Dark level restoring circuit for television receiver

A dark level restoring circuit for a television receiver which receives a video signal, clamps a pedestal portion of the video signal to a reference pedestal level by a pedestal clamper, compares the output of the pedestal clamper with a reference dark level, and amplifies the compared result by a gain control amplifier. The output of the pedestal clamper and the amplified compared result are synthesized, the dark peak level is held, and the held dark peak level and the reference pedestal level are compared wherein the gain of the gain control amplifier is controlled by the compared result. A blanking signal is received, and a mute signal is generated which corresponds to a non-video signal portion when the effective raster size of the video signal is smaller than the size of a face plate of a cathode ray tube to which the video signal is supplied. The blanking signal and the mute signal are synthesized and a control signal is generated which controls the gain control amplifier such that its gain is minimized and the holding of the dark peak level is inactive.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to a dark level ring circuit for use with a 
television receiver. 
2. Description of the Prior Art 
An example of a conventional dark level restoring circuit (U.S. Pat. No. 
4,631,589) will be described below with reference to FIG. 1 of the 
accompanying drawings. 
As shown in FIG. 1, a video signal applied to an input terminal 1 from a 
video signal source (not shown) is supplied to a pedestal clamp circuit 2, 
in which pedestal level of the input video signal is clamped to reference 
pedestal level of a reference voltage source 3. A video signal from the 
pedestal clamp circuit 2 is supplied to a dark detector/dark expander 
(dark level detector/dark level expander) 5 and a synthesizer (adder) 4. 
The dark detector/dark expander 5 comprises a level comparator 6 and a 
variable gain amplifier (gain controller) 8. The video signal from the 
pedestal clamp circuit 2 is supplied to the level comparator 6 which 
detects a dark level signal portion below the reference dark level by 
comparing the level of the video signal supplied thereto with a reference 
dark level from a reference voltage source 7. The dark level signal 
portion is supplied to a variable gain amplifier (gain controller) 8, in 
which the dark level signal portion is lowered and expanded. A gain of the 
variable gain amplifier 8 is controlled in response to a compared output 
level from a level comparator 14 which will be described later on. 
The dark level signal portion that had been expanded by the variable gain 
amplifier 8 in the dark detector/dark expander 5 is added to or replaced 
with the dark level signal portion of the video signal clamped by the 
pedestal clamp circuit 2 so that a video signal whose dark level portion 
is expanded is output from an output terminal 13. This output video signal 
is processed by some suitable video signal processing means (not shown) 
and supplied to a cathode ray tube (CRT), not shown. Simultaneously, this 
output video signal is supplied to a dark peak hold circuit 12. The gain 
of the variable gain amplifier 8 is optimized in response to the addition 
or replacement of the dark level signal portion. 
The dark peak hold circuit 12 detects and holds the dark peak level of the 
dark level signal portion of the video signal supplied thereto from the 
adder 4. The dark peak level held by the dark peak hold circuit 12 is 
supplied to the level comparator 14, in which it is compared with the 
reference pedestal level of the reference voltage source 3. A compared 
output from the level comparator 14 is supplied to the gain controller 8 
and the gain of the variable gain amplifier 8 is controlled in response to 
the compared output level of the level comparator 14 such that the dark 
peak level becomes coincident with the reference pedestal level. 
A blanking signal from an input terminal 10 is supplied to the variable 
gain amplifier 8 and the dark peak hold circuit 12, whereby the gain of 
the variable gain amplifier 8 is made zero and the dark peak level 
detection in the dark peak hold circuit 12 is made inactive. Therefore, a 
last dark peak level in the video signal portion before the blanking 
period is maintained. Consequently, the signal below the detected dark 
level in the video signal portion is lowered to the pedestal level and 
changed to a dark level signal, thereby emphasizing a contrast. Thus, it 
becomes possible to improve a picture quality from a visual sense 
standpoint. 
Recent high-definition television receivers use a CRT having a face plate 
with an aspect ratio of 16:9. When a conventional television signal with 
an aspect ratio of 4:3 is received and reproduced by the high-definition 
television receiver, the following problems arise. 
