Video tape recorder for recording a video signal with an additional time-base reference signal

An apparatus for recording a video signal onto tracks of a recording tape includes circuitry for producing a first time-base reference signal which is a color burst signal composed of a first predetermined number of sinewave cycles and a second time-base reference signal which is a burst signal composed of a second predetermined number of sinewave signals. The second predetermined number of cycles is larger than the first predetermined number of cycles. The apparatus further includes superimposing circuitry for superimposing the first time base reference signal on an input video signal at intervals of a horizontal scanning period of the video signal and for superimposing the second time base reference signal on the input video signal at intervals of a predetermined number of horizontal scanning periods of the video signal. The apparatus further includes a modulator for modulating an output signal of the superimposing circuitry to obtain a modulated signal and a recording arrangement for recording the modulated signal onto the tracks of the recording tape.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a video tape recorder (VCR) of the helical 
scanning rotary head type for recording and reproducing video signals. 
2. Description of Prior Art 
A known component type VCR for recording and reproduction of video signals 
employs a method in which a timebase reference for the video signal is 
produced by inserting several sinewaves of color burst (referred to as a 
burst wave) after a negative synchronizing (sync) pulse for one horizontal 
scanning period, prior to frequency modulation of the video signal for 
recording. 
In reproduction of the video signal with this method, a zero-cross point of 
the burst wave in a demodulated video signal is detected for producing a 
playback pulse following a timebase variation. The playback pulse then 
actuates a PLL circuit to produce a playback clock signal. 
The demodulated video signal containing a timebase variation is converted 
to a digital signal by an A/D converter controlled by the playback clock 
and stored in a memory. It is then read out from the memory by a reference 
clock which carries no timebase variation so that a resultant reproduced 
video signal is free from timebase variation. 
However, the burst wave which is used for timebase correction is inserted 
in each horizontal scanning period and the timebase frequency of the NTSC 
system becomes as low as about 15 KHz. If the video signal is timebase 
extended for each horizontal scanning period and is recorded on a 
plurality of channels, the timebase frequency will further be decreased to 
about 7.5 KHz. 
When the timebase correction is carried out at such a low frequency, its 
response speed will remain low. Accordingly, an abrupt timebase change 
caused by the switching of heads affects video data recorded in the 
beginning of each recording track. This develops a duration, equal to a 
multiple of H (H: horizontal scanning period), where no timebase variation 
can be corrected throughout several horizontal scanning periods. As the 
result, a visual failure known as skew distortion will appear in the upper 
portion of a reproduced image. 
For eliminating the above failure, a modified method has been proposed as 
disclosed in laid-open Japanese Patent Application No. S63-61577, in which 
a negative signal which is longer in duration than a negative sync pulse 
provided in each horizontal scanning period is inserted in the beginning 
of each track prior to recording, and this negative signal is detected for 
timebase correction during reproduction. 
However, even if the duration of the negative signal is adequately long, 
only one edge is used as a timebase reference. This provides a precision 
of timing almost equal to that given by the negative sync pulse in each 
horizontal scanning period and the effect of skew distortion will hardly 
be eliminated. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to avoid the generation of visual 
error known as skew distortion which appears in the upper portion of a 
reproduced image due to an abrupt timebase change in the beginning of a 
track caused by the switching of the heads. 
According to an improved method of the present invention, in addition to a 
burst signal of sinewaves provided in each horizontal scanning period and 
acting as a first timebase reference signal, a series of sinewaves which 
is longer in duration than the burst signal is inserted into a video 
signal as a second timebase reference signal in 1H period prior to 
recording. During reproduction, a zero-cross point of the burst wave is 
detected for the correction of timebase variations and also, the second 
timebase reference signal is utilized for timebase correction of the 
beginning of each track at a high speed. 
More particularly, the response of the timebase corrector during a timebase 
correcting action can be optimized in speed at the beginning of each 
track. Consequently, skew distortion resulting from an abrupt timebase 
change in the beginning of a track and appearing in the portion of a 
reproduced image will be minimized.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a waveform diagram showing three scanning line periods of a video 
signal recorded at the beginning of each recording track using a VCR of 
the present invention. 
FIG. 1 shows a horizontal sync signal 1, a first timebase reference signal 
2, a second timebase reference signal 3 which is added to only at the 
beginning region of each track, and a picture signal 4. Waveforms after 
the second scanning period are identical to that of the second scanning 
period which comprises the first timebase reference signal 2 and the 
picture signal 4. 
FIG. 2 is a waveform diagram showing in more detail the first timebase 
reference signal 2, in which denoted by 5 is a burst wave consisting of 
four sine-wave cycles. Although the picture signal is a luminance signal 6 
in this embodiment, it may be a chrominance signal or TDM signal formed by 
timebase multiplexing of two, chrominance and luminance, signals. 
FIG. 3 is a waveform diagram showing the second timebase reference signal 
3, in which denoted by 7 is a burst wave consisted of a greater number of 
sine-wave cycles than that of the first timebase reference signal 2 for 
timebase reference. 
