The small office/home office is becoming a more attractive alternative for many people, particularly with recent advances in home automation equipment. Still, the need for a copier, document scanner, printer and fax machine is an expensive and space-intensive problem. That's where the multi-function peripheral (MFP), which contains a subset of all those office functions in a single compact unit, comes in. In addition to the space saved there are other clear advantages. Documents are scanned once, then printed many times; copy quality is often much better than that of standard office copiers, and greater flexibility is provided by having all the functions in a single unit.
The internal circuitry of the MFP is more complex than that of a standalone scanner or printer, but many of the blocks common to both functions are combined to achieve a compact solution. The principal part of any imaging application is the image sensor, which receives the light reflected off the page being scanned and converts the acquired charge into a voltage that can then be processed. Most scanning applications today use a charge-coupled device (CCD) or a contact image sensor (CIS) to perform this operation. The image sensor provides red, green and blue data to the next block of circuitry, the analog front end (AFE).
The analog front end is a three-channel IC that takes the red, green and blue data from the image sensor, performs analog signal processing and converts the signal to a digital word. The digital data is then further processed in the digital domain and stored in memory or sent to a host computer. Each channel of a typical analog front end contains several circuit blocks, starting with the input clamp, which shifts the CCD output signal to a level that can be processed by standard CMOS ICs running on 5-V supplies. Each channel also contains a correlated double sampler to extract the pixel information from the CCD signal, programmable offset stage to provide course offset correction and a programmable gain amplifier (PGA) to boost each channel to a level that will utilize the full dynamic range of the analog-to-digital converter.
The outputs of the three PGAs are fed into a three-channel analog multiplexer, which drives the input of a high-speed A/D converter, processes the red, green and blue data in succession and outputs the digital words in an RGB format.
The AFE is programmed via a standard serial port interface that easily interfaces with most off-the-shelf microcontrollers. The AFE, as well as its programmable gain amplifier and offset control registers, can be set into different modes of operation using the serial port to set internal registers. There are slightly different AFE requirements specific to the scan and copy functions of the MFP, which will be discussed in more detail below.
The digital portion of an MFP consists of several blocks that perform timing generation, motor control, image processing, compression and interfacing to either a print engine or a host computer. A microcontroller such as an 80C51 provides control of the scan operation and print operation as well as programming capability for all the programmable blocks. The motor controller sends a pulse-width modulated waveform to a stepper motor that moves the sensor past the paper or, in a sheet-fed scanner, the paper past the sensor. The vertical resolution of the scan is determined by the speed at which the paper moves past the sensor.
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The timing generator provides all the timing required by the image sensor as well as all the timing for the analog front end. Clocks required for the image sensor are the transfer gate pulses, reset pulse and charge transfer clocks; the analog front end requires correlated double sampler clocks as well as an analog-to-digital converter clock.
The image-processing unit is the most involved and complex piece of digital circuitry on an MFP; many stages of digital image processing take place here. The image-processing unit could either receive data from the AFE of an image that has just been scanned or data from a host computer that needs to be printed. There is also a memory interface that links the image- processing unit and host computer with a block of DRAM. This DRAM acts as a data buffer between processing steps as well as a store for all the pixel offset, shading and gamma correction coefficients.
For scanned images, several steps of digital processing must be performed before the image can either be printed or sent to a PC. The first two steps are pixel-rate offset and gain correction, which are used to calibrate out the fixed pattern noise and shading errors inherent in imaging systems. The next step is a gamma correction-because of imperfections in the human eye, subtle changes in color are not detected in a mathematically linear fashion as a color changes from dark to light. The gamma correction block in an MFP compensates for this.
At this point, the data can be further processed and sent to a print engine or it can be sent to a host computer. If the data is to be printed, more digital processing will be done on it, such as zoom and/or dot-per-inch adjustments and color space conversion from RGB to CMYK. The data is then sent to a printer engine. If the data is to be sent to a host computer, then the data will skip these last steps and travel directly to the PC via the parallel port or a USB port. If the scanned image was very large then there may be a compression of the data before it is sent to the host.
Though this is a large amount of circuitry, the decreasing geometries of foundry CMOS and advances in technology have enabled IC makers such as Analog Devices to put all the functions described above on just a few ICs. The system described here can be done with a CCD, a high-performance, high-speed analog front end such as the Analog Devices AD9822, a DRAM and a digital ASIC that integrates the timing generator, microcontroller, DRAM controller, image-processing unit and IEEE-1284 host interface.
Just like a flatbed
The analog front end needed for an MFP application is very similar to the one used in a flatbed scanner. Both applications require AFEs that can interface with a variety of linear image sensors, including different-resolution CCDs and CIS modules. The AFE must have sufficient A/D converter resolution to avoid degrading the CCD's signal-to-noise performance, which is typically 10 to 12 bits. The A/D converter resolution must also meet today's scanner expectations, requiring 12 to 14 bits of resolution per color to achieve 36- to 42-bit combined (RGB) color. Overall performance is also an important consideration and the AFE used in the MFP must be able to achieve image quality comparable to that of a standalone flatbed scanner. Finally, the consumer scanner market demands low cost, so the AFE for scanner and multifunction peripheral applications must be competitively priced.
One area where the two applications (flatbed scanners and MFPs) differ is in the sampling rate. The scan speed for a standalone scanner has historically been limited by the host interface, whether it's an enhanced printer port, USB or even SCSI. With the scanner's ability to transmit a maximum of only a few megabytes per second to the host, the image sensor and AFE only need to operate at a couple of megapixels per second or fewer. Most AFEs for scanners offer sample rates of 6 MHz, which is an effective sample rate of 2 MHz/color.
One AFE that can satisfy the requirements for MFP applications is the AD9822 from Analog Devices. It has all the necessary features for high-quality scanning-operating modes for both CCD and CIS image sensors, true 14-bit A/D conversion, good linearity and time domain performance, and an attractive price. Most important for MFPs, it offers a typical sample rate of 15 MHz (5 MHz/color). Higher-speed operation, up to 30 MHz, is possible with reduced performance. This allows the AD9822 to be used for high-quality scanning while providing the higher speed needed for the copy function in the MFP.
Today's MFPs are helping people work more effectively at home without sacrificing valuable office equipment or space. Combining a full-function digital processing core with scanner and printer hardware enables high-quality scanning, printing and copying in a single box. Further advances in AFE technology allow fast scanning, enabling MFPs to serve as high-speed, high-quality copiers at an affordable price.