Video synchronizing signal separator

An improved video synchronizing signal separator for separating luminance and synchronizing signal components, respectively, from a composite video signal. Separate luminance and synchronizing channels are supplied with the composite video signal, each channel including a biased diode and an output attenuator. The diodes are poled in opposite directions to conduct when the particular threshold level established by the bias voltage applied thereto is exceeded. As an example, the diode in the luminance channel may be positively poled to conduct when the luminance signal component exceeds a positive threshold level and the diode in the synchronizing channel may be negatively poled to conduct when the synchronizing signal component exceeds a negative threshold level. The positive-going luminance signal component serves to cut off the diode in the synchronizing channel and the negative-going synchronizing signal component serves to cut off the diode in the luminance channel.

BACKGROUND OF THE INVENTION 
This invention relates to a video synchronizing signal separator and, more 
particularly, to a relatively simple yet effective separator circuit for 
separating the luminance and synchronizing signal components of a 
composite video signal. 
In video signal processing circuits, such as those used in television 
receivers, video recorders, video signal enhancers, and the like, the 
usual periodic horizontal synchronizing signal that is included in the 
composite video signal is separated and separately processed. Desirably, 
the informational components of the composite video signal, such as the 
chrominance and luminance components, should not interfere with the 
separated synchronizing signal. Similarly, those informational components 
should be separately processed without interference from the synchronizing 
signal. Thus, synchronizing signal separator circuits have been developed 
for separating the horizontal synchronizing signal from the composite 
video signal. 
Typical of horizontal synchronizing separator circuits heretofore used are 
the so-called "sync clipper" circuits formed of a diode connected in shunt 
with the video path, the diode being conductive to shunt the information 
components to ground yet being cut off in the presence of the horizontal 
synchronizing signal, thus passing the horizontal synchronizing signal to 
further circuitry. A similar diode has been used to clamp the 
informational components, and particularly the luminance signal, to a 
predetermined level, such as the black level. This clamping diode is 
conductive to shunt the synchronizing signal to ground. 
Although the aforementioned sync clipper circuits have proven to be 
successful in operation and are relatively inexpensive, there have been 
proposals for more complicated sync processing circuits, such as described 
in U.S. Pat. Nos. 3,881,055, 4,057,826, 4,424,528 and 4,453,183. 
It is desirable to provide a sync separator circuit of relatively simple 
and inexpensive construction, yet is reliable in separating the 
synchronizing signal component from its composite video signal. 
OBJECTS OF THE INVENTION 
Therefore, it is an object of the present invention to provide a relatively 
inexpensive and reliable synchronizing signal separator circuit. 
Another object of this invention is to provide a sync separator for 
separating the horizontal synchronizing signal of a composite video signal 
to one channel and at least the luminance signal component to another 
channel. 
A further object of this invention is to provide a sync signal separator 
that can be used with color or black-and-white television signals. 
An additional object of this invention is to provide an improved 
synchronizing signal separator having two channels, one channel carrying 
only the synchronizing signal, to the exclusion of the luminance component 
of the composite video signal, and the other channel carrying only the 
luminance component, to the exclusion of the synchronizing signal. 
Various other objects, advantages and features of the present invention 
will become readily apparent from the ensuing detailed description, and 
the novel features will be particularly pointed out in the appended 
claims. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a synchronizing signal separator is 
provided for separating the luminance and synchronizing signal components 
from a composite video signal. Separate luminance and synchronizing 
channels are provided, the luminance channel including a diode biased for 
conduction in one direction and the synchronizing channel including a 
diode biased for conduction in the opposite direction, with the respective 
bias voltages establishing threshold voltages. As an example, the diode 
included in the luminance channel is cut off when the composite video 
signal is less than the positive threshold level determined by the bias 
voltage applied to that diode; and the diode included in the synchronizing 
channel is cut off when the composite video signal is less than the 
negative threshold level determined by the bias voltage supplied to that 
diode. Respective attenuators are included in each channel to adjust the 
amplitude of the luminance and synchronizing signal components that are 
passed by the respective diodes.

Referring now to the drawing, the illustrated circuit may be used in a 
television receiver, a video recorder, a video disk player or, in general, 
a video processor. The illustrated processing circuitry includes a 
chrominance-luminance separator 14, a chrominance processing circuit 16, a 
synchronizing signal separator 10, a mixer 22, a mixer 26, and output 
terminals 32 and 34. Chrominance-luminance separator circuit 14 is coupled 
to an input terminal 12 and is adapted to separate the chrominance 
component from the luminance and synchronizing signal components normally 
included in a composite color video signal. Input terminal 12 is adapted 
to receive the composite color video signal or, alternatively, to receive 
a composite black-and-white signal. As is appreciated, a "composite" video 
signal includes both video information, comprised of the luminance 
component and the chrominance component, and synchronizing information 
comprised of the horizontal synchronizing signal. The composite video 
signal also includes, as is conventional, the vertical synchronizing 
signal. The chrominance-luminance separator circuit may be of conventional 
construction, such as a filter, and is adapted to supply to chrominance 
processing circuit 16 the chrominance component separated from the 
composite video signal, and to supply to synchronizing signal separator 10 
the luminance and synchronizing signal components included in the 
composite video signal. 