These problems will be described below with reference to FIGS. 2A through 
2D. As shown in FIG. 2A, when a television signal with an aspect ratio of 
4: 3 is supplied to and reproduced by a CRT having a face plate with an 
aspect ratio of 16:9, non-video signal portions (non-picture portions) are 
produced on both sides of the video signal portion on the picture screen 
of the CRT. Accordingly, a signal of one horizontal period of that video 
signal is at the pedestal level in the non-video signal portions as shown 
in FIG. 2B and becomes lower than the reference dark level (detected dark 
level). 
In this case, as shown in FIG. 2C, when the variable gain amplifier 8 and 
the dark peak hold circuit 12 are made active within a period T1 including 
the video signal portion and the non-video signal portions provided at 
both sides of the video signal portion in which the blanking signal is at 
low level, the dark peak hold circuit 12 detects and holds the level of 
the video signal in the non-video signal portions as the dark peak level 
and the gain of the variable gain amplifier 8 is controlled such that the 
level of the video signal in the non-video signal portions becomes 
coincident with the reference pedestal level. As a consequence, the video 
signal (see FIG. 2D) output from the output terminal 13 is equal to the 
video signal (see FIG. 2A) from the pedestal clamp circuit 2. Thus, the 
dark signal portion below the dark detected level of the video signal is 
not expanded. 
SUMMARY OF THE INVENTION 
In view of the aforesaid aspect, it is an object of the present invention 
to provide a dark level restoring circuit for a television receiver in 
which a dark level signal portion of a video signal can effectively be 
expanded even when the effective raster size of the video signal becomes 
smaller than the size of a face plate of a cathode ray tube to produce a 
non-video signal portion corresponding to a portion outside the effective 
raster size of the face plate of the cathode ray tube. 
According to an aspect of the present invention, there is provided a dark 
level restoring circuit for a television receiver which is comprised of a 
video signal source, a pedestal clamp circuit connected to the video 
signal source and for clamping a pedestal portion of the video signal to a 
reference pedestal level, a first comparator connected to the pedestal 
clamp circuit and for comparing the output of the pedestal clamp circuit 
and a reference dark level, a gain control amplifier connected to the 
first comparator and for amplifying the output of the first comparator, a 
synthesizer for synthesizing the outputs of the pedestal clamp circuit and 
the gain control amplifier, a dark peak hold circuit connected to the 
synthesizer for holding the dark peak level, a second comparator for 
comparing the output of the dark peak hold circuit and the reference 
pedestal level, the output of the second comparator controlling the gain 
of the gain control amplifier, a blanking signal source, a mute signal 
circuit for generating a mute signal corresponding to a non-video signal 
portion when the effective raster size of the video signal is smaller than 
the size of a face plate of a cathode ray tube to which the video signal 
is supplied, and a logic circuit for synthesizing the blanking signal and 
mute signal and for generating a control signal, the control signal 
controls the gain control amplifier such that the gain of the gain control 
amplifier is minimum and the dark peak hold circuit such that the holding 
operation of the dark peak hold circuit is inactive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A dark level restoring circuit according to an embodiment of the present 
invention will hereinafter be described with reference to the drawings. 
FIG. 3 shows a circuit arrangement of the dark level restoring circuit 
according to the present invention. In FIG. 3, like parts corresponding to 
those of FIG. 1 which shows the example of the prior art are marked with 
the same references and therefore need not be described in detail. As 
shown in FIG. 3, a blanking signal (see FIG. 4C) applied to the input 
terminal 10 and a non-video area signal, i.e., mute signal (see FIG. 4D) 
applied to an input terminal 11 and which indicates the non-video signal 
portions corresponding to the portions outside the effective raster size 
of the face plate of the CRT obtained when the video signal from the input 
terminal 1 is displayed on the effective raster size of the face plate 
smaller than the face plate of the CRT are supplied to an OR gate 9 and an 
output signal (see FIG. 4E) of the OR gate 9 is supplied to the variable 
gain amplifier 8 and the dark peak hold circuit 12, whereby the gain of 
the variable gain amplifier 8 is made zero in the blanking period and the 
non-video signal portions and the dark peak detection in the dark peak 
hold circuit 12 is made inactive. 
