signal processing circuit; Fig. 3 is a circuit diagram showing a video signal processing circuit according to an embodiment of the present invention; and Fig. 4 is a circuit diagram showing a video signal processing circuit according to another embodiment of the present invention. As shown in Fig. 3, a video reproduction signal output from magnetic head 11 is amplified by preamplifier 12. An output of preamplifier 12 is input to LPF 13, which extracts a chroma signal component. An output of preamplifier 12 is also input through FM-AGC (FM-automatic gain control) circuit 14 to HPF 15. HPF 15 is of a low degree and sharpness (Q). Hence, HPF 15 outputs a reproduced FM signal including a chroma signal components having frequencies around 629 KHz. The output of HPF 15 is supplied to chroma signal removing circuit 18. Chroma signal removing circuit 18 includes HPF 19 which receives the output of HPF 15, amplifier 20 which receives the output of HPF 19, LPF 21 which receives the output of amplifier 20, and subtracter 22 for subtracting the output of LPF 21 from the output of HPF 15. In chroma signal removing circuit 18, HPF 19 removes signal components of the frequencies lower than that of the chroma signal component from the output of HPF 15. The output of HPF 19 is supplied to amplifier 20 which adjusts the amplitude of the chroma signal component appropriately. LPF 21 removes signal components of the frequencies higher than that of the chroma signal component from the output of amplifier 20. Hence, LPF 21 outputs only the chroma signal component of the frequency around 629 KHz. Subtracter 22 subtracts the output of LPF 21 from the output of HPF 15 and outputs a signal excluding the chroma signal component but including the low-pass component of the FM signal. The output of chroma signal removing circuit 18 is amplitude-limited by limiter 16 and supplied to demodulator 17, in which a luminance signal is extracted. In the above embodiment, although two high-pass filters, HPFs 15 and 19 are used, both filters can be of low degree and sharpness. Hence, increase in cost is prevented. In addition, since HPF 15 has low degree and sharpness, the phase shift is rarely made and can be compensated easily. Further, since a high-pass component is maintained in the demodulated luminance signal, an image of high frequency characteristics, in which shade gradient is great, can be obtained. The present invention is not limited to the above-described embodiment. For example, according to the above embodiment, the output of HPF 15 is supplied to HPF 19; however, the output of FM-AGC circuit may be supplied to HPF 19. Also, according to the above embodiment, the signal processing circuit comprises a chroma signal removing circuit including an HPF, an amplifier, an LPF, and a subtracter as shown in Fig. 3; however, as shown in Fig. 4 which illustrates another embodiment of the invention, the chroma signal removing circuit may be replaced by transversal filter 23 which is constructed so as to remove the chroma signal component having the frequency of around 629 KHz from the output of HPF 15. As has been described above, according to the image signal processing circuit of the present invention, a high-pass component is maintained in the demodulated luminance signal, and a chroma signal component is excluded from the luminance signal component. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A video signal processing circuit comprising: a first low-pass filter (13) for extracting a chroma signal component from a video reproduction signal including the chroma signal component and a luminance signal component; a first high-pass filter (15) supplied with the video reproduction signal; and a chroma signal component removing circuit (18) for extracting a chroma signal component from an output of the first high-pass filter and for removing the chroma signal component from the output of the first high-pass filter, characterised by a demodulator (17) for receiving the output of said chroma signal component removing circuit (18) and reproducing a luminance signal. A video signal processing circuit according to claim 1, characterised in that said chroma signal component removing circuit (18) comprises: a second high-pass filter (19) which receives the output of said first high-pass filter (15); an amplifier (20) which receives the output of said second high-pass filer (19); a second low-pass filter (21) which receives the output of said amplifier (20); and a subtracter (22) for subtracting the output of said second low-pass filter (21) from the output of said first high-pass filter (15). A video signal processing circuit according to claim 1, characterised in that said chroma signal component removing circuit comprises a transversal filter (23) for extracting the chroma signal component from the output signal of said first high-pass filter (15).

Inventors: HAGINO HIDEYUKI C O INTELLECTU, HAGINO, HIDEYUKI, C/O INTELLECTUAL PROPERTY
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H04N9
IPC codes: H04N9