Clamp circuits

A circuit for providing a signal which is clamped to a desired DC voltage level comprises an amplifier for receiving an input signal and a feedback loop for developing an offset signal for combination with the input signal by the amplifier to provide an output signal at the desired DC voltage level. The feedback loop includes a noise detector for detecting the level of noise present in the output signal, and adjusting the time constant of the feedback loop in dependence thereon.

This invention relates to clamp circuits and more particulary, but not 
excusively, to feedback video clamp circuits. 
It is known to use clamp circuits in the field of video signal processing. 
The clamp circuit (otherwise known as a DC restorer) is employed to clamp 
the composite video signal to the correct DC level for further processing 
by other circuitry and to remove low-frequency noise, including mains hum 
at 50 Hz or 60 Hz, from the video signal. A video clamp circuit operates 
by identifying a portion of the video waveform which recurs and ideally is 
of constant voltage, or is a known AC component superimposed on a constant 
voltage, for example the back porch of the horizontal synchronizing 
interval or the horizontal sync tip, and clamps this selected portion of 
the video waveform to a predetermined voltage potential level. 
Video clamp circuits may be classified by the speed of the clamping action. 
The speed may be fast, so as to clamp in several TV lines, or it may be 
slow and take many TV frames in order to clamp. A clamp circuit is 
designed to operate with a particular speed in order to optimize a 
certrain function, such as removing mains hum. 
It has been found that a conventional video clamp circuit will not 
satisfactorily remove all commonly encountered low frequency distortions. 
Slow clamps are not able to adequately remove mains hum. Fast clamps can 
do an adequate job of removing mains hum, and even more abrupt changes in 
DC level such as might be encountered when switching between two different 
signal sources, but may cause other problems when there is noise on the 
incoming signal. One such problem is a phenomenon known as "streaking", 
which is so named because when the affected signal is viewed on a picture 
monitor bright and dark streaks will be seen. This "streaking" is much 
more objectionable than the inadequately removed mains hum which arises 
through use of a slow clamp. Note that by noise persons skilled in the art 
generally mean random impulsive and continuous signals with frequency 
content predominatly above 15 kHz and which would be translated to lower 
frequency noise (i.e. streaking) by the action of a conventional fast 
clamp. 
According to the present invention there is provided a circuit for 
providing a signal which is clamped to a desired DC voltage level, 
comprising amplifier means for receiving an input signal, means for 
developing an offset signal for combination with the input signal by the 
amplifier means to provide an output signal at the disired DC voltage 
level, the signal developing means having a variable time constant and 
operating, in use, to bring the output signal to the desired DC voltage 
level in an interval dependent upon the value of said time constant, and 
means for detecting the level of noise present in the output signal and 
adjusting the value of said time constant in dependence thereon. 
The present invention may be used to provide a video clamp circuit which 
adapts automatically to variation in the noise level of the incoming video 
signal. As the noise level of the incoming signal increases, the speed at 
which the signal is clamped is reduced. This ability of the clamp circuit 
to adapt its clamp speed to the noise level enables the clamp circuit to 
provide good DC level restoration without introducing other problems.

