Canon CANOSCAN N1220U Service Manual

Also see for CANOSCAN N1220U: User guide User guide Getting started guide Getting started

CHAPTER 22 - 19COPYRIGHT  2000 CANON INC. CANOSCAN N650U/N656U/N1220U REV.0 JUNE 2000 PRINTED IN JAPAN (IMPRIME AU JAPON)Input Buffer RAM I (for writing)Buffer RAM II (for reading) OutputFixed volume has been writtenBuffer RAM I (for reading) OutputInput Buffer RAM II (for writing)Sampling pointMoving directionScanning unitFigure 2-194. CalibrationWhen the contact image sensor reads a document of an even density, the image signalcorresponding to each pixel is not uniform for the following reasons.1) Light intensity of LED is not uniform.2) There is variation in the sensitivity of the phototransistors.3) There is a slight output from the phototransistors even when there is no input.These variations are corrected by the calibration. Calibration data is used as standarddensity data when scanning a document. Scanned image data is compared to the standarddensity data for the image data correction. The calibration converts the image data from 14bits to 12 bits each color.5. Gamma correctionCalibrated red, green and blue image data are divided uniformly into 4096 gradationsaccording to the document density. The contrast and density of this image data are adjustedby the gamma correction. The gate array writes gamma curve specified by the control programinto the buffer RAM before scanning a document. Image scan starts and the image data inputto the buffer RAM is converted to 8 bits by the gamma curve data and is output.6. PackingProcessed image data is temporarily stored in the buffer RAM before sent to the hostcomputer. The buffer RAM is divided into the writing block and reading block. While data iswritten into the writing block, the data is read from the reading block. When fixed volume hasbeen written, the writing switches to the reading.Figure 2-20When decreasing the resolution, the scanning unit moves at a fast speed to read less lines.