Analog-to-Digital Converter: How it Works
An analog-to-digital converter helps to establish communication between the processor of the scanner and its matrix. The ADC measures the input voltage and describes it in numbers that are then sent to the processor.
For example, we’ve got 4V at the input of the ADC, then this value changes to 9V. It means we get a string of 00000100, then 00001001 at the output. These are binaries for numbers 4 and 9. The number of zeroes and ones used by the ADC to represent the value is referred to as bit capacity (measured in bits). It is an important parameter of a scanner that indicates the precision of the input signal as measured by the ADC.
Today you can come across an inexpensive scanner that uses a converter with a bit capacity of 24 – 48bit. In theory, you should go for a scanner with a higher bit capacity, but there is one thing to remember about: sometimes the manufacturers write “48 bit” in big letters on the box, with a small print somewhere in the corner that reads “software 48bit, hardware 36bit”. In this case, the real bit capacity of the used ADC is 36bit and you should take this number into account. I should admit that it’s next to impossible to distinguish between scans taken on a 36-bit and 42-bit scanner (the human eye can distinguish between about 16.7 million colors or 24bit). In our case, the bit capacity of the converter and the color depth is the same thing as the converter just calculates the colors of dots the image is made of. The higher the bit capacity, the more true-to-life image you receive.
Modern scanners come equipped with special-purpose processors. This processor makes all the circuits and units work in tune. It also comes up with the image data to be sent to the CPU. In some scanners, the processor also functions as an interface controller.
The list of software instructions for the processor is stored in a ROM chip; it is written into the chip at the production stage. The contents of the chip are called firmware. Some professional scanners allow updating their firmware, but it is not necessary for SOHO scanners.
Besides the ROM chip, scanners use 1MB or 2MB of RAM as a buffer. This is where the scanned information is sent to. After the contents of the buffer are sent to the computer, the scanner processor zeroes the buffer for a new scan. Instructions to be executed by the scanner processor are also stored in local RAM (the processor has a few kilobytes of its own memory). The processor memory is organized like a pipeline: the processor executes instructions one by one, so that the second instruction comes to the place of the first one, and the new instruction comes instead of the last one, as it shifts ahead.
The amount of the local memory used to be indicated in the technical specifications. Today it is generally omitted, as it doesn’t practically affect the scanner speed. It is also omitted when a particular scanner uses some portion of the computer main RAM, which is implemented in the driver.