This invention relates to a recording and reproducing system for color video signals, and more particularly to an improved chroma signal recording and reproducing system which minimizes deviation of hues from the original as well as that of color saturation from the original and eliminates drop-out or missing of color information, thereby ensuring a high quality of reproduced pictures.
Various types of chroma signal recording and reproducing systems have hitherto been proposed. Two types of the low-band conversion type and the color-difference line-sequential FM type are typical of those commonly employed in a magnetic picture recording and reproducing apparatus for commercial use such as video tape recorders. The basic principle, merits and demerits of these two types will now be described.
(1) Low-band conversion type:
According to this type, a subcarrier is subjected to quadrature two-phase balanced modulation by two color difference signals to obtain a modulated chrominance signal commonly used in the NTSC system or PAL system, the frequency band of the chrominance signal being then converted into that lower than the FM band of a luminance signal, and this low-band converted chrominance signal is superposed on the FM luminance signal to be recorded together with the luminance signal. In the playback mode, the low-band converted chrominance signal is separated from the reproduced signal to be then restored to the original high-band chrominance signal by a frequency converter.
Therefore, the low-band conversion type exhibits the following merits (a-1) to (a-4) among others.
(a-1) By virtue of the quadrature two-phase balanced modulation, two color difference signals can be recorded in multiplex without widening the frequency band occupied.
(a-2) There occurs a time base variation in the reproduced signal when the speed of the magnetic recording medium moving relative to the magnetic head varies. Because this time base variation causes merely a phase variation of the chrominance signal of a low frequency, the frequency band conversion after reproduction of the chrominance signal into its original high-frequency band alleviates the effect of the time base variation.
(a-3) The simple procedure of converting the frequency band of the chrominance signal is merely required for recording and reproducing color TV signals used in the NTSC system or PAL system.
(a-4) As the carrier chrominance signal is provided by modulation according to an AM mode, no beat interference occurs even when adjoining two tracks are scanned simultaneously by the magnetic head in the playback mode, and crosstalk can be easily eliminated by means of phase shift (PS) or phase inversion (PI). Thus, this type is suitable for guardband-less recording.
On the other hand, the low-band conversion type has demerits listed below.
(b-1) Since modulation is according to an AM mode, a level variation occurring in the reproduced signals cannot be removed, resulting in deviation of color saturation from the original.
(b-2) Since the carrier chrominance signal does not include the carrier component, a continuous reference carrier having the same frequency and phase must be provided and used for synchronous detection during demodulation of the color signal. However, when a time base variation is present in the reproduced chrominance signal, the reference carrier fails to completely follow up the time base variation resulting in deviation of hue from the original, even when, an APC circuit is incorporated to produce the reference carrier on the basis of the color burst signal.
(b-3) Since the low-band converted color signal is superposed on the FM luminance signal to be recorded together, when a distortion of third order typical of magnetic recording appears in the recording and reproducing system, a beat interference (moire) due to cross modulation appears in the reproduced luminance signal. In order to alleviate this moire interference, it is necessary to select the frequency of subcarrier converted into the low band at a special value to utilize the interleaving effect or to precisely restrict the level of the low-band converted color signal relative to the FM luminance signal.
In regard to the level variation discussed in (b-1), incorporation of a conventional AGC circuit which controls the gain on the basis of the detected burst level is effective for compensation of sensitivity difference between magnetic heads and for correction of the mean level in each field. However, it has been difficult for the AGC circuit to suppress an instantaneous level variation.
(2) Color-difference line-sequential FM type:
According to this type, two color difference signals are subjected to frequency modulation (FM), and FM color difference signals are alternately recorded at a time interval of one horizontal scanning period (1H). In the playback mode, in lieu of the color difference signal omitted in the record mode, the signal is obtained by delaying the reproduced signal by 1H.
This color-difference line-sequential FM type exhibits the following advantages (c-1), (c-2). (c-1) By the use of frequency modulation (FM), a level variation which may occur can be removed by a limiter, and any deviation of color saturation does not occur. (c-2) Similarly by the use of the frequency modulation, hue deviation does not occur even in the presence of a time base variation.
On the other hand, the color-difference line-sequential FM type has the following disadvantages of (d-1), (d-2).
(d-1) According to its basic principle, the color signal information drops out at alternate horizontal scanning line. Therefore, the vertical resolution of color becomes degraded.
(d-2) Since the frequency band used for recording FM color signals is generally set to be lower than the FM band of the luminance signal, even when an inclined azimuth arrangement is employed, simultaneous scanning the adjoining two tracks by the magnetic head results in beat interference. Therefore, this is not suitable for guardband-less recording, and it is difficult to increase the recording density.