1. Field of the Invention
The present invention relates to a color signal processing apparatus in a video signal recording and reproducing system such as a video tape recorder.
2. Description of the Prior Art
Heretofore, in recording a color television signal, it has been frequently practiced to record a carrier color (chrominance) signal of the color television signal by frequency converting or frequency shifting the same. For example, in a home video tape recorder, the carrier color signal in the color television signal was mixed with an output signal from a local oscillator in a frequency converter comprised of a balanced demodulator or the like for the frequency conversion to a low frequency band signal, which was recorded on a magnetic tape with a luminance signal.
When the carrier color signal which had been frequency converted to the low frequency band signal was to be reproduced, a color signal processing apparatus as shown in FIG. 1 was used. FIG. 1 is a block diagram which shows only that portion which processes the carrier color signal of the video tape recorder with a luminance signal processing circuit, a color demodulation circuit and succeeding circuit being omitted. A signal read by a magnetic head, not shown, is applied to a separation circuit, also not shown, for separating a low frequency carrier color signal from the luminance signal. The separated low frequency carrier color signal is applied to an input terminal 1 and then amplified by an amplifier 2 as required. The amplified low frequency carrier color signal from the amplifier 2 is supplied to a frequency converter 3, to which a signal for frequency conversion generated in a manner to be described later is also applied through a band pass filter 4. The frequency converter 3 thus converts the low frequency carrier color signal to a high frequency band signal which is equivalent to a standard color televison signal. The frequency converter 3 includes a non-linear circuit such as a multiplier circuit comprised of a balanced modulator or a differential amplifier, as is well known in the art. The carrier color signal which has been converted to the high frequency band signal by the frequency converter 3 is then applied to a band pass filter 5 where spurious frequency components are eliminated, thence to an output terminal 6. Other processes such as demodulation or the like may be carried out as required. The signal for the frequency conversion to be supplied to the frequency converter 3 is generated in the following manner. The high frequency carrier color signal from the band pass filter 5 is applied to a burst gate 7, which gates a color burst signal during a horizontal retrace period under the control of a horizontal synchronizing signal supplied from a synchronizing signal input terminal 8. A fixed frequency oscillator 9 oscillates at a frequency (f.sub.C) which is approximately equal to that of a carrier wave of the carrier color signal which has been converted to the high frequency band signal. The oscillation output from the oscillator 9 and the color burst signal from the burst gate 7 are applied to a phase detector 10, which produces a voltage corresponding to a phase difference between those two signals. The output voltage of the phase detector 10 controls an oscillation frequency of a voltage controlled oscillator 11 which has a center frequency which is equal to a carrier frequency (f.sub.LC) of the carrier color signal which has been converted to the low frequency band signal during recording. The oscillation frequency of the voltage controlled oscillator 11 changes in accordance with the output voltage of the phase detector 10. The output of the voltage controlled oscillator 11 and the output of the fixed frequency oscillator 9 are applied to the frequency converter 12 where they are mixed together to produce a frequency component equal to a sum of both frequencies (f.sub.C + f.sub.LC) through the band pass filter 4. The sum frequency (f.sub.C + f.sub.LC) signal is used as a frequency conversion signal for the carrier color signal. A loop including the phase detector 10 and the voltage controlled oscillator 11 constitutes an automatic phase control (APC) loop, which has been known to reduce a time base variation included in the reproduced color signal.
In the APC loop shown in FIG. 1, however, since the color burst signal is repetitively inserted at a horizontal scan frequency (i.e. at approximately 15,734 KHz hereinafter abbreviated as f.sub.H, in the NTSC system) as is well known in the art, the width of variation of the oscillation frequency of the voltage controlled oscillator 11 must be limited within .+-. f.sub.H /2 around the frequency f.sub.LC. This is because the color burst signal is not a continuous signal as described above but has side band components at the interval of the frequency f.sub.H so that if the width of variation of the frequency is too wide the phase lock will occur not only at a normal carrier wave frequency but also at a frequency which is an integral multiple of the frequency f.sub.H.
For this reason it has been difficult to mass produce an oscillator which has a narrow frequency variation width with a circuit including conventional L - C components.
Furthermore, the apparatus of FIG. 1 had a drawback of complexity in the circuit configuration in that two frequency converters were required and the filter for passing the frequency conversion signal was required.