Image signal processing

Abstract: An image signal processing apparatus wherein the black level of an analog image signal is adjusted by controlling the clamp level of the inputted analog image signal. The clamped analog image signal is digitalized. Using the digital image signal offset to a first level during the blanking period, the black level adjustment of the analog image signal is carried out. The first level during the blanking period of the digital image signal corresponding to the analog image signal with the black level having been adjusted, is changed to a second level, allowing a correct and easy black level adjustment using a simple circuit arrangement.

Description: The present invention relates to a method and apparatus for adjusting a clamping device in image signal processing apparatus. A known image signal processing apparatus for processing an analog image signal after digitization has a structure such as shown in Fig. 1. In Fig. 1, reference numeral 1 represents a clamp circuit, 2 represents an A/D converter, 3 represents a digital signal processing circuit, 4 represents a D/A converter, and 5 represents a variable voltage source. The operation of this apparatus will be described below. Referring to Fig. 1, an analog image signal inputted from an input terminal a is clamped to a clamp level by the clamp circuit 1 at the timing when a clamp pulse is entered from an input terminal b . The clamp level is preset by the variable voltage source 5. The analog image signal clamped by the clamp circuit 1 is converted into a digital data by the A/D converter 2 synchronously with a clock pulse inputted from an input terminal c , and thereafter the digital data is supplied to the digital signal processing circuit 3. The digital signal processing circuit 3 is supplied with the clock pulse inputted from the input terminal c as well as a blanking pulse inputted from an input terminal d . The digital signal processing circuit 3 converts the digital data supplied from the A/D converter 2 into a data which indicates "0" during the blanking period designated by the blanking pulse, and performs various digital signal processing. The processed signal is supplied to the D/A converter 4. The digital data processed by the digital signal processing circuit 3 is converted into an analog image signal synchronously with the clock pulse inputted from the input terminal c , and thereafter the analog image signal is outputted from an output terminal e . In the image signal processing circuit shown in Fig. 1, a so-called black level adjustment is carried out by adjusting a preset voltage level of the variable voltage source 5 and changing the clamp level so as to make the blanking level coincide with the black level of an inputted image signal. However, even if the black level of an inputted image signal becomes lower than the blanking level, the digital data outputted from the A/D converter will not lower below "0", resulting in difficulty in adjusting the black level. Furthermore, even if the minimum reference potential of the A/D converter 2 is arranged to be adjusted so as to match the level of an image signal to be inputted to the A/D converter 2, an error is likely to occur and the circuit becomes complicated,agai resulting in difficulty in adjusting the black level. An image signal processing apparatus for processing an analog color image signal after digitization has the structure such as shown in Fig. 2. In Fig. 2, reference numeral 11 represents a clamp circuit, 12 represents an A/D converter, 13 represents a digital signal processing circuit, 31 and 35 represent a switch, 32 represents a luminance matrix circuit, 33 represents a chrominance matrix circuit, 34 represents an adder, 14 represents a D/A converter, and 15 represents a variable voltage source. The operation of the apparatus shown in Fig. 2 will be described below. Referring to Fig. 2, analog R, G, and B signals inputted from input terminals are clamped to clamp levels by the clamp circuits 11 at the timing when a clamp pulse is entered from an input terminal. The clamp levels are preset by variable voltage sources 15. The analog R, G, and B signals clamped by the clamp circuits 11 are converted into digital data by the A/D converters 12 synchronously with a clock pulse inputted from an input terminal, and thereafter the digital data is supplied to the digital signal processing circuit 13. The digital signal processing circuit 13 is supplied with the above-described clock pulse as well as a blanking pulse inputted from an input terminal. The digital signal processing circuit 13 converts the digital data supplied from the A/D converters 12 into data which indicates "0" during the blanking period designated by the blanking pulse by connecting the switches 31 controlled by the blanking pulse to B terminals shown in Fig. 2. The luminance matrix circuit 32 and chrominance matrix circuit 33 generates a digital luminance signal and R-Y and B-Y digital chrominance signals using the