CCD image sensors have hitherto been used in video cameras. The CCD image sensor generates pixel data for one screen, which is acquired as two-dimensional image data. A vertical register and a horizontal register read the pixel data, which is converted into one data stream. The data stream is output from one channel. FIG. 1 shows an CCD image sensor of one-channel output type that transfers data, and also a conventional image-signal processing apparatus of one-channel output type, for use in the CCD image sensor.
As depicted in FIG. 1, the conventional image-signal processing apparatus 101 is designed to process, for example, the image signals output from a CCD image sensor 110. The image sensor comprises a vertical register 111 and a horizontal register 112. The vertical register 111 transfers charges accumulated in an imager in the vertical direction, in units of lines. The horizontal register 112 transfers the charges transferred by the vertical register, in the horizontal direction in units of lines.
The image-signal processing apparatus 101 comprises an analog front-end circuit 121, a delay line 122, a Y/C separating circuit 123, a Y process circuit 124, and a C process circuit 125. The front-end circuit 121 receives a signal output from the horizontal register 112 of the CCD image sensor 110, performs gain control and A/D conversion on the signal and outputs a digital image signal. The delay line 122 delays the digital pixel data by a predetermined time so that the pixel data may be subjected to the Y/C separation that will be carried out later. The Y/C separating circuit 123 receives the pixel data input as a RGB signal or a complementary color signal and separates the pixel data into a luminance (Y) component and a chroma (C) component. The Y process circuit 124 effects a prescribed process on the luminance (Y) component of the pixel data and outputs luminance data. The C process circuit 125 carries out a specific process on the chroma (C) component of the pixel data and outputs chroma data.
In the image-signal processing apparatus 101 thus configured, the image signal output in one channel from the CCD image sensor 110 is converted to a digital signal and the digital signal is separated into a luminance (Y) component and a chroma (C) component. Therefore, the apparatus 101 can output digital image data that consists of these components. The CCD image sensor 110 may have about 1,000,000 pixels for one screen. In this case, the analog front-end circuit 121 can carry out analog processes, such as A/D conversion, at an operating frequency of about 33 MHz.
In recent years, CCD image sensors having high resolution of more than one million pixels for one screen have come into use. If the image-signal processing apparatus 101 designed to read an one-channel output is to read an image signal from a CCD image sensor of such a high resolution exceeding one million pixels, the process of the analog signal, such as A/D conversion, must be performed at an operating frequency exceeding 40 MHz. At such a high frequency, the analog-signal process such as A/D conversion is inevitably unstable. To make the process stable, it is necessary to use very expensive components such as an IC.
In order to solve this problem, CCD image sensors with a plurality of output channels have been proposed in recent years. Since image signals are supplied from many output channels, these analog signals can be processed (or converted to digital signals) at an operating frequency lower than is required when the CCD image sensor has only one channel. The analog process is performed on the signals of all channels, converting them to digital signals, and the digital signals are combined into one signal for one channel. Thus, the analog process is stable as is desired.
FIG. 2 illustrates a conventional image-signal processing apparatus 201 configured to process image signals output from two channels.
This image-signal processing apparatus 201 processes image signals output from a CCD image sensor 210. The CCD image sensor 210 comprises a vertical register 211 and two horizontal register 212 and 213. The vertical register 211 transfers charges, generated from light and accumulated in an imager in the vertical direction, in units of lines. The horizontal registers 212 and 213 transfer the charges transferred by the vertical register, in the horizontal direction in units of lines. In the CCD image sensor 210, the charges are transferred from the vertical register 211 to the first horizontal register 212 and the charges are transferred from the first horizontal register 212 to the second horizontal register 213. Hence, the sensor 210 outputs image data items for two lines, respectively, at the same time. For example, the first horizontal register 212 outputs the pixels forming an odd-numbered line, while the second horizontal register 213 outputs the pixels forming an even-numbered line.
The image-signal processing apparatus 201 comprises a first analog front-end circuit 221, a second analog front-end circuit 222, a delay line 223, a Y/C separating circuit 224, a Y process circuit 225, and a C process circuit 226. The first front-end circuit 221 receives a signal output from the first horizontal register 212, performs gain control and A/D conversion on the signal and outputs a digital image signal of the first channel. The second front-end circuit 222 receives a signal output from the second horizontal register 213, performs gain control and A/D conversion on the signal and outputs a digital image signal of the second channel. The delay line 223 combines the digital image signals of the first and second channels, into a one-channel image signal. The delay line 223 then delays the one-channel image signal by a predetermined time so that the image signal may be subjected to the Y/C separation to be performed later. The Y/C separating circuit 224 receives the image signal input as a RGB signal or a complementary color signal and separates the image signal into a luminance (Y) component and a chroma (C) component. The Y process circuit 225 carries out a prescribed process on the luminance (Y) component of the image signal and outputs luminance data. The C process circuit 226 effects a specific process on the chroma (C) component of the image signal and outputs chroma data.
In the image-signal processing apparatus 201 thus configured, the image signals output in two channels from the CCD image sensor 210 are converted to a digital signal. The digital signal is separated into a luminance (Y) component and a chroma (C) component. Thus, the apparatus 201 can output a digital image signal that contains a luminance (Y) component and a chroma (C) component. Since the CCD image sensor 210 outputs image signals in two channels, the analog front-end circuits 221 and 222 only need to have a low operating frequency. This renders the analog-signal process stable.
Here arises a problem in manufacturing the CCD images sensor that outputs image signals in two channels. It is very difficult to provide two horizontal registers that are identical in characteristics. Consequently, the signals output in two channels may differ in terms of gain. Further, they may differ in terms of the black-level offset. To adjust the gain difference and the black-level offset difference between channels, the horizontal registers hold a pilot signal each, and the pilot signal corrects the gain and black level of each channel. The pilot signals may have an error, however. Inevitably, it is difficult to adjust the gain difference or the black-level offset difference as is desired.