Video signal processing circuit and method for blanking signal insertion with transient distortion suppression

A video signal processing circuit and method for the insertion of a blanking data signal including blanking pulse, horizontal sync and color burst signal components into an input video signal to provide a modified video signal. The modified video signal is subjected to signal shaping to reduce the slopes of the front and rear edges of the blanking pulse portion in order to suppress transient overshoot and ringing distortion when the resultant output video signal is transmitted through a low pass filter. The signal shaping is preferably provided by a voltage controlled amplifier whose gain control terminal is connected to a control voltage which varies during the blanking pulse and is provided at the output of a slew limited amplifier or a non-linear low pass filter with a sine squared pulse response. The input of the slew limited amplifier is connected to an electronic switch operated by a timing circuit for selectively switching between different gain voltage sources. The timing circuit also applies a switching signal to another electronic switch providing a blanking data insertion means which selectively applies the input video signal or the blanking data signal to the input of the voltage controlled amplifier. An analog to digital converter can be employed to convert the input video signal to a digital signal for storage in a memory and further processing before insertion of digital blanking data.

BACKGROUND OF INVENTION 
The present invention relates generally to video signal processing by the 
insertion of a separately generated blanking data signal including 
blanking pulse, horizontal sync and, in the case of color television video 
signals, color burst signal components, into an input video signal without 
the introduction of transient signal distortion. In particular, the 
invention is directed to such a video signal processing circuit and method 
employing a signal shaping means for reducing the slopes of the front and 
rear edges of the blanking pulse portion to enable the modified video 
signal produced after insertion of the blanking data signal to be 
transmitted through a low pass filter without transient overshoot and 
ringing distortion of the output video signal. The video signal processing 
circuit of the present invention is especially useful in a television 
frame synchronizer or a television timebase correction system. 
A television signal processor employing a separately generated digital 
blanking data signal which is inserted into a digital input video signal 
for greater accuracy and less distortion of the horizontal sync and color 
burst components of such blanking data, is described in the article 
"Digital Processing In the DPS-1" by J. Lowry et al in Society of Motion 
Picture and Television Engineers Digital Video, Volume 2, pages 43-63 
published March 1979. Previous television frame synchronizer circuits 
employing digital techniques for the insertion of separately generated 
digital blanking data signals have the disadvantage that they introduce 
transient distortion including overshoot and ringing in the blanking pulse 
portion of the output video signal. The video signal processing circuit of 
the present invention overcomes this problem. 
The video signal processing circuit and method of the present invention 
provides an externally generated blanking data signal which is inserted 
into the input video signal to provide more accurate horizontal sync and 
color burst signal components by replacing these signal components in the 
input video signal. The blanking data signal may be a digitally generated 
blanking data signal which is inserted into a digital input video signal 
produced when the analog input video signal is transmitted through an 
analog to digital converter and a digital processor circuit with memory. 
The resulting modified video signal is then transmitted through a signal 
shaping circuit such as a voltage controlled amplifier which reduces the 
slope of the front and rear edge portions of the blanking pulse portion to 
provide a shaped video signal which is transmitted through a low pass 
filter without transient signal distortion such as ringing and overshoot 
of the output video signal as is produced by prior digital blanking data 
insertion circuits. Thus, the step transition times of the front and rear 
edges of the blanking pulse portion of the video signal are increased so 
that they are greater than the period of the upper limit frequency of the 
low pass filter, in order to prevent such transient signal distortion of 
the output video signal. 
The control voltage terminal of the voltage controlled amplifier used to 
shape the video signal may be connected to the output of a slew limited 
amplifier, such as that shown in U.S. Pat. No. 4,163,948 issued Aug. 7, 
1979 to M. L. Rieger et al, or the output of a low pass filter with a sine 
squared pulse response such as that described in the article "A New 
Sine-Squared Pulse and Bar Shaping Network" by A. Kastelein from IEEE 
Transactions of Broadcasting, pages 84-89 Volume BC-16, No. 4, December 
1980. The input of the slew limited amplifier or the low pass filter with 
sine squared response is selectively connected by an electronic switch to 
different gain control voltages including a blanking gain voltage source 
which is applied when the blanking data signal portion is received at the 
input of the voltage control amplifier. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide an improved video signal 
processing circuit and method for the insertion of an externally generated 
blanking data signal into an input video signal to provide more accurate 
horizontal sync and color burst signal components in the output video 
signal without transient signal distortion of such output video signal. 