inputted digital data. The digital chrominance signals are supplied to the D/A converters 14 via switches 35 which are controlled by an adjustment signal supplied from an input terminal and connected to D terminals shown in Fig. 2 during the black level adjustment. During the period other than the blanking period, the digital luminance signal is supplied to the D/A converter 14 via the adder 34 and switch 31 controlled by the blanking pulse, the digital luminance signal being added with a data (e.g., a digital data representative of a voltage level "7") set by an offset data a entered from an input terminal so that the black level of the luminance signal relative to the blanking level is adjusted. The digital luminance signal, R-Y and B-Y digital chrominance signals processed by the digital signal processing circuit 13 in the above manner are converted into analog image signals synchronously with the clock pulse inputted from the input terminal, and thereafter the analog image signals are outputted from output terminals. In the image signal processing circuit shown in Fig. 2, during the black level adjustment, the R, G, and B digital signals indicating "0" during the blanking period are supplied to the D/A converters 14 by changing the connection of the switches 35 by the adjustment signal, and converted into analog image signals to be outputted. The voltage setting levels of the variable voltage sources 15 are adjusted by the levels of the outputted analog R, G, and B signals to thereby change the clamp levels. In this manner, the blank level is made to become coincident with the black levels of the analog R, G, and B signals inputted from the input terminals, to thereby conduct the black balance adjustment between the luminance signal outputted from the luminance matrix circuit 32 and the chrominance signals outputted from the chrominance matrix circuit 33. As described above, in the image signal processing apparatus shown in Fig. 2, the voltage setting levels of the variable voltage sources 15 are adjusted to change the clamp levels such that the blanking level is made to become coincident with the black levels of the inputted analog R, G, and B, thereby conducting the black balance adjustment between the outputted luminance signal and analog R-Y and B-Y chrominance signals. However, even if the black levels of the inputted analog R, G, and B signals become lower than the blanking level during the black balance adjustment because of setting particular clamp levels, the digital data outputted from the A/D converter 12 will not lower below "0", resulting in difficulty in adjusting the black level. Furthermore, even if the minimum reference potentials of the A/D converter 12 are arranged to be adjusted so as to match the levels of analog R, G, and B signals to be inputted to the A/D converters 12, an error is likely to occur and the circuit becomes complicated, again resulting in difficulty in adjusting the black level. European Patent Specification No. EP-A-0,220,894 discloses apparatus for adjusting an analog clamping device and in which a predetermined value is applied during blanking periods to an ADC converter. Similarly, U.S. Patent Specification No. US-A-4,642,696 and UK Patent Specification GB-A-2,228,643 disclose clamping circuits which suffer from the problem with which the present invention is concerned. It is a concern of the present invention to provide an image signal processing apparatus capable of solving the above-described problems. According to one aspect of the present invention there is provided an image signal processing apparatus as set out in claim 1. In order that the present invention may be more readily understood various embodiments thereof will now be described by way of example and with reference to the accompanying drawings in which: Fig. 1 shows the outline of the structure of a conventional image signal processing apparatus. Fig. 2 shows the outline of the structure of another conventional image signal processing apparatus. Fig. 3 shows the outline of the structure of an image signal processing apparatus according to a first embodiment of the present invention. Fig. 4 shows the outline of the structure of an image signal processing apparatus which is included by way of example. A preferred embodiment of the present invention will be described below. Fig. 3 shows the outline of the structure of an image signal processing apparatus according to an embodiment of the present invention. In Fig. 3, like elements to those shown in Fig. 1 are represented by using identical reference numerals, and the detailed description thereof will be omitted. Referring to Fig. 3, an analog image signal inputted from an input terminal a is clamped by a clamp circuit 1 and supplied to an A/D converter 2, in a similar manner as of Fig. 1. The digital data converted by the A/D converter is supplied to terminal A of a switch 6. An offset data representative of a predetermined voltage level (e.g., a digital data representative of