Raster input scanners typically employ one or more arrays such as CCDs for scanning. The array converts each scanned image line into a series of charges which, following suitable processing, are output as image signals or pixels to an end user. The scanning array, for example, may be carried on a carriage which traverses back and forth under a platen to provide the necessary relative motion between the image and array. Other arrangements such as a lamp scanning array with a movable document, etc., may also be utilized. An optical system focuses the reflected image onto the array, and one or more lamps provide illumination of the image.
In a typical scanning process, the image area viewed by each array of photosensors is converted into a charge potential representative of the image's gray level. Scanning takes place during an integration period of a preset duration. Following integration, the image charges are transferred to a pair of analog shift registers, the operating sequence being such that during the integration period, the image charges (image data) from a previous scanline are clocked from the shift registers, leaving the shift registers free to receive the image charges from the next integration period. The duration of the integration period, which must be sufficiently long to fully integrate the image line being scanned yet not so long as to allow the array of of photosensors to become saturated, is measured by a periodic fixed rate clock signal or shift pulses.
When the scanner operates in a synchronized manner, relative scanning movement between the array and images are at a fixed rate. This in turn permits timing of the shift pulses in the signal requesting the next line of image signals, referred to as the integration signals, to be synchronized with one another.
However, when a scanner operates asynchronously; i.e., when the internal transfer rate of the image input terminal is faster than the data transfer rate of the device receiving the image data; the relative scanning movement between the array and image is not fixed, but random and changes with demand. In other words, the signals are produced in response to the movement of the array or document. As a result, the timing of the integration signal timing can vary and not be in synchronism with the fixed rate shift pulses. This can reduce the integration period, resulting in an incomplete integration of the image line being scanned.
Many methods and devices have been developed which provide asynchronous transfer when there is a difference in the transfer rate between the sending and receiving devices and also address the incomplete integration problem. These various approaches will be briefly discussed below.
U.S. Pat. No. 4,541,061 to Schoon discloses an approach wherein the operating clock signal of a scanning apparatus is provided at a rate that matches the varying velocity of the scanning mirror through the use of a memory in which various clock rates are stored based on the known repetitive movement of the mirror. The entire contents of this U.S. Patent are hereby incorporated by reference.
Another approach has been proposed in U.S. Pat. No. 4,587,415 to Tsunekawa et al. This U.S. Patent discloses a photodetector with a timing control unit for controlling the information storage and information read out process of the detector. The entire contents of this U.S. Patent are hereby incorporated by reference.
Moreover, U.S. Pat. No. 4,628,368 to Kurata et al. discloses a system for controlling the scanning rate of a document reader in which the speed, acceleration, and the deceleration of the reader are set according to image information stored in a buffer memory. The entire contents of this U.S. Patent are hereby incorporated by reference.
U.S. Pat. No. 5,043,827 to Beikirch disclose a scanning system which corrects for incomplete integration. The scanning system utilizes an interpolator to provide a composite line of image signals when an integration period is prematurely terminated. The entire contents of this U.S. Patent are incorporated by reference
Furthermore, U.S. Pat. No. 4,878,119 to Beikirch et al. discloses a process for operating a scanning array asynchronously. The array has at least one row of sensors for scanning an image viewed by the array during an integration period and a shift register for receiving the signal charges developed by the sensor following the integration period. This process includes periodically generating, at a constant clock rate, integration pulses defining a succession of predetermined integration intervals and generating a start integration signal in response to a demand for a line of image signals where the time at which the start integration signals occurs is different than the time at which the integration pulses occur. The process interrupts the current integration interval in response to a start integration signal to commence a new integration interval to provide the asynchronous transfer of a line of image signals. The entire contents of this U.S. Patent are hereby incorporated by reference.
By using asynchronous stop/stop scanning, the optical system or documents are slowed down or stopped completely until an output buffer in the image input terminal is empty enough to resume scanning. The video data timing is not synchronous to a fixed clock, but is dependent upon the motion control system. This involves a more complex control system and has motion and image quality issues associated therewith.
In addition to an asynchronous transfer system, a large memory buffer can be used in the image input terminal to address the problem of differing transfer rates. In this situation, a complete page can be stored in resident memory and transferred at the rate utilized by the interface of the host. However, this technique increases the cost to the system in terms of additional components call on power consumption, and the physical space required by the page memory.
A problem exists when attempting to transfer scanned video data between an image input terminal and the host terminal when the internal data transfer rate of the image input terminal exceeds or is capable of exceeding the transfer rate of the host terminal. An example of such a situation is when the output of the scanner is gray video which is many times the volume of a threshold binary image. This produces a data rate mismatch, which can result in a truncated or corrupted page.
Another problem associated with transferring of data is the limited bandwidth of a synchronous clock image input terminal/host interface when used with an image input terminal which is capable of producing a wide range of data volumes and rates. An example is an image input terminal capable of producing binary and gray video data at various resolutions. In a binary mode, the data rates can be managed; however, when used in a gray output mode, the data rates become prohibited. This is especially true when the interface is governed by the host terminal which receives the data. Also, when the image input terminal produces video data which exceeds the synchronous host interface rate, such as in a gray output mode, the output video page is corrupted.
To prevent or avoid these problems; i.e., the mismatching of transfer rates, the present invention scans at a slower speed with the same integration time; i.e., a higher resolution, and periodically discards or skips a selected number of scanlines of valid image data to cause the scanner to realize a virtual or apparent data transfer rate which is equal to or less than the data transfer rate of the host terminal. In other words, the internal data transfer rate of the scanner is masked such that the transfer rate as perceived externally is equal to or less than the data transfer rate of the host terminal.