Another object of the invention is to provide such a video signal 
processing circuit and method including signal shaping for reducing the 
slope of the front and rear edges of the blanking pulse portion of the 
video signal sufficiently to prevent transient signal distortion when such 
shaped video signal is transmitted through a low pass filter 
A further object of the invention is to provide such a video signal 
processing circuit in which the signal shaping of the video signal is 
provided by a voltage controlled amplifier whose control terminal is 
connected to a source of variable control voltage for properly shaping the 
slopes of the front and rear edge portions of the blanking pulse portion 
in a predetermined accurate manner. 
An additional object of the invention is to provide such a video signal 
processing circuit in which the control terminal of the voltage controlled 
amplifier is connected to a slew limited amplifier whose input is 
selectively connected to a blanking gain voltage source when the blanking 
pulse portion of the video signal is applied to the input of the voltage 
controlled amplifier for more accurate shaping of the blanking pulse 
portion without adversely effecting the waveform of the horizontal sync 
and color burst components or the active video portion of the video 
signal. 
Still another object of the invention is to provide such a video signal 
processing circuit and method for inserting an externally generated 
digital blanking data signal into a digital input video signal and for 
shaping the modified video signal to reduce the slope of the front and 
rear edge portions of the blanking pulse portion of the video signal so 
that their transition time is greater than or at least substantially equal 
to the period of the upper limit frequency of a low pass filter through 
which the shaped video signal is transmitted in order to prevent transient 
signal distortion of the output video signal. 
A still further object of the invention is to provide an improved video 
data signal processing method for the insertion of an externally generated 
blanking data signal into a video input signal and for shaping the 
resultant modified video signal to reduce the slopes of the front and rear 
edge portions of the blanking pulse before such video signal is 
transmitted through a low pass filter in order to prevent overshoot and 
ringing distortion of the output video signal.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A video signal processing circuit in accordance with the present invention 
is shown in FIG. 1. An analog input video television signal is applied to 
an input terminal 10 which is connected to the input of an analog to 
digital converter 12. The analog to digital converter converts the analog 
input video signal into a digital input video signal which is applied to a 
digital processor circuit 14 containing a digital memory for storing and 
processing such digital video signal. A read signal applied to the read 
input 16 of the digital processor circuit causes the digital input video 
signal to be read out of the memory and transmitted from the output of the 
digital processing circuit 14 to one input terminal of a two-position 
electronic switch 18. The other input terminal of the switch 18 is 
connected to a blanking data source 20 which externally generates a 
digital blanking data signal including blanking pulse, horizontal sync 
and, in the case of color television video signals, color burst signal 
components. The blanking data signal may be an analog signal if it is to 
be inserted in an analog input video signal, but is a digital signal 
preferably and is inserted into the digital input video signal by 
switching the movable contact of the switch 12 into the lower position 
when a blanking insert signal is applied to a switch control output 22 of 
a timing signal generator circuit 24 connected to such switch. The timing 
circuit 24 also produces the read signal applied to the read input 16 of 
the digital processor circuit 14. 
The electronic switch 18 functions as a signal insertion means for 
insertion of the digital blanking data signal from source 20 into the 
digital input video signal produced at the output of the digital processor 
circuit 14 to provide a modified video signal. This modified video signal 
is applied to the input of a digital to analog converter 26 which converts 
the digital modified video signal into an analog signal and applies the 
analog modified video signal to the input of a voltage controlled 
amplifier 28. 