FIG. 4 illustrates the recording of those signals onto the recording tracks 
of a video tape 9. In particular, the second timebase reference signal 3 
is allocated to a region 8 on a recording track 10 of the video tape 9. 
Each recording track contains at the beginning region thereof a video 
signal component X containing the second timebase reference signal 3. 
Also, a series of video signal components A containing the first timebase 
signals 2 are recorded onto succeeding regions 11 of the recording track 
10 respectively. It is assumed that the scanning with a recording head is 
carried out in the direction denoted by the arrow 12. 
FIG. 5 shows a block diagram of a recording section of the VCR of the 
present invention. In operation, an input video signal containing the 
first timebase reference signals 2 is fed through an input terminal 13 to 
a sync separator circuit 14 where sync pulses are separated from the input 
video signal. The sync pulses are transmitted to a timing generator 
circuit 15 which in turn actuates a switch 16 for switching from the 
output of a memory 17 to the input video signal at the timing of the front 
end of each recording track. Each group of video signal components headed 
by the signal X is then fed through an FM modulator 18 to a rotary head 19 
for recording onto a video recording tape 20. 
FIG. 6 is a block diagram of a timebase corrector (TBC) in the VCR of the 
present invention for a reproducing operation. 
In action, a video signal read out from a recording tape 21 with a rotary 
head 22 is fed to an FM demodulator 23 where it is frequency demodulated. 
The resultant frequency demodulated video signal carrying timebase 
reference components is delivered to a first timebase reference signal 
detector 24 (referred to as a first detector hereinafter) for detecting 
the first timebase reference signal and a second timebase reference signal 
detector 25 (referred to as a second detector) for detecting the second 
timebase reference signal. The first detector 24; upon detecting the first 
reference signal for one horizontal scanning period produces a pulse whose 
duration is equal to the horizontal scanning period. 
The frequency of the pulse is commonly about 15.73 KHz in the NTSC system. 
The pulse is transferred to a first phase comparator 26 where it is phase 
compared in each horizontal scanning period with the output of a voltage 
controlled oscillator (VCO) 28 which has been divided to about 15.73 KHz 
by a frequency divider circuit 27. The resultant output signal is then fed 
through an error amplifier 29 and a loop filter (low-pass filter) 30 (LPF 
1) to the VCO 28. 
Similarly, the second detector 25; upon detecting the second timebase 
reference signal of each track; produces a pulse which is equal in period 
to the reference sine wave. The pulse is transferred to a second phase 
comparator 31 where it is phase compared in each horizontal scanning 
period with the output of the VCO 28 which has been divided by a divider 
circuit 32. The resultant output signal is then fed through an error 
amplifier 33 and a loop filter 34 (LPF 2) to the VCO 28. When the 
frequency in the reference sine wave is a few megahertz higher than the 
frequency in the horizontal scanning period, the second loop filter (LPF 
2) 34 can allow a higher frequency signal to pass than that of the first 
loop filter (LPF 1) 30; thus providing a faster response speed. 
Accordingly, the response to skew distortion developed in the front end of 
each track signal will be speedier. 
A switch 35 is actuated to switch the loops because the second timebase 
reference signal is contained within the beginning region of each track 
signal. Its action is timed by a signal derived from the second detector 
25 and passed through a pair of monostable multivibrators 36 and 37. 
Also, the video signal is converted by an A/D converter 38 into a digital 
form which is then stored in a memory 39. The action of the A/D converter 
38 and the memory 39 is triggered by a sampling clock signal and a write 
clock signal respectively which are both output in the form of CK outputs 
of the VCO 28. A signal retrieved from the memory 39 using a clock signal 
CKR which is constant in the phase contains no timebase variation and will 
be transmitted further from an output terminal 40. 
When the frequency in the horizontal scanning period is 15.73 KHz, the 
frequency of an output of the VCO 28 becomes about 14.3 MHz after passing 
through the first divider circuit 27 having a dividing ratio of 910:1. If 
the second divider circuit 32 has a dividing ratio of 182:1, the frequency 
of the burst wave becomes 2.86 MHz. 
It would be understood that the foregoing values are given as examples. 
When the video signal is a or HDTV signal of different scanning lines, 
or a recorded signal which is divided into a plurality of channels for 
reduction of a frequency band and extended with time thus having a 
scanning period frequency different from that of its original signal, it 
will be processed with equal success by the foregoing arrangement of the 
present invention. 
FIG. 7 is a block diagram of a detector employed for detection of the first 
timebase reference signal. An input video signal carrying the first 
timebase reference signal, whose waveform is shown in FIG. 8-A, is fed to 
an input terminal 41 and then, transferred to a first gate circuit 44 
which is controlled by the combination of a sync separator circuit 42 and 
a timing circuit 43. The first gate circuit 44 converts a sinewave 
component of the video signal into a burst signal of a sinewave form shown 
in FIG. 8-B. The burst signal is waveform modulated to into a series of 
pulses by a comparator 45 provided for detection of a zero-cross point of 
the sinewave. One of the pulses is picked up by a second gate circuit 46 
and delivered from an output terminal 47 as an output signal of the 
detector whose waveform is shown in FIG. 8-D. 