Chrominance processing circuit 16 may be of conventional construction and 
is adapted to separate and separately process, as by phase enhancement, 
amplitude adjustment, and the like, the usual color burst signal and the 
color information components normally considered to be a part of the 
chrominance component. The burst signal which is processed by chrominance 
processing circuit 16 is supplied, by way of terminal 28, to mixer 26 
whereat the burst signal is combined, or mixed, with the horizontal 
synchronizing signal separated by sync separator circuit 10. Similarly, 
the color information components separated from the chrominance component 
by the chrominace processing circuit are supplied, by way of terminal 24, 
to mixer 22 whereat the color information components are combined, or 
mixed, with the luminance component separated from the composite video 
signal by the snyc separator circuit. Chrominance processing circuit 16 
and mixers 22 and 26 are of conventional construction. Mixer 22 is coupled 
to output terminal 32 to supply the combined luminance and color 
information components, and mixer 26 is coupled to output terminal 34 to 
provide the combined horizontal synchronizing and burst signal components. 
An additional mixer circuit (not shown) may be coupled to output terminals 
32 and 34 for the purpose of re-combining the horizontal synchronizing 
signal, the burst signal, the luminance signal and the color informational 
signals so as to re-form a composite color video signal. It is 
appreciated, however, that each of the aforementioned components included 
in the re-formed composite color video signal may be separately processed 
prior to the combination thereof, thus enhancing each component, as 
desired. 
Synchronizing signal separator 10 is coupled by way of amplifier 18 to 
chrominance-luminance separator 14 to receive the combined luminance and 
horizontal synchronizing signal components included in the composite video 
signal. The purpose of the synchronizing signal separator is to separate 
the luminance component from the synchronizing signal component and to 
supply these separated components individually to mixers 22 and 26, 
respectively. 
In the illustrated embodiment, synchronizing signal separator 10 is 
comprised of a luminance channel and a synchronizing channel to pass the 
luminance signal component and the synchronizing signal component, 
respectively. The luminance channel is comprised of a unilateral 
conducting device 42, such as a diode or diode-connected transistor, and a 
bias circuit 46. The bias circuit is coupled to the unilateral conducting 
device, referred to hereafter simply as the diode, to establish a 
threshold or turn-off level. If the amplitude of the positive-going 
composite video signal supplied to synchronizing signal separator 10 is 
less than this threshold or turn-off level, diode 42 is rendered 
non-conductive. Conversely, when the positive-going amplitude of the 
composite video signal exceeds the threshold or turn-off level established 
by bias circuit 46, diode 42 is rendered conductive. 
In the illustrated embodiment, bias circuit 46 is comprised of a 
potentiometer 48, or other adjustable resistance, coupled across a 
suitable voltage source +V by means of resistors 50 and 52. The 
potentiometer is connected to the cathode of diode 42. This bias circuit 
establishes a positive threshold level which must be exceeded by the 
composite video signal in order to render diode 42 is conductive. 
The synchronizing channel is comprised of a unilateral conducting device 
44, such as a diode or a diode-connected transistor, and a bias circuit 
56. The unilateral conducting device, referred to simply as a diode, is 
poled in a direction opposite to that of diode 42, and the anode of diode 
42 is connected in common with the cathode of diode 44 to receive the 
composite video signal. Bias circuit 56 is coupled to the anode of diode 
44 and is adapted to supply a bias voltage which establishes a threshold, 
or turn-off, level for the diode. In the illustrated embodiment, diode 44 
may be thought of as being negatively poled and is rendered conductive 
when the threshold or turn-off level established by bias circuit 56 is 
exceeded. More particularly, when the amplitude of the negative-going 
portion of the composite video signal exceeds this threshold or turn-off 
level, diode 44 conducts. When the negative-going amplitude is less than 
this threshold level, the diode is rendered non-conductive. 
Bias circuit 56 is similar to bias circuit 46 and is comprised of a 
potentiometer 58, or other adjustable resistance, connected across a 
voltage source +V by means of resistors 60 and 62. The voltage sources 
coupled to bias circuits 46 and 56 may be equal or, in the alternative, 
may be separate and distinct sources of operating potential. For example, 
the voltage source to which bias circuit 56 is coupled may be a negative 
DC voltage and the voltage source to which bias circuit 46 is coupled may 
be a positive DC voltage. 
The luminance channel includes a luminance output circuit comprised of an 
adjustable attenuator 74 coupled to ground (or another reference 
potential) by way of a resistor 76. Attenuator 74 is coupled to receive 
the luminance signal component that flows through diode 42; and the 
junction defined by this attenuator and resistor 76 is coupled to mixer 22 
to supply an attenuated luminance signal component thereto. 