The voltage controlled amplifier 28 acts as a signal shaping means for 
shaping the modified video signal by reducing the slopes of the front edge 
and the rear edge of the blanking pulse portion of the modified video 
signal. As a result, such edges are no longer vertical transitions, but 
have transition times which are greater than or at least substantially 
equal to the period of the upper cutoff frequency of the low pass filter 
connected to the output of such amplifier to prevent transient distortion 
as hereafter described. A gain control terminal 30 of the voltage control 
amplifier 28 is connected to the output of a slew limited amplifier 32 
whose input is connected through a gain selector switch 34 to three 
different D.C. voltage sources. The slew limited amplifier may be of the 
type shown in U.S. Pat. No. 4,163,948 issued Aug. 7, 1979 to M. L. Rieger. 
Thus, the gain selector switch 34 is a three-position electronic switch 
having a first terminal S1 connected to the movable contact of a 
potentiometer 36 whose end terminals are connected between a +V positive 
D.C. voltage source and ground to provide a video gain control for varying 
the gain of the active video or T.V. picture signal portion of the 
modified video signal applied to the input of the voltage control 
amplifier 28. The second terminal S2 of the gain selector switch 34 is 
connected to ground to apply zero volts to the input terminal of the slew 
limited amplifier 32 for a zero gain setting of the voltage controlled 
amplifier 28. The third terminal S3 of such switch is connected to a 
blanking gain voltage source 38 which applies a fixed voltage to the input 
terminal of the slew limited amplifier 32 which sets a gain of about one 
for the voltage controlled amplifier when the blanking data portion of the 
modified video signal is applied thereto, in a manner hereafter described 
with respect to FIG. 2. The gain selector switch 34 is switched between 
the three switch position S1, S2 and S3 by a gain selector signal at 
output 40 of the timing generator circuit 24. 
It should be noted that the slew limited amplifier 32 may be replaced by a 
low pass filter with sine squared pulse response such as the type shown in 
the above-referenced article by A. Kastelein. The voltage controlled 
amplifier 28 may be of a conventional type or it may be similar to that 
described in the article "A Four-Quadrant Analog Divider/Multiplier with 
0.01% Distortion" by Barry Gilbert, published in Digest of Technical 
Papers of 1983 IEEE International Solid State Circuits Conference, pages 
248 and 249 published Feb. 25, 1983. However, any other suitable voltage 
controlled amplifier can be employed whose gain varies in a predetermined 
manner dependent upon the voltage applied to the control terminal 30 of 
such amplifier. 
The timing signal generator 24 generates a horizonal sync and color burst 
timing signal at output 42 which is applied to the blanking data circuit 
20 to cause such circuit to generate the blanking data signal. The 
blanking data signal includes the blanking pulse, the horizontal sync and 
the color burst signal components which are inserted into the input video 
signal by the signal insertion switch 18 in the lower position of such 
switch. Thus, the sync and burst timing signal at output 42 is properly 
timed with respect to the blanking insert timing signal at output 22 and 
the gain selection signal at output 40 as well as the read output signal 
16 of the timing generator 24 for proper operation of the video signal 
processing circuit of the present invention. 
The operation of the circuit of FIG. 1 will be apparent from the waveforms 
of the signals produced therein which are shown in FIG. 2. These signals 
include a modified video signal 44 produced at the output of the digital 
to analog converter 26 as indicated by reference number 1 on FIG. 1. The 
modified video signal 44 includes a blanking data portion comprised of a 
negative rectangular blanking pulse having a front edge 46 and a rear edge 
48, and a minimum amplitude voltage level 50 of approximately zero volts 
which blanks or cuts off the cathode ray tube of the television receiver 
for horizontal line retrace during such blanking pulse. A horizontal sync 
pulse 52 in the form of a negative rectangular pulse is added to the 
blanking pulse to further reduce the amplitude thereof during the width of 
such sync pulse. A color burst signal component 54 in the form of a high 
frequency sine wave is added to the "back porch" portion of the blanking 
pulse immediately following the horizontal sync pulse 50 in the case of a 
conventional video color television signal. It should be noted that the 
blanking data signal portion exists between the front edge 46 and the rear 
edge 48 of the blanking pulse and is followed by a positive-going active 
video signal portion 56 containing the television picture signal 
components which has a much greater amplitude than the 0 volts level 50 so 
that the cathode ray tube is unblanked or turned on condition to display 
such active video signal portion. 