Therefore, as shown in FIG. 4E, the variable gain amplifier 8 and the dark 
peak hold circuit 12 are made active only in the video signal portion of 
the period T2 so that the dark peak hold circuit 12 detects and holds the 
top of the dark level signal portion below the reference dark level within 
the video signal portion of the video signal shown in FIG. 4B as the dark 
peak level. Then, the gain of the variable gain amplifier 8 is controlled 
such that the dark peak level becomes coincident with the reference 
pedestal level. Thus, in the output signal (video signal) shown in FIG. 4A 
obtained from the output terminal 13, the dark level signal portion of the 
video signal (see FIG. 4A) from the pedestal clamp circuit 2 is expanded. 
A circuit for generating a mute signal will be described below with 
reference to FIG. 5. As shown in FIG. 5, there is provided a counter 19 
which clears a count value in response to a horizontal synchronizing 
signal HD supplied thereto from an input terminal 18. The counter 19 is 
cleared at the timing of the horizontal synchronizing signal HD supplied 
thereto and then counts a clock supplied thereto from an input terminal 
20. There is provided a ROM (read-only memory) 21 which is operated by the 
output of the counter 19 supplied thereto as an address signal. The mute 
signal shown in FIG. 4D is obtained from the ROM 21 at every horizontal 
period. Specifically, in the ROM 21 are written data which goes to low 
level when the count value of the counter 19 ranges from 0 to x.sub.1 
(count value corresponding to a timing tl in FIG. 4D), data which goes to 
high level when the count value of the counter 19 ranges from x.sub.1 to 
x.sub.2 (count value corresponding to a timing t2 in FIG. 4D), data which 
goes to low level when the count value of the counter 19 ranges from 
x.sub.2 to x.sub.3 (count value corresponding to a timing t.sub.3 in FIG. 
4D), data which goes to high level when the count value of the counter 19 
ranges from x.sub.3 to x.sub.4 (count value corresponding to a timing 
point t.sub.4 in FIG. 4D) and data which goes to low level when the count 
value of the counter 19 ranges from x.sub.4 to 0 (timing at which the 
horizontal synchronizing signal HD is supplied one more time and the 
counter 19 is cleared by the horizontal synchronizing signal HD supplied 
thereto). 
While the arrangement of the mute signal generating circuit has been 
described so far, the present invention is not limited thereto and various 
variants thereof may be considered but need not be described herein. 
The OR gate 9 is supplied with the blanking signal and the mute signal from 
the input terminal 10 and 11 and generates the control signal by which the 
gain of the variable gain amplifier 8 is made zero during the blanking 
period and the non-video signal portion and the dark peak level detection 
in the dark peak hold circuit 12 is made inactive. Therefore, it is 
needless to say that the OR gate or other suitable logic circuits may be 
used in accordance with polarities of the blanking signal and the mute 
signal. 
According to the present invention, since the gain of the variable gain 
amplifier is made zero and the dark peak detection in the dark peak hold 
circuit is made inactive during the non-video signal portions 
corresponding to the portions outside the effective raster size of the 
face plate of the cathode ray tube not only during the blanking period but 
also during the period in which the effective raster size of the face 
plate becomes smaller than the raster size of the face plate of the 
cathode ray tube to which the video signal is supplied, it is possible to 
obtain a dark level restoring circuit for a television receiver in which a 
dark level signal portion of the video signal can effectively be expanded 
even if there occur the non-video signal portions which correspond to the 
portions outside the effective raster size of the face plate of the 
cathode ray tube. 
Having described a preferred embodiment of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited thereto and that various changes and modifications could be 
effected therein by one skilled in the art without departing from the 
spirit or scope of the invention as defined in the appended claims.