FIG. 9 is a block diagram of a detector employed for detection of the 
second timebase reference signal. An input video signal carrying the 
second timebase reference signal, of which waveform is shown in FIG. 10-A, 
is fed to an input terminal 48 and transferred to a gate circuit 49 where 
a sinewave component of the video signal is gated to produce a signal 
whose waveform is shown in FIG. 10-B. The resultant signal is then 
transferred to a comparator 50 where a zero-cross point of the signal is 
detected to produce a zero-cross signal shown in FIG. 10-C. The zero-cross 
signal is transmitted further from an output terminal 51. In action, the 
gate circuit 49 is controlled by a timing pulse produced by the 
combination of a sync separator circuit 52 and a timing circuit 53. 
A timebase corrector in accordance with another embodiment of the present 
invention will now be described referring to FIG. 11. In operation, an 
input video signal which has been frequency demodulated and contains 
timebase reference signal components is fed to an input terminal 54 and 
transferred to both a first timebase reference signal detector 55 for 
detection of the first timebase reference signal and a second timebase 
reference signal detector 56 for detection of the second timebase 
reference signal. 
Upon detecting the first timebase reference signal, the first timebase 
detector 55 having an arrangement similar to that shown in FIG. 6 produces 
a pulse whose duration is equal to the horizontal scanning period. The 
pulse is transferred to a first phase comparator 56 where it is phase 
compared in each horizontal scanning period with the output of a voltage 
controlled oscillator (VCO) 58 which is divided by a frequency divider 57. 
A resultant output is transferred through an error amplifier 59 and a loop 
filter (LPF) 60 to the VCO 58. 
Similarly, the second timebase detector 56 upon detecting the second 
timebase reference signal carried in the beginning end of each track 
signal produces a pulse in each track. The pulse is fed to a phase control 
terminal 61 of the VCO 58 and to a preset terminal 62 of the divider 57. 
Consequently, the VCO 58 is reset in phase for oscillation and the divider 
57 is preset in phase for dividing action. Also, the VCO 58 produces an 
output CK which serves as a sampling clock for an A/D converter 63 and a 
write clock for a memory 64, similar to that shown in FIG. 6. An output 
signal from the memory 64 is delivered from an output terminal 65. 
As the result, the timebase corrector can provide a quicker response to the 
generation of skew distortion. Because the second timebase reference 
signal is longer in duration than the first timebase reference signal, the 
resultant pulses are less affected by unwanted signal components such as 
noise; thus exhibiting a higher accuracy. Hence, no phase error will be 
developed even if the response speed is high. 
FIG. 12 is a block diagram of a detector for detection of the second 
timebase reference signal. In action, an input video signal carrying a 
second timebase signal component which waveform is shown in FIG. 13-A is 
fed to an input terminal 66 and transferred to a gate circuit 67 where a 
sinewave component of the input video signal is gated to produce a signal 
shown in FIG. 13-B. The resultant signal is then filtered into a signal 
waveform shown in FIG. 13-C by a bandpass filter 68 which can pass 
frequencies of the second timebase reference signal. 
The signal waveform of FIG. 13-C exhibits an incremental shape of an 
envelope since the bandpass filter 48 allows a narrow range of frequencies 
to pass. 
Then, the filtered signal is converted to a zero-cross signal, shown in 
FIG. 13-D, by a comparator 69 which can detect a zero-cross point in the 
signal. The zero-cross signal is transferred to a gate circuit 70 where it 
is gated to produce a detection pulse shown in FIG. 13-E for delivery from 
an output terminal 71. In action, the two gate circuits 67 and 70 are 
controlled by timing pulses produced by a sync separator circuit 72 and a 
timing circuit 73. 
FIG. 14 is a timing chart showing the actions of primary parts of the 
timebase corrector shown in FIG. 11. More particularly, FIG. 14-A 
illustrates a waveform of a signal contained within the beginning region 
of each track signal of the input video signal which contains skew 
components. FIGS. 14-B and 14-C respectively represent two outputs of the 
first and second detector circuits 55 and 56. FIG. 14-D shows a preset 
amplitude of the divider 57 determined by the output of the second 
detector circuit 56. It should be noted that the preset amplitude P is 
equivalent to a clock difference (denoted by P in FIG. 14-C) between the 
pulse output of the first detector 55 and the pulse output of the second 
detector 56. This results from the fact that the pulse output of the 
second detector 56 is delayed by P from the pulse output of the first 
detector 55. 
Although the number of recording channels is not specifically defined in 
this embodiment, it may arbitrarily be determined for VCR multi-channel 
recording operation in which a video signal of each channel can 
successfully be processed according to the present invention.