1. Field of the Invention
The present invention relates to an image signal processing apparatus which is used for an electronic still camera or the like, and in which the transfer control of an image signal sent from an image pickup element is improved in connection with the encoding of the image signal.
2. Description of the Prior Art
As an apparatus for converting an optical image into an image signal through an image pickup element and for storing the image signal as digital data into a storage device, there is, for example, a digital-type electronic still camera. In the electronic still camera, image data compression is essential in order to store a bulky amount of digital image data in a storage device of a given capacity. As the method of image data compression, conventionally, there have been proposed various methods. For example, known methods are a DPCM system using a correlation between picture elements adjacent to each other, a DCT system in which a picture area is divided into n x n picture element areas (blocks) composed of n columns of picture elements and rows of picture elements and a correlation in which every block is used.
FIG. 9 is a block diagram showing an example of the signal processing in an electronic still camera employing the DPCM system. This example will be described hereunder.
An image signal produced from an image pickup element 11 such as a CCD or the like is amplified by an amplifier 12, and the amplified signal is separated into three color signals, for example, red, green, and blue (R, G, B) color signals, by a color separation circuit 13. Then, the color signals R, G, and B are processed by gamma circuits 14 and white balance circuits 15 respectively, converted into digital signals by A/D conversion circuits 16 respectively, and stored in a field memory 17. The field memory 17 is used as a buffer memory for a DPCM encoding circuit 18.
The DPCM encoding circuit 18 serves to encode the image data which is converted into the digital signal and read from the field memory 17 in the order of color frames or in the order of color lines. FIG. 10 shows the configuration of the DPCM encoding circuit 18. That is, in the DPCM encoding circuit 18, a predicted value X.sub.0 (obtained by predicting a value at the present point of time on the basis of a sampled value/values at a sample point/points before the present point of time) is subtracted from a sampled value x.sub.0 at the present point of time by means of a subtracter 81 to thereby obtain a prediction error signal .epsilon..sub.0. The prediction error signal .epsilon..sub.0 obtained as a result of the subtraction is outputted after being non-linearly converted by a non-linear converter 82. The predicted value may be obtained, for example, by a method using a sampled value at a sampling point one picture element before the present point of time (hereinafter referred to as a previous-sample-prediction method).
Reference numeral 83 designates a local decoder portion which serves to prevent accumulation of errors generated in the decoding operation. In the local decoder portion 83, a non-linear reverse converter 84 serves to reversely convert an output of the non-linear converter 82, and an adder 85 serves to add a prediction error signal which is an output of the non-linear reverse converter 84 to the above-mentioned predicted value, the result of addition being supplied to a predictor 86.
The image data encoded in the DPCM encoding circuit 18 is written, through a buffer memory 19, into an image data storage medium such as an IC memory card or the like in a storage device 20. The buffer memory 19 serves to finally write the image data into a storage medium 20a while controlling the rate of data transfer between the DPCM encoding circuit 18 and the image data storage medium 20.
In FIG. 9, a CCD driving circuit 21 serves to operate a CCD used as the image pickup element 11. Further, a timing generation circuit 22 is controlled by a system control circuit 23 so as to supply timing signals at predetermined points of time to various circuits 13 through 21 to thereby operate those circuits 13 through 21.
In such a configuration, the gamma circuit 14, the white balance circuit 15, and the A/D conversion circuit 16 are required for each of the color signals of R, G, and B. Further, all the color signal R, G, and B separated by the color separation circuit 13 are simultaneously outputted from the color separation circuit 13. Accordingly, if the field memory 17 were not provided, it would be necessary to provide the DPCM encoding circuit 18 for each of the color signals of R, G, and B. It is however difficult to actually realize such a configuration, and, therefore, it is necessarily required to provide, in a camera, a buffer memory, such as a field memory 17 as mentioned above or a frame memory, of a large capacity.
Thus, in the conventional case, it has been necessary to provide three sets of the gamma circuits 14, the white balance circuits 15, and the A/D conversion circuits 16 for the respective R, G, and B color signals and the field memory 17 or the like which is a large-capacity buffer memory. As a result, the circuit scale and cost is prevented from being reduced so that the apparatus cannot be made inexpensive.