a voltage level "15") is supplied from an input terminal f to terminal B of the switch 6 which is activated by a blanking pulse entered from an input terminal d . The switch 6 is connected to terminal A during the period other than the blanking period so that the digital data from the A/D converter 2 is outputted. During the blanking period, the switch 6 is connected to terminal B so that the offset data is outputted. In this manner,the digital data output from the switch 6 has a voltage level set to a predetermined offset voltage level during the blanking period. Therefore, this digital data (in this case, the digital data representative of "15") may have a lower level than the blanking level. The digital data outputted from the switch 6 is supplied to terminal A of a switch 8 and to the positive terminal of a digital subtracter 7. To the negative terminal of the digital subtracter 7, there is supplied thesame offset data entered from the input terminal f . The digital data outputted from the digital subtracter 7 therefore has a level of the digital data outputted from the switch 6 and subtracted by the offset data. This digital data is supplied to a terminal B of the switch 8. In the normal state, the switch 8 is connected to terminal B . Therefore, the digital data supplied to the digital signal processing circuit 9 has a level indicating "0" during the blanking period because the digital data at the switch 6 was offset and subtracted by the offset level by the digital subtracter 7. The digital signal processing circuit 9 performs various digital signal processing, and the processed signal is converted into an analog image signal by a D/A converter and outputted from an output terminal e . During the black level adjustment of an inputted image signal, the switch 8 is connected to terminal A upon reception a control signal entered via an input terminal g from a system controller (not shown). Therefore, the digital data at the switch 6 offset by the offset level during the blanking period is supplied to the digital signal processing circuit 9, and the analog image signal is outputted from the D/A converter 4. During the black level adjustment, it is possible to monitor a digital data level having a lower level than the blanking level (i.e., "15"), allowing correct and easy black level adjustment. In the above embodiment, although the blanking level is set to "15", this value may be set as desired. Furthermore, although the clamp circuit operates at the timing of a clamp pulse, this timing may be set in a different manner. For example, the clamp circuit may operate at the timing when a minimum potential such as a sink chip level of an inputted image signal is detected. As described above, according to the present embodiment, the black level adjustment of an image signal can be performed correctly and easily using a simple circuit arrangement. Fig. 4 shows the outline of the structure of an image signal processing apparatus included by way of example. In Fig. 4, like elements to those shown in Fig. 2 are represented by using identical reference numerals, and the detailed description thereof will be omitted. Referring to Fig. 4, analog R, G, and B signals inputted from input terminals are clampled to clamp levels by clamp circuits 11 at the timing when a clamp pulse is entered from an input terminal. The clamp levels are preset by variable voltage sources 15. The analog R, G, and B signals clamped by the clamp circuits 11 are converted into digital data by A/D converters 12 synchronously with a clock pulse inputted from an input terminal, and thereafter the digital data is supplied to a digital signal processing circuit 16. The digital signal processing circuit 16 is supplied with the above-described clock pulse as well as a blanking pulse inputted from an input terminal. The digital signal processing circuit 16 replaces the digital data supplied from the A/D converters 12 by another data during the blanking period by connecting the switches controlled by the inputted blanking pulse to B terminals shown in Fig. 2. This data is an offset data b (e.g., a digital data representative of "15") entered from an input terminal during the blanking period. During the black balance adjustment of a luminance signal and chrominance signals, switches 65 are connected to terminals D upon reception of an adjustment signal entered from an input terminal. Therefore, during the adjustment, digital R, G, and B signals indicating "15" during the blanking period are supplied to D/A converters 14 by changing the connection of the switches 65 to terminals D shown in Fig. 4, and converted into analog R, G, and B signals to be outputted. The voltage setting levels of variable voltage sources 15 are adjusted by the levels of the outputted analog R, G, and B signals to thereby change the clamp levels. In this manner, the blank level is made to become coincident with the black levels of the analog R, G, and B signals inputted from the input terminals, to thereby