A shaped video signal 58 is produced at the output of the voltage 
controlled amplifier 28, as indicated by reference number 2 at the output 
thereof. The shaped video signal 58 is similar to the modified video 
signal 44 in the timing position and waveform shape of the horizontal sync 
pulse 52', the color burst signal component 54', and the active video 
signal portion 56'. However, the front edge 46A and the rear edge 48A of 
the blanking pulse portion of the shaped video signal 58 are of reduced 
slope from the vertical step transitions forming the front edge 46 and the 
rear edge 48 of the blanking pulse portion of the modified video signal 
44. This reduction in the slope of the front and rear edges 46A and 48A of 
the blanking pulse portion is due to the change in gain of the voltage 
controlled amplifier 28 by control voltage signal 60 produced at output 3 
of the slew limited amplifier 32 when the selector switch 34 connects the 
input of the slew limited amplifier through the third switch terminal S3 
to the blanking gain voltage source 38 during the time period between 
T.sub.2 to T.sub.5 of the gain selection signal 40 of FIG. 2. Thus, the 
gain control signal 60 produced at the output of the slew limited 
amplifier 32 and applied to the control terminal 30 of the voltage 
controlled amplifier 28 provides such voltage controlled amplifier with a 
constant gain of approximately one during portion 62 of the blanking data 
signal when the horizontal sync pulse 52 and the color burst 54 are 
produced, and also provides a constant gain of approximately one during 
portion 64 corresponding to the active video signal portion 56' of the 
shaped video signal. However, it should be noted that the voltage of the 
gain control signal portion 64 may be increased or decreased relative to 
the voltage of portion 62 depending upon the required amplitude of the 
active video signal portion 56', and this variation of voltage is 
indicated by the double headed arrow 66. 
The voltage of the control signal as shown at 68A from a value 
corresponding to a gain of one to a value corresponding to a gain of zero 
during the time period T.sub.1 to T.sub.2 immediately preceding the front 
edge 46 of the blanking pulse of the modified video signal 44 resulting in 
a lower sloped front edge 46A of the shaped video signal 58. It should be 
noted that even though the gain increases from 0 to 1 during the time 
period T.sub.2 to T.sub.3 from the end of the control signal portion 68A 
to the beginning of control portion 62, there is no change in the voltage 
of the corresponding portion of the shaped video signal 58 during this 
time because of the fact that the blanking pulse amplitude 50 is zero 
volts for the modified signal 42, so there is no amplification at such 
time. The control voltage signal 60 reduces the gain of the amplifier 28 
from one to zero during time period T.sub.5 to T.sub.6 from the end of 
portion 62 to the beginning of control voltage portion 70A, but here 
again, there is no change in the amplitude of the shaped video signal 58 
during this time because the modified video signal 44 is then at zero 
volts. However, as the control voltage increases during time period 
T.sub.6 to T.sub.7, as shown by signal portion 70A, the rear edge 48 of 
the blanking pulse is reduced in slope to provide waveform portion 48A of 
the shaped video signal. As a result of these changes in gain, the shaped 
video signal 58 is produced at the output of the voltage controlled 
amplifier 28. It should be noted that for a wider blanking pulse interval, 
the shaped video signal portions 46B and 48B are produced in response to 
the changes in the control voltage during signal portions 68B and 70B, 
respectively. 
The blanking insert signal 22 applied to the blanking data signal insertion 
switch 18 is produced with a negative-going rectangular pulse portion 72 
during time period T.sub.2 to T.sub.6 corresponding to the blanking data 
signal including horizontal sync and color burst components which is equal 
in width to the blanking pulse portion bounded by edges 46 and 48. The 
blanking insert signal 22 also includes a select digital video data 
portion 74 which is a positive-going rectangular signal corresponding to 
the time period of the active video portion 56 of the modified video 
signal 44. Similarly, the gain selection signal 40 applied to the selector 
switch 34 includes a variable gain portion 76 corresponding to the switch 
position S1 and to the setting of the video gain control potentiometer 36. 
