Horizontal AFC circuit in a television receiver usable with a video signal recording and reproducing apparatus

A horizontal AFC circuit comprising a phase detector circuit supplied with a horizontal synchronizing signal separated from a television video signal and with a comparison signal and carrying out phase comparison, a filter circuit for filtering the output of the phase detector circuit, a horizontal oscillator circuit supplied with the output of the filter circuit and oscillating with an oscillation frequency controlled thereby, a horizontal deflection circuit for forming the output signal of the horizontal oscillator circuit into a horizontal deflection pulse, a wave shaping circuit operating upon being supplied with the output pulse of the horizontal deflection circuit to wave shape this output pulse and to supply the resulting output signal thereof as said comparison signal to the phase detector circuit, means for supplying a control pulse of a pulse width corresponding to a vertical blanking period of the television video signal, and loop gain control means supplied with the control pulse and operating to cause the loop gain of the horizontal AFC circuit to be relatively large in the pulse width duration and to cause the loop gain to be relatively small in a period other than said pulse width duration.

BACKGROUND OF THE INVENTION 
The present invention relates generally to horizontal AFC circuits 
(horizontal automatic frequency control circuits), and more particularly 
to a horizontal AFC circuit in a television receiver for receiving a 
reproduced video signal from an apparatus for magnetically recording and 
reproducing a video signal. 
In general, there is a video signal magnetic recording and reproducing 
apparatus of a type which records and reproduces video signals on and from 
slant tracks on a magnetic tape for each field alternately by means of two 
rotating magnetic heads, for example. In a video signal magnetic recording 
and reproducing apparatus of this type, the reproduced video signals from 
the rotating heads are switched with vertical periods and are led out as a 
continuous series of reproduced video signals. 
In this video signal magnetic recording and reproducing apparatus, however, 
if there is an occurrence such as timing lag of the switching time 
instants of the output signals of the two heads or stretching or 
contraction of the magnetic tape, phase lag will occur in the series of 
signals at the signal switching instant, whereby skew distortion will 
develop in the reproduced picture of the television receiver. The phase 
lag in that event develops in the manner of a step function. 
When a phase lag of a step function occurs in this manner in a reproduced 
signal supplied to a television receiver, a transient response develops in 
the phase characteristic of the television receiver. In order to reduce 
the effect of the distortion in the reproduced picture which accompanies 
this phase lag, it is necessary to shorten the time of the above mentioned 
transient response. This transient response may be shortened by increasing 
the AC loop gain of the horizontal AFC circuit of the television receiver. 
However, when the AC loop gain is made large, the passing quantity of the 
high-frequency band component of the filter provided in the horizontal AFC 
circuit increases. For this reason, the degree to which the horizontal AFC 
circuit system is disturbed by the noise component becomes large. 
Especially in the case of reception of television broadcasting waves in an 
area of weak electric field, the disorder in the horizontal direction of 
the reproduced picture becomes large. Consequently, it is difficult, in 
principle, to attain simultaneously both shortening of the transient 
response time and improvement of the noise characteristic. 
Accordingly, in a horizontal AFC circuit of the prior art, an AC loop gain 
changeover switch is provided. By operating this switch, the AC loop gain 
is caused to be of relatively low value in order to improve the noise 
characteristic in the case of reception of television broadcast waves 
since the skew distortion is small in this case. In the case where a 
signal reproduced from a video signal magnetic recording and reproducing 
apparatus is reproduced into a picture, the switch is operated to cause 
the AC loop gain to be a relatively large value in order to shorten the 
transient response time since the skew distortion is large in this case. 
However, in either case, improvement of both the transient response 
characteristic and the noise suppression characteristic cannot be 
achieved, and in either mode of picture reproduction, one of these two 
characteristics is unavoidably sacrificed. In this known horizontal AFC 
circuit, furthermore, it has been necessary to carry out troublesome 
manipulation of the above mentioned changeover switch in accordance with 
the mode of picture reproduction. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the invention to provide a novel and 
useful horizontal AFC circuit in which the above described problems in the 
prior art have been overcome. 