conduct the black balance adjustment between the luminance signal and the chrominance signals. During the black balance adjustment, the digital data having a level lower than the blanking level (i.e., voltage level "15") can be monitored so that it is easy to confirm that the black levels of the inputted analog R, G, and B signals are lower than the blanking level because of particular clamp setting levels. In this manner, the black balance adjustment can be performed correctly and easily. The digital R, G, and B signals with the black balance adjusted are supplied to a luminance matrix circuit 62 and chrominance matrix circuit 63 to generate a digital luminance signal and R-Y and B-Y digital chrominance signals. The digital chrominance signals R-Y and B-Y are supplied to D/A converters 14 via switches 65 which are controlled by the adjustment signal and connected to C terminals shown in Fig. 4. The digital luminance signal is supplied to an D/A converter 14 via a subtracter 64 and a switch 61 controlled by the blanking pulse, the digital luminance signal being set to a predetermined offset level during the blanking period to adjust the black level of the luminance signal relative to the blanking level in the similar manner as of Fig. 2. The predetermined offset level is calculated from an offset data (e.g., a digital data representative of a voltage level "7") c entered from an input terminal and the offset data b . In this case, the predetermined offset level is a digital data representative of a voltage level "8" ="15" - "7". The digital luminance signal, R-Y and B-Y digital chrominance signals supplied to the D/A converters 14 are converted into analog image signals synchronously with the clock pulse inputted from an input terminal and outputted externally. During the black level adjustment, it is possible to monitor a digital data having a lower level than the blanking level (e.g., a voltage level "15"), allowing correct and easy black level adjustment. In the above embodiments, although the blanking level offset is set to "15" and the adjustment level of the black level of the luminance signal relative to the blanking level is set to "7", these values may be set as desired.

Claims: An image signal processing apparatus for processing an analog image signal having active and blanking periods, the apparatus comprising: clamping means (1) for clamping said image signal to a desired level; means (5) whereby said level can be varied; analog/digital converting means (2) for converting the clamped image signal into a digital signal, and signal processing means (4, 9) for digitally processing the output of said analog/digital converting means and providing an analog image output signal; and wherein said apparatus has a normal operational mode and a black level adjustment mode; said apparatus further comprising first switch means (6) adapted in said normal operational mode and during said active periods to supply to said signal processing means the output of said analog/digital converting means less a predetermined offset value, and during the blanking periods a signal of a first predetermined level which replaces the output of said analog/digital converter means so as to set during the blanking periods a first predetermined level at the input of the signal processing means; and second switch means (8) adapted in said black level adjustment mode and during said blanking periods to replace said signal of a first predetermined level with a signal of a second predetermined level and during said active periods to remove the offset applied to the image signal. Apparatus according to claim 1, wherein said first switch means comprise a first switch (6) having two input terminals (A, B), one terminal (A) being connected to the output of said analog/digital converting means, and the other terminal (B) being connected to a source of offset data which provides said predetermined offset value, said first switch having a control input adapted to receive a blanking pulse whereby during blanking periods said switch connects said predetermined offset value to said signal processing means. Apparatus according to claim 2, wherein the output of said first switch is connected both to one input (A) of a second switch (8) and to an input of a subtractor (7) the output of which subtractor is connected to a second input (B) of said second switch, and wherein said subtractor has a second input connected to the source of said predetermined offset value, and wherein the operation of said second switch is such that during said normal operation mode said second switch connects the output of said subtractor to said signal processing means, and during said black level adjustment mode said second switch connects the output of said first switch to said signal processing means.

Inventors: OHARA EIJI, TAKAIWA KAN, OHARA, EIJI, TAKAIWA, KAN, OHARA, EIJI, CANON KABUSHIKI KAISHA, TAKAIWA, KAN, CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H04N5
IPC codes: H04N9, H04N5