In addition, the gain selection signal 40 includes a ground voltage level 
78 at switch position S2 of the selector switch 34. The ground voltage 
gain selection signal level 78 between times T.sub.1 and T.sub.2 
corresponds to control voltage portion 68A in which the gain reduces from 
1 to 0 in the control voltage 60, and level 78 between the time T.sub.5 to 
T.sub.6 corresponds to the gain reduction from 1 to 0 between the end of 
signal portion 62 and the start of portion 70A. When this gain selection 
signal 40 increases the time of ground voltage level 78 to the time period 
T.sub.0 to T.sub.2 and the time period T.sub.5 to T.sub.7, the control 
voltage 60 is widened to add portion 68B and the 0 gain portion prior to 
portion 70B. This in turn widens the shaped video signal 58 to produce 
edge portions 46B and 48B rather than edge portions 46A and 48A. Finally, 
the gain selection signal 40 also includes a fixed voltage portion 80 at 
the third switch position S3 corresponding to the voltage of the blanking 
gain voltage source 38. It should be noted that the corresponding portions 
of the signals 44, 58, 64, 22 and 40 are in vertical time alignment in 
FIG. 2 as indicated by vertical dashed lines at the times T.sub.0 to 
T.sub.7, and are maintained in this time relationship for proper operation 
of the circuit of FIG. 1. 
The shaped video signal 58 produced at the output of the voltage controlled 
amplifier 28 is transmitted through a low pass filter 82 and through an 
output amplifier 84 to an output terminal 86 from which the output video 
signal is transmitted. As shown in FIG. 3A, a distorted output video 
signal 88 corresponding to the rear edge 48 of the blanking pulse portion 
of the modified video signal 44 is produced with transient distortion 
including overshoot 90 at the top of its leading edge and ringing 
distortion 92 along the top of such output signal at the output of the low 
pass filter 82 when applying the modified video signal directly to such 
low pass filter without first transmitting it through the voltage 
controlled amplifier 28 for signal shaping in the manner of the present 
invention. In contrast, an undistorted video output signal 94 
corresponding to the rear edge 48A of the blanking pulse portion of the 
shaped video signal 58 is produced at the output of the low pass filter 
and at output terminal 86 when the shaped video signal produced in 
accordance with the present invention is applied to the input of the low 
pass filter 82. Thus, it can be seen that the undistorted video output 
signal 94 has suppressed transient distortion with substantially no 
overshoot or ringing distortion in its waveform. When the low pass filter 
has a high frequency cutoff of about 4.0 megahertz, corresponding to a 
period of 250 nanoseconds, the slopes of the front edge 46A and the rear 
edge 48A of the blanking pulse portion of the shaped video signal 58 must 
be reduced so that the step transition time of such edges is greater than 
or at least substantially equal to 250 nanoseconds, in order to prevent 
transient signal distortion. Similarly, when a low pass filter is used 
having an upper cutoff frequency of 7.0 megahertz, corresponding to a 
period of 140 nanoseconds, the transition time of the front edge 46A and 
the rear edge 48A of the blanking pulse portion of the shaped video signal 
58 must be greater than or at least substantially equal to 140 nanoseconds 
in order to prevent such transient signal distortion. 
It will be obvious to those having ordinary skill in the art that many 
changes may be made in the preferred embodiment of the present invention 
without departing from the spirit of the invention. For example, the 
blanking data signal source 20 can be an analog signal source to provide 
an analog blanking data signal which is inserted into an analog input 
video signal, in which case the analog to digital converter 12, the 
digital processing circuit 14 and the digital to analog converter 26 can 
be eliminated. Alternatively, the voltage controlled amplifier 28 may be 
employed as a digital signal shaping amplifier having its output connected 
to the input of the digital to analog converter 26. Therefore, the scope 
of the present invention should only be determined by the following claims 
.