Another and more specific object of the invention is to provide a 
horizontal AFC circuit adapted to carry out an operation wherein the AC 
loop gain is substantially made relatively large only in the vertical 
blanking period within a video signal (composite video signal) and is made 
relatively small in other video information signal period. In the 
horizontal AFC circuit according to the invention, the transient response 
time of the phase lag in the vertical blanking period is short, and, even 
when there is a phase lag at the switching point of the reproduced signals 
from a video signal magnetic recording and reproducing apparatus, there is 
substantially almost no occurrence of picture bending in the reproduced 
picture. Furthermore, in the video information signal period, the noise 
characteristic is good, and a good picture which is almost completely free 
of the effect of noise is obtained. 
Still another object of the invention is to provide a horizontal AFC 
circuit in which, through the use of the output of a vertical deflection 
circuit or a vertical oscillation circuit, the AC loop gain is 
automatically made relatively large only in the vertical blanking period 
in the video signal.

DETAILED DESCRIPTION 
FIG. 1 illustrates one example of a horizontal AFC circuit of a television 
receiver of general type to which the present invention can be applied. A 
horizontal synchronizing signal which has been separated from a reproduced 
video signal enters the circuit through an input terminal 11 and is 
supplied to a phase detector 12. Here, the difference in the phase of the 
horizontal synchronizing signal and of a comparison signal from a wave 
shaping circuit 16 described hereinafter is detected. A signal of a level 
corresponding to this phase difference is led out of the phase detector 
12. This phase difference signal is supplied through a filter circuit 13 
of voltage transmission type or current transmission type to a horizontal 
oscillator circuit 14 and controls the output oscillation frequency 
thereof. The output signal of the horizontal oscillator circuit 14 is 
supplied to a horizontal deflection output circuit 15. The horizontal 
deflection pulse produced as output by this circuit 15 is led out through 
a terminal 17 and, at the same time, is fed to the above mentioned wave 
shaping circuit 16. The horizontal deflection pulse is wave formed by the 
wave shaping circuit 16 and is supplied as a comparison signal to the 
phase detector 12. 
As is known, the oscillation of the horizontal oscillator circuit 14 is 
controlled by the circuit of the above described organization in a manner 
such that phases of the input horizontal synchronizing signal and of the 
comparison signal obtained from the horizontal deflection pulse will 
coincide. In this manner the horizontal synchronization of the reproduced 
picture is stably maintained. 
In the above described horizontal AFC circuit, the quantity of variation of 
the oscillation frequency of the horizontal oscillator circuit 14 per 
radian of phase lag between the horizontal synchronizing signal and the 
output horizontal deflection pulse is generally called the loop gain. In 
the case where a filter circuit of voltage transmission type or current 
transmission type is used for the filter circuit 13, the frequency 
characteristic of the loop gain is as indicated in FIG. 2. In FIG. 2, the 
ordinate represents loop gain, and the abscissa represents angular 
frequency. The loop gain characteristic for a direct-current signal of the 
output of the phase detector 12 becomes as indicated by the DC loop gain 
of the curve part I in FIG. 2. The loop gain characteristic for an 
alternating-current signal of the output of the phase detector 12 is as 
indicated by the AC loop gain of the curve part II. 
In a video signal magnetic recording and reproducing apparatus, in general, 
the reproduced video signals from respective rotating magnetic heads are 
switched at a time instant or point Sp which is approximately five 
horizontal scanning periods (5H) in front of a vertical blanking period 
V.BLK as indicated in FIG. 3(A). In FIGS. 3(A) through 3(D), the ordinate 
represents a time axis and corresponds to the direction of advancing of 
the field. In general, as a consequence of causes such as timing lag of 
the switching time point and stretching and contraction of the magnetic 
tape, phase lag occurs at the signal switching point, and the phase 
characteristic thereof varies in a step function at the signal switching 
point Sp as indicated in FIG. 3(B). The following description is set forth 
with respect to the case where the phase is delayed at the signal 
switching point Sp. The case wherein the phase is advanced is similar, the 
waveforms in FIG. 3(B), 3(C), and 3(D) merely becoming symmetrical left 
and right with respect to the time axis. 
