1. Field of the Invention
This invention relates generally to a video camera having a solid-state optical sensor and, more particularly, to a digital signal processing circuit by which the camera output signal frequency band is limited.
2. Description of the Background
In a known color video camera having three solid-state image transducing devices, the image of an object that falls on the solid state image transducing device for generating the green color signal is typically displaced in the horizontal direction. Such displacement is generally one-half the alignment pitch of the picture elements relative to the image of the object that is projected on the solid-state image transducing devices for generating the red and blue signals. This displacement of the green transducing device is provided in order to increase the resolution of the video camera and this technique is generally referred to as "special offset" or "special displacement". Solid-state television cameras employing this technique are shown in U.S. Pat. Nos. 3,975,760 and 4,672,430, both of which are assigned to the assignee hereof.
FIG. 1 represents the frequency spectrum of the output signal obtained using the conventional solid-state camera in which the "special offset" or "special displacement" technique is employed. The most widely used image sensing element for the solid-state camera is the charge coupled device (CCD). In FIG. 1, fl represents the sampling frequency of the CCD and, as described in U.S. Pat. No. 3,975,760, an output signal of the CCD is provided to an optical low-pass filter to limit the frequency band of such output signal.
Referring to FIG. 1, a baseband component of the output signal of the solid-state image sensing device is represented by solid line 31, a primary sideband component of the green signal is represented by broken line 32G, and primary sideband components of the red and blue signals are represented by the same broken line 32RB. In this case, the generation of so-called aliasing noise caused by the sideband components is prevented because the phase of the green signal and the phase of the red and blue signals are opposite each other and, thus, the aliasing noises offset each other. On the other hand, in the case of a digital video camera or a monochromatic or color image processing apparatus in which digital signal processing is employed, in-phase or so-called "same phase" sampling is mainly used because the application of the "special offset" or "special displacement technique" is difficult.
FIG. 2 represents a case of the in-phase or same phase sampling and because the sideband component shown by broken line 33 has the same phase as the output signals of the three solid-state image transducing devices, the sideband component 33 will extend into the baseband signal shown by solid line 31. This extension or overlapping between the sideband component and the baseband signal will produce an aliasing noise shown as the slanted line shaded portion in FIG. 2.
In order to overcome this aliasing noise problem, the use of an optical low-pass filter is known in conventional systems to depress a high-frequency component of the baseband component as shown by solid line 34A in FIG. 3A. Such optical low pass filter can prevent the generation of the aliasing noise because the frequency band component of the sideband component from 0 to 1/2 f1 is depressed, as shown by the broken line 34B in FIG. 3A. In the case of digital signal processing, a low-pass filter known as a pre-filter that has a frequency characteristic, as shown by the solid line 35 in FIG. 3B may be used to limit the frequency band of the baseband signal.
Although the optical low-pass filter as described above, which has a frequency characteristic generally as in FIG. 3A, can adequately prevent generation of the aliasing noise, there is the disadvantage that the modulation transfer function (MTF) at the high-frequency band may deteriorate due to attenuation of the high-frequency band having an effective frequency from 0 to 1/2 f1, because such optical low-pass filters generally do not have a sharp cutoff characteristic. The modulation transfer function is known in solid-state or diode type video cameras as the modulus of the optical transfer function and is generally synonymous with the sine amplitude response.