FIG. 1 illustrates the section of a composite vidio signal that is between 
successive horizontal lines of the raster. At the end of one line (the 
right side of the television screen when viewed from the front) the video 
signal drops to the blanking level and remains briefly at the blanking 
level before the horizontal sync pulse occurs (the so-called front porch 
of the synchronizing interval). The sync pulse is a negative-going pulse, 
and the base portion of the pulse is known as the sync tip. After the sync 
pulse, the signal returns to the blanking level for the back porch of the 
synchronizing interval. A subcarrier reference color burst occurs during 
the back porch of the synchronizing interval. After the back porch, the 
next horizontal line of the video signal commences. 
The circuit illustrated in FIG. 2 comprises an input amplifier 10 which 
receives the composite video input signal. The output of the amplifier 10 
is used to generate a noise-sensitive DC signal which is fed back to the 
amplifier 10 as a DC offset signal. The feedback loop by which the DC 
offset signal is generated functions by sampling the output of the 
amplifier 10 during the burst time on the back porch of the video signal. 
The sampling is accomplished by means of a burst filter 12 and a back 
porch sampler 14. The burst filter 12 is a combination band reject, low 
pass filter tuned to the frequency of the color burst signal. The output 
of the burst filter is fed to the back porch sampler, which performs an 
averaged sample and hold function during burst time. The back porch 
sampler operates under control of a timing circuit 16 which receives 
horizontal sync pulses from a time-pulse generator 18. The output of the 
back porch sampler 14 is representative of the average DC level of the 
output signal during burst time, and is applied to a variable time 
constant amplifier 24 and to a noise detection circuit 22. The output of 
the amplifier 24 is applied to the amplifier 10 as a DC offset voltage, 
which brings the DC level of the output video signal to the desired level. 
Referring to FIG. 3, the signal received by the noise detection circuit 22 
from the back porch sampler is first processed by a high pass filter 221 
in order to remove the DC and low frequency information which are present 
at the output of the back porch sampler and are systematic effects which 
should not be detected as noise. The ouput of the high pass filter 221 is 
fed to two analog switches 222 and 223 which are in parallel and determine 
whether the output of the high pass filter is passed to the RMS converter 
224. The high pass filter output also is used by the large transient 
detector 225, which controls the time out circuit 226. The output of the 
time out circuit 226 controls the analog switch 222 and is fed to the time 
out duty cycle monitor 227. Finally the time out duty cycle monitor 227 is 
used to control the analog switch 223. 
In normal operation, that is to say when the incoming signal is received 
without large amounts of noise or large DC level transients, the output of 
the burst sampler contains HF information which is almost solely the 
result of noise present on the incoming signal. Under this condition the 
output of the high pass filter is allowed to pass through the switch 222 
into the RMS converter 224 which produces a DC output proportional to the 
RMS value of its input and hence to the noise on the incoming signal. 
In the case of an input signal which contains large amounts of noise or 
large DC level transients, the large transient detector 225, time out 
circuit 226, and time out duty cycle monitor 227 come into use. If a large 
DC level shift occurs such as might be encountered when switching between 
different signals, the clamp will react to correct this level shift and in 
doing so cause a large high frequency transient to appear at the output of 
the high pass filter 221. The transient should not be detected as noise, 
since it is just the clamp acting to correct a systematic error. Therefore 
the large transient detector 225 determines whether the amplitude of the 
transient exceeds an amount which would cause unacceptable errors in the 
output of the RMS converter and causes the switch 222 to open and prevent 
the large transient from reaching the RMS converter if its amplitude is 
excessive. The large transient detector does this by triggering the time 
out circuit 226 which causes the switch 222 to open for a short fixed time 
period. This time period is long enough to prevent the bulk of the 
recovery transient from reaching the RMS converter. 
Unfortunately DC level transients cannot be distinguished from large 
amounts of noise solely on the basis of amplitude information. The time 
out duty cycle monitor distinguishes between DC level transients and large 
amounts of noise at an additional level of discrimination by using the 
assumption that DC level transients are unlikely to occur on a very 
frequent basis. The time out duty cycle monitor does this by determining 
the duty cycle of time out circuit 226, i.e., the aggregate time for which 
the circuit 226 has held the switch 222 open within a predetermined 
period, e.g. a few TV lines. If the duty cycle exceeds a limit which is 
established in advance and may be, for example, 25%, the duty cycle 
monitor 227 detects this and overrides the switch 222 by causing the 
switch 223 to close and allow the output of the high pass filter 221 to 
reach the RMS detector 224, since based on the assumption of infrequent DC 
level transients the large transient detector is responding to noise and 
not to large transients. In this way a satisfactory determination of noise 
is achieved even in the presence of large DC transients. 
As noted above, the noise detector 22 generates a voltage which is 
proportional to the amplitude of the noise present on the output of the 
burst sampler. The output signal from the noise detector 22 is applied as 
a control voltage to the variable time constant amplifier 24. The time 
constant of the amplifier 24, and hence the speed of the feedback loop 
formed by the filter 12, sampler 14 and amplifier 24, is electrically 
controlled by the output signal from the noise detector, so that a video 
signal which is relatively free of noise is clamped rapidly, and noisier 
video signals are clamped progressively more slowly. In this manner the 
consequences of sampling the back porch of the output video signal during 
an interval when its level is determined by noise, rather than systematic 
effects, and thus providing an input to the amplifier 24 which reflects 
the noise level rather than the level of systematic effects, are 
minimized: by causing the signal to be clamped slowly, the feedback loop 
effectively integrates the sampled level of the back porch over an 
extended period and thus minimizes the effect of noise. 
It will be appreciated that the invention is not limited to the particular 
clamp circuit which has been shown and described, since variations may be 
made therein without departing from the scope of the invention as defined 
in the appended claims. For example, instead of sampling the output signal 
from the amplifier 10 during the burst time on the back porch of the 
horizontal synchronizing interval in order to determine the DC level of 
the signal and to generate the DC offset voltage, the signal may be 
sampled during sync tip to generate the DC offset voltage. In addition, 
the invention is not limited to the general class of DC restorers using 
feedback. For example, the invention can also be implemented as a 
feedforward DC restorer. Although the invention has been described in 
connection with a conventional analog color video signal, the principles 
of the invention are also applicable to digital video signals (using 
either hardware or a software algorithm), to inverted video signals and to 
monochrome video signals.