Similarly, the synchronizing channel includes a synchronizing signal output 
circuit comprised of an adjustable attenuator 84 that is connected by way 
of resistor 86 in a shunt path to the separated synchronizing signal 
component. A resistor 82 couples the synchronizing signal component which 
flows through diode 44 to this shunt path, and the junction defined by 
resistor 82 and the shunt path is connected, by way of an emitter-follower 
transistor 90 and a coupling capacitor 92, to mixer 26. Attenuators 74 and 
84 are adjustable to selectively attenuate the amplitudes of the luminance 
and synchronizing signal components, respectively, as desired. It is 
appreciated that each attenuator may comprise an adjustable potentiometer. 
In operation, let it be assumed that chrominance-luminance separator 14 
supplies the composite video signal represented by the illustrated 
waveform to the common terminal to which the luminance and synchronizing 
channels are connected. It is assumed that the luminance signal component 
is relatively positive (or positive-going) and the synchronizing signal 
component is relatively negative (or negative-going), as shown. Bias 
circuit 46 connected to the cathode of diode 42 establishes threshold 
level T.sub.L and bias circuit 56 connected to the anode of diode 44 
establishes threshold level T.sub.S. When the amplitude of the composite 
video signal is positive and exceeds threshold level T.sub.L, diode 42 is 
rendered conductive. At this time, however, diode 44 is seen to be back 
biased by the positive amplitude of the composite video signal and is 
rendered non-conductive. The threshold level T.sub.L may be equal to the 
bias voltage derived from bias circuit 46 plus the inherent V.sub.be 
voltage across diode 42. Preferably, the threshold level T.sub.L is 
clamped to the black level. 
As the amplitude of the composite video signal decreases, the conductivity 
of diode 42 correspondingly decreases. Eventually, when the amplitude of 
the composite video signal falls below the threshold level T.sub.L, diode 
42 is rendered non-conductive, that is, it is cut off. 
It is appreciated that, during the synchronizing signal interval of the 
composite video waveform, the negative-going synchronizing pulse exceeds 
the threshold level T.sub.S established by bias circuit 56. This threshold 
level T.sub.S may be substantially equal to the bias voltage derived from 
bias circuit 56 plus the inherent V.sub.be voltage across diode 44. When 
the negative-going portion of the composite video signal exceeds the 
threshold level T.sub.S, diode 44 is rendered conductive. It is 
appreciated that, at this time, diode 42 is back biased and is 
non-conductive. Accordingly, diode 44 conducts substantially throughout 
the horizontal synchronizing pulse duration; and when the composite video 
signal once again crosses the threshold level T.sub.S in the positive 
direction, diode 44 is turned off. 
The luminance signal component which flows through diode 42 while that 
diode is rendered conductive, that is, for the duration that the positive 
portion of the composite video signal exceeds threshold level T.sub.L, is 
attenuated by attenuator 74, and the attenuated luminance component is 
supplied to mixer 22 substantially free of the synchronizing signal 
component. Similarly, the synchronizing signal component which flows 
through diode 44 during the interval that the negative portion of the 
composite video signal exceeds threshold level T.sub.S is attenuated by 
attenuator 84, and the attenuated synchronizing signal component is 
supplied by way of emitter-follower 90 to mixer 26 substantially free of 
the luminance signal component. Since the video information portion, or 
luminance information component, does not coincide in time with the 
negative-going horizontal synchronizing pulse, diodes 42 and 44 are 
rendered conductive mutually exclusively. Bias circuits 46 and 56 serve to 
make certain that the positive-going luminance information component 
passes through diode 42 to the exclusion of the negative-going horizontal 
synchronizing pulse, and that the negative-going horizontal synchronizing 
pulse passes through diode 44 to the exclusion of the positive-going 
luminance information component. 
While the present invention has been particularly shown and described with 
reference to a preferred embodiment, it will be readily appreciated by 
those of ordinary skill in the art that various changes and modifications 
may be made. For example, in the event that the luminance information 
component is a negative-going signal and the horizontal synchronizing 
pulse is a positive-going pulse, the poling of diodes 42 and 44 may be 
reversed. Nevertheless, the diode included in the luminance channel is 
conductive during the luminance signal interval and is biased to 
non-conduction in response to the horizontal synchronizing signal 
component; and the diode included in the synchronizing channel is 
conductive during the synchronizing signal interval and is biased to 
non-conduction in response to the luminance signal component. 
It is intended that the appended claims be interpreted as covering not only 
the illustrated embodiment but those variations and modifications 
mentioned above. It is recognized that the synchronizing signal separator 
circuit need not be limited solely to two-terminal diode devices but, 
rather, may cover diode-connected transistors or other known 
uni-directional conducting circuit elements.