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Scanner System-On-A-Chip Is Focused
The Design Is Not Just Putting Bits Together For A Single IC — It’s Re-thinking The Way To Do The Job
By Paul McGoldrick

Even with only a few dozen color flatbed/sheet-fed document scanner manufacturers in the world it is not a cozy marketplace. This year has seen the scanner become a commodity product with prices diving below $100: And that means that the OEM has to differentiate his product to keep or take market share; price, features, performance. National Semiconductors has re-thought the process — and has produced a winner in the LM9830.

Price Drives Performance?

This market, perhaps more than any other, is driven by pennies saved. That means that being able to offer the maximum integration possible for the whole solution would offer the OEM the biggest savings on silicon; for National it made sense for the company to use their designers to integrate the analog front-end, generation of the sensor clock, motor control, and data buffering.

But getting price down through integration is only one step, a step that doesn’t win that much for the manufacturer in its marketin war. By looking at the performance gaps that exist in currently-made equipment National was able to see ways of making some fairly dramatic improvements, without major cost complications.

Current scanners typically scan at a constant rate — like 5 ms per line — which makes both the mechanics and the electronics really simple, although full-stepping (on/off) the scanning motor means less motor torque, higher currents and shorter motor life. But a larger problem occurs because the I/O to the computer is the limiting factor in the scanning speed. Typically, PCs will have an I/O rate between 50 Kbytes/s and 1 Mbytes/s depending on what else the machine is working on at the time. Many will max. out at about 800 Kbytes/s but will fall really low if the machine is busy processing something else.

With current technology the constant scan at 5 ms per line may mean that the output could well have been faster to maximize the I/O capability. In other cases the constant output may end up finding itself unable to feed the congested I/O so everything stops. When there is capacity again the scanner has to retrace its steps a little before carrying on.

In either case the interface speed to the PC is not maximal. National turns this scenario upside down by looking at the data transfer rate on the parallel interface and then matching the scanner system clock frequency. In that way the output is always consistent for the particular I/O throughput available.

Additionally, the LM9830 (Merlin, the bird, was the in-house project title) has a system which averages pixels in both the horizontal and vertical directions instead of the current industry practice of dropping pixels; the latter can give a sharper look in the resulting scan but it is only because information is viewed lying against positions where information is missing.

The Analog Front-End

The technology of the front-end is basically that in current National products. The CCDs (or Contact Image Sensors) are set to look for a reference white and black that are physically located in the scanner on an overscan area to the document itself. The R, G and B signals are ac coupled (see Fig. 1) to the IC and fully clamped at the beginning of each line (to 1.5 V for CIS and 3.5 V for CCD.) An inverting or non-inverting amplifier is switched in (white and black being at different polarities for the two sensor types) and the signals then go through the, now standard, correlated double-sampling.

Figure 1

Press Release:
National Announces Industry’s First Complete Scanner-on-a-Chip

National Semiconductor Corporation introduced the industry’s first system-on-a-chip solution for color image and document scanners. The LM9830 incorporates all the functions of a high-performance color scanner, including analog front end, sensor clock generation, microstepping motor control, data buffering and parallel port interface, into a single integrated circuit.

Using a 6MHz, 12-bit ADC with pipelined architecture, National’s LM9830 operates at a 6 million pixels/second conversion rate and scans images in full 36-bit color. The LM9830 incorporates digital offset and shading correction, as well as all the analog processing functions that National built into earlier imaging products, such as gain offset and correlated double sampling. In addition, digital pixel processing and individual RGB gamma tables enable the new part to provide a variety of high-quality resolutions from 50 to 600 dpi horizontal. Vertical resolutions are also 50-600 dpi in one dpi increments.

The LM9830 has fine control over system clock rates, which allows matching the scanner clock rate to the parallel port interface speed. This improvement results in scan speeds that are as much as 4X faster than current parallel scanners. By zooming in on a subset of CCD pixels, the device can transmit an arbitrary range of pixels to speed up the scanning of smaller items such as business cards. Pixel depths of 1, 2, or 4 bits are packed into bytes for faster scans of line art and low pixel depth images.

The microstepper motor controller determines the effective vertical resolution and moves the sensor over the image at a constant speed. A PWM current-control option for the stepper motor delivers the high torque and smooth movement needed for faster scans and superior images. This feature also allows use of a less expensive motor with a lower current rating.