In the horizontal AFC circuit of the television receiver which has been 
supplied with a signal of the phase characteristic indicated in FIG. 3(B), 
a transient response of the signal occurs as indicated in FIG. 3(C). 
Furthermore, on the reproduced picture screen of the receiver, the picture 
suddenly bends in the lateral or horizontal direction at the above 
mentioned switching point Sp near the end of one field as indicated in 
FIG. 3(D), that is, at the lower part of the picture screen, and this 
bending continues also in the initial part of the next field succeeding to 
the blanking period, that is, the upper part of the picture screen. The 
period T of bending of the picture screen indicated in FIG. 3(D) 
corresponds to the transient response time T indicated in FIG. 3(C). 
With respect to the above mentioned bending of the picture, screen, since 
the lowermost part of the picture is almost invisible on an actual picture 
screen, this bending poses almost no problem in actual practice. 
Furthermore, within the vertical blanking period V.BLK, the picture does 
not appear on the picture screen, and, for this reason, this bending does 
not give rise to any problems. However, bending of the picture appears at 
the upper part of the picture screen of the succeeding field, and the 
picture screen becomes disturbed. The longer is the above mentioned 
transient response time T, the larger is the part of the picture screen in 
which the bending of the picture occurs. Accordingly, the part of the 
upper part of the picture screen in which picture bending occurs can be 
reduced by shortening the transient response time T. 
The expedient of increasing the aforementioned AC loop gain in order to 
shorten this transient response time T can be considered. However, merely 
increasing the AC loop gain will result in an increase in the quantity of 
the high-frequency component passing through the filter circuit 13, 
whereby the degree of disturbance of the horizontal AFC circuit by the 
noise component becomes great. Therefore, as mentioned hereinbefore, it is 
difficult in principle to achieve simultaneously both a shortening of the 
transient response time and an improvement in the noise characteristic. 
The present invention contemplates overcoming the above described problem. 
More specifically, in accordance with the invention, the AC loop gain is 
automatically caused to be large only in the vertical blanking period as 
described below. For this reason, since the AC loop gain becomes large in 
the vertical blanking period, the transient response time of the phase lag 
accompanying the switching of the reproduced signals of the video signal 
magnetic recording and reproducing apparatus can be shortened. Therefore, 
the bending of the picture at the upper part of the reproduced picture 
screen of the succeeding field can be greatly decreased. In this 
connection, the noise characteristic during the vertical blanking period 
at this time becomes poor, but since the vertical blanking period does not 
appear on the reproduced picture screen even if there is noise, there is 
no deleterious effect due to noise in actual practice. Since the AC loop 
gain is made small in the video information signal periods other than the 
vertical blanking period, the noise characteristic is good, and a good 
picture without any effect of noise is obtained. 
The AC loop gain may be increased by directly causing it to increase. 
Furthermore, since the AC loop gain increases as a result of an increase 
in the DC loop gain as indicated by curves I' and II' in FIG. 2, the DC 
loop gain may be caused to increase in order to increase the AC loop gain. 
In the first embodiment of the invention described hereinbelow, the AC 
loop gain is increased by increasing the DC loop gain. 
The circuit of one example of the phase detector 12 constituting an 
essential part of a first embodiment of the horizontal AFC circuit of the 
invention is shown in FIG. 4. The parts in this horizontal AFC circuit of 
the invention other than the phase detector are the same as those of the 
block system illustrated in FIG. 1 and are constituted by known circuits. 
A horizontal synchronizing signal through the input terminal 11 is applied 
to the base of a transistor Q1, which is thereupon placed in its operative 
state. On one hand, a comparison signal of sawtooth waveform resulting 
from the wave shaping of a horizontal deflection pulse by the wave shaping 
circuit 16 is applied to a terminal 21. This comparison signal is applied 
by way of a capacitor C11 to the base of an NPN transistor Q3 and, at the 
same time, further by way of resistors R11 and R12 to the base of an NPN 
transistor Q2. The transistors Q2 and Q3 constitute a differential 
amplifier. The average voltage of the comparison signal becomes a DC bias 
voltage applied to the junction between the resistors R11 and R12. 
Here, it is assumed that the comparison waveform signal indicated in FIG. 
5(A) and the horizontal synchronizing signal indicated in FIG. 5(B) have a 
phase relationship as indicated in the figures. Furthermore, for the sake 
of simplifying the description, it will be assumed that the resistance 
values respectively of resistors R13, R14, R15, and R16 are equal, and 
that the resistance values respectively of resistors R17 and R18 are also 
equal. 
As indicated in FIGS. 5(A) and 5(B), in the period .tau.1 wherein the 
horizontal synchronizing signal exists and the comparison signal is 
negative, the transistors Q1 and Q2 are in their operative states, and a 
PNP transistor Q4 connected between the collector of the transistor Q2 and 
a direct-current power source terminal 22 operates. On the other hand, the 
transistor Q3 and a PNP transistor Q5 connected between the collector of 
the transistor Q3 and the terminal 22 are in their inoperative states. 
Consequently, the collector output of the transistor Q2 is applied to the 
base of a PNP transistor Q6, which thereupon operates, while the collector 
output of the transistor Q3 is not applied to the base of a PNP transistor 
Q7, which therefore is in its inoperative state. As a result of the 
operation of the transistor Q6, NPN transistors Q8 and Q9 operate, and a 
current substantially equal to the collector current of the transistor Q1 
flows from the filter circuit 13 toward the collector side of the 
transistor Q9. 
Then, in a period .tau.2 wherein the horizontal synchronizing signal is 
present and the comparison signal is positive as indicated in FIGS. 5(A) 
and 5(B), the transistors Q1 and Q3 are in their respective operative 
states, and the transistor Q5 operates, whereby the transistor Q7 
operates. At this time, the transistor Q6 does not operate, whereby the 
transistors Q8 and Q9 do not operate. For this reason, a current 
substantially equal to the collector current of the transistor Q1 flows 
out of the collector of the transistor Q7 to the filter circuit 13. 
The above mentioned periods .tau.2 vary in accordance with the phase 
difference of the comparison signal and the horizontal synchronizing 
signal, whereby the flowing-in and flowing-out periods of the currents 
flowing into and flowing out of the filter circuit 13 vary. As a 
consequence of this variation, the average DC level of the output of the 
filter circuit 13 varies, and phase detection is accomplished. 
On one hand, the DC loop gain of the AFC system has a proportional 
relationship to the collector current of the transistor Q1. Accordingly, 
in the circuit of the present invention, the vertical deflection pulse 
from a vertical deflection circuit 24 is formed to have a positive 
polarity by a wave shaping circuit 25, and the resulting output pulse is 
applied to an input terminal 23. This pulse which has been introduced 
through the input terminal 23 is applied to the base of a transistor Q10, 
which thereupon is rendered operative. Consequently, the transistor Q10 
assumes its conductive state only in this period during which the above 
mentioned pulse is entering the circuit, that is, the vertical blanking 
period. 
When the transistor Q10 becomes conductive, the value of the emitter 
resistance of the transistor Q1 becomes the resistance value of the 
parallel resistance of a resistor R19 and a resistor 20, and the 
resistance value becomes less than that in the case wherein the transistor 
Q10 is nonconductive and only the resistor 19 constitutes the emitter 
resistance. For this reason, the collector current of the transistor Q1 
increases, and the DC loop gain increases as indicated by curve I' in FIG. 
2. As a result, the AC loop gain also increases as indicated by curve II' 
in FIG. 2 responsive to the increase in the DC loop gain only during the 
vertical blanking period, and the transient response time of the phase lag 
in this vertical blanking period is shortened. 