The LM9830 is powered by a single 5V source and typically dissipates just 350mW. It includes a low-power standby mode for powerdown capability. National also provides complete TWAIN driver software and support.

A gain boost — which with the later programmable gain amps gives an overall gain on each channel of between x0.93 to x3.0 — can be switched to either x1 or x3. In contrast with display technology, where red is the least sensitive, blue is normally the weaker channel in pick-ups. The static offset DACs set up coarse channel offset prior to the ADC, with each DAC being 5-bit plus sign.

The 12-bit 6-MHz ADC (pipeline architecture) is fed from a three-channel multiplexer to give a maximum pixel conversion rate of 2 MHz per channel. 12 bits is the current marketing buzzword for the industry to be able to put on their cartons even though, as we will see, this is just hype.

Digital Processing

The main signal path output of the ADC is truncated to 10 bits while a feed representing the gain and offsets for each pixel (as a coefficient) are passed Data/Normal mode switch (see Fig. 2) to be stored in external SRAM. 2 bytes are used with usually 6 bits for offset and 10 bits for gain when processing CCD signals. The coarse correction made in the analog stages are finely corrected in the pixel-rate offset subtraction and pixel-rate gain multiplier stages.

Figure 2

The coefficients are accessed at the pixel rate and the subtractor saturates at 0 while the multiplier saturates at 1023.

The pixel-processing stage (see Fig. 2, again) adjusts the incoming pixel resolution to the lower, popular resolutions of 400, 300, 200, 150, 100, 75 and 50 dpi (dots per inch) even though the optical sensor might have a 600 dpi resolution. Most current products just throw pixels away resulting in a scan that has a "dimpled" look to it. The LM9830 averages pixels adjoining one another — both horizontally and vertically — so that all the information that was scanned is used. For a 100 dpi output, for example, three adjacent pixels from a 300 dpi sensor would be averaged. The processing is much faster with the lower resolutions chosen, so that a typical A4 document would be scanned in <10 s at 150 dpi, <40 s at 300 dpi and <160 s at 600 dpi (note the square x4 factor.)

So, the 12-bit scan and DAC — advertised by the OEM product manufacturer as a "36-bit scanner" — is truncated to 10 bits for the processing and is then processed down to 8 bits in the gamma-correction stage so that the signals are compatible with the majority of image manipulation software applications used to clean up, and doctor, images. The gamma correction tables for each of the channels will be 1 Kbytes x 8 bits and can be any arbitrary user-defined curves loaded through the DataPort.

The 8-bit gamma-corrected outputs are concatenated as necessary for lower bits/pixel for such things as line art (one bit/pixel) by packing the desired MSBs of multiple pixels together in one byte, increasing the transmission speed for 1, 2 and 4 bits/pixel. The synchronous output feeds the external SRAM which feeds it back at an asynchronous, non-constant, rate to the EPP/Nibble Mode Interface (see Fig. 2, again) which is a parallel port interface FIFO to the PC.

This feed is also tightly associated to the system clocks and the stepper motor controller so that the mechanical speed of the scan is controlled to match the data rate. If the buffer should fill completely then scanning would have to be temporarily stopped. The use of a fully-controlled stepping allows the OEM manufacturer to continue to use low-cost drive transistors with smaller motors. The PWM generators are driven by the same 50-MHz oscillator as used for the system clocks. The crystal itself is external with a few components but the maintaining amplifier is on-chip.

Competition

National tested the LM9830 by pulling the electronics from a stock scanner and replacing them with its own (same motors, sensors, etc.) Compared to the original electronics the new set made the scanner faster (timed as "button push" to "scan in window" on the PC) by a factor of more than 3 on the "speed" scan position and 6 on the "quality" position. Similar, impressive, improvements of over 3 times were obtained against another stock product.

Click here for competition results.

This is a well thought out product which has the right marketing features and the improvement of speed which will be sought by the OEMs. The price does not, of course, reflect deliveries for the huge numbers of devices which the OEMs handle for their product lines.

The LM9830 is in production and is in a 100-pin TQFP. It is priced at $10.00 in 1000-piece lots. Evaluation boards will be available. National Semiconductor Corporation, 2900 Semiconductor Drive, Santa Clara, CA 95052. Tel: +1 (408) 721-5000.

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