In the period during which the pulse is not applied to the input terminal 
23, that is, the time period other than the vertical blanking period, the 
transistor Q10 becomes nonconductive, and the emitter resistance of the 
transistor Q1 becomes only the resistance value of the resistor R19. The 
emitter resistance thereby becomes greater than that in the case of 
parallel resistance of the resistors R19 and R20, and the collector 
current to the transistor Q1 decreases. As a consequence, the DC loop gain 
decreases as indicated by curve I in FIG. 2. For this reason, in the video 
information signal period other than the vertical blanking period, the AC 
loop gain also decreases as a result as indicated by curve II in FIG. 2, 
and the noise resistance characteristic improves. 
The pulse to be applied to the input terminal 23 need not be limited to a 
pulse obtained as in the above described embodiment of the invention but 
may be any pulse having a cyclic period and a pulse width corresponding to 
the vertical blanking period, examples of suitable pulses being the output 
pulse of the vertical oscillator circuit, a pulse obtained by wave forming 
this output pulse, or a pulse obtained from the vertical synchronizing 
signal and wave formed to a specific pulse width. 
Next, examples of the filter circuit 13 constituting an essential part of a 
second embodiment of the horizontal AFC circuit of the invention will now 
be described with reference to FIGS. 6 and 7. In this horizontal AFC 
circuit of the invention, the parts thereof other than the filter circuit 
are the same as those in the circuit shown by block diagram in FIG. 1 and 
can be constituted by known circuits. 
In a filter circuits 13a shown in FIG. 6 as the first example of the filter 
circuit 13 according to the invention, a pulse of negative polarity and a 
pulse width corresponding to the vertical blanking period is applied to a 
terminal 31. This pulse introduced through the terminal 31 is applied 
through a resistor R23 to the bases of transistors Q11 and Q12. Normally, 
with a positive voltage applied to their bases, these transistors Q11 and 
Q12 are in their conductive states, and the opposite terminals of a 
resistor R22b are short-circuited. The relationship between the AC loop 
gain and the DC loop gain in this case becomes as follows. 
##EQU1## 
The AC loop gain assumes a relatively small specific value as indicated by 
curve II in FIG. 2. Therefore, picture reproduction is accomplished with 
good noise characteristic in the video information signal period. 
Next, when the operation enters the vertical blanking period, a negative 
pulse of a pulse width corresponding to this period is applied to the 
terminal 31, whereby the transistors Q11 and Q12 become nonconductive. For 
this reason, the resistor R22b becomes connected in series with the 
resistor R22a in effect, whereby the combined impedance of the resistors 
R22a and R22b becomes large, and the AC loop gain becomes large. 
Therefore, the transient response time of the phase lag within the 
vertical blanking period becomes small. 
The filter circuit 13b as the second example of the filter circuit 13 
according to the invention is illustrated in FIG. 7. In this filter 
circuit, there is provided a switching circuit 32 which comprises 
transistors similarly as in the example illustrated in FIG. 6 and is 
connected in parallel with a resistor R21b. A pulse of positive polarity 
and having a pulse width corresponding to the vertical blanking period is 
applied through a terminal 33 to this switching circuit 32. During period 
other than the vertical blanking period, the switching circuit 32 is 
nonconductive, whereby a resistor R21a is in a state of being connected in 
series with the resistor R21b, and the AC loop gain is small. In the 
vertical blanking period, the switching circuit 32 becomes conductive, and 
the ends of a resistor R21b are short-circuited. The combined resistance 
of the resistors R21a and R21b becomes small, and the AC loop gain becomes 
large. 
The circuit of one example of the connection part between the filter 
circuit 13 and the horizontal oscillator circuit 14 constituting essential 
parts of a third embodiment of the horizontal AFC circuit of the invention 
is illustrated in FIG. 8. Other constituent parts of the circuit are the 
same as in the prior art. A signal which has passed through the filter 
circuit 13 comprising resistors R21 and R22 and a capacitor C21 is 
supplied through resistors R24a and R24b to the horizontal oscillator 
circuit 14. A switching circuit 34 is connected in parallel with the 
resistor R24, and a pulse of a width corresponding to the vertical 
blanking period is applied through a terminal 35 to this switching circuit 
34. 
The switching circuit 34 is rendered conductive by the pulse supplied 
through the terminal 35 to short-circuit the resistor R24b. As a 
consequence, the combined impedance of the resistors R24 and R24b becomes 
small relative to the input impedance of the horizontal oscillator circuit 
14. Therefore, the proportion (control sensitivity) of the quantity of 
oscillation frequency variation of the horizontal oscillation circuit 14 
with respect to the unit variation quantity at a point 36 of the signal 
which has passed through the filter circuit 13 becomes large, and, as an 
effective result, the AC loop gain becomes large. 
Next, examples of the wave shaping circuit 16 constituting an essential 
part of a third embodiment of the horizontal AFC circuit of the invention 
will be described. Other constituent parts of the circuit are the same as 
in the prior art. 
In the first example of the wave shaping circuit 16a shown in FIG. 9, the 
output signal of the horizontal deflection circuit 15 is applied to a 
terminal 41. This wave shaping circuit 16a comprises resistors R31 and 
R32, capacitors C21 and C22, and a switching circuit 42. The signal 
applied to the terminal 41 is wave shaped into a sawtooth waveform in 
accordance with the charging and discharging of the capacitor C21 and is 
supplied as a comparison signal through a terminal 43 to the phase 
detector 12. 
The switching circuit 42 is connected in parallel with the resistor R32 and 
is controlled in ON-OFF operation by a pulse of a width equal to the 
vertical blanking period applied to a terminal 44. At times when no pulse 
is being applied thereto, the switching circuit 42 is in "OFF" state, 
whereby the resistor R32 is not short-circuited, and the time constant of 
charging and discharging of the capacitor C21 determined by the resistance 
values of the resistors R31 and R32 and the capacitance value of the 
capacitor C21 is large. Consequently, the amplitude of the sawtooth wave 
led out through the terminal 43 is small, whereby the loop gain is small. 
On the other hand, the switching circuit 42 becomes "ON" during the width 
period of the applied pulse, and the terminals of the resistor R32 are 
short-circuited. As a consequence, the charging and discharging time 
constant of the capacitor C21 becomes small, and the amplitude of the 
sawtooth wave led out through the terminal 43 becomes large. Therefore, 
the loop gain becomes large. 
The circuit 16b of a second example of the wave shaping circuit is 
illustrated in FIG. 10. The output signal of the horizontal deflection 
circuit 15 introduced through a terminal 45 is applied through a resistor 
R33 to the base of a transistor Q13. The emitter of this transistor Q13 is 
grounded (earthed), and between its collector and a bias power source 49, 
resistors R34 and R35 are connected. A switching circuit 46 to which a 
pulse of a width equal to the vertical blanking period is applied through 
a terminal 47 is connected in parallel with the resistor 34. 
At a time when a pulse is not being applied thereto, the switching circuit 
46 is in its "OFF" state, and the charging time constant of the capacitor 
C23 determined by the resistance values of the resistors R34 and R35 and 
the capacitance value of a capacitor C23 is large. Consequently, the 
amplitude of the sawtooth wave led out through a capacitor C24 and a 
terminal 48 and supplied to the phase detector 12 is small, and the loop 
gain is small. On the other hand, the switching circuit 46 assumes its 
"ON" state in the width period of the pulse applied thereto, and the 
terminals of the resistor 34 are short-circuited. As a consequence, the 
charging time constant of the capacitor C23 becomes small, and the 
amplitude of the sawtooth wave led out of the terminal 48 becomes large. 
Therefore, the loop gain becomes large. 
While, in each of the above described embodiments of the invention, the 
switching circuit is connected in parallel respectively with the resistors 
R22b, R21b, R24b, R32, and R35, it is also possible to connect the above 
named resistors R22b, R21b, R24b, R32 and R34 in parallel respectively 
with the resistors R22a, R21a, R24a, R31, and R35 and to connect the 
switching circuit in series with the resistors R22b, R21b, R24b, R32, and 
R34. In this case, the polarity of the control pulse to be applied to the 
switching circuits is so selected that the same loop gain control 
operation in effect as that described above can be achieved. 
Further, this invention is not limited to these embodiments but various 
variations and modifications may be made without departing from the scope 
and spirit of the invention.