Image Processing Reference
In-Depth Information
Analog-to-digital converter
Digital signal
processing
Image
sensor
ADC
Analog
signal
Digital
signal
Analog signal
Sampled and digitized signal
Quantization
levels
Digitization
Time
Time
FIGURE 5.52
Digitization of signals.
Analog output from sensors was processed* and recorded to videotapes as an analog
voltage signal until the mid-1990s when digital video cameras appeared on the market.
But digital signal systems were strongly required from viewpoints of interoperability (it
is easier to expand to different systems), media, and lossless copies. Furthermore, since a
digital signal is superior to analog processing with respect to variety functions and sta-
bility, analog sensor output was digitized by an analog-to-digital converter (ADC) before
signal processing, as shown in Figure 5.52.
This is desirable from an SNR viewpoint. Because analog can take any value, noise can
be added to analog signals. Digital signals are allowed to take only one of the quantized
values
†
in the system. Thus, digital signal systems are tolerant to noise addition, which is
also an advantage as no new noise occurs after conversion to digital signal.
‡
In that sense,
digitization at an earlier stage is favorable. Some new functions such as high-speed read-
out described below can be added, depending on the digitization stage and method.
Sampling and quantization are necessary for analog-to-digital (A/D) conversion as shown
in the bottom right of Figure 5.52. Sampling means to sample analog signal in a specific fre-
quency. In “L” bit digital systems, the discrete value of only the number of 2
L
is allowed to be
taken. Quantization means to replace the sampled analog value to the nearest discrete value in
2
L
values. There are three stages at which light intensity information is digitized, as follows:
40,41
1. Chip-level ADC sensor. This is a configuration in which independent com-
monly used ADC devices are formed on the same chip with sensors, as shown in
Figure 5.53a. No heavy development subject is necessary. This type needs analog
output of sensor and ADC having the same range of frequency (about 30 MHz)
with a case of independent chip configuration. There is no function or bandwidth
advantage, while high-frequency analog output from a chip is not necessary.
2. Column-level ADC sensor. This type has been widely put to practical use in con-
sumer applications. An ADC converter is formed at each column, as shown in
*
There was a time when analog signal was converted to digital signal to apply highly functional digital signal
processing, and then converted back to analog signal for analog recording.
†
Of course adequate resolution for the specific application is necessary.
‡
Although it is possible for the noise component rate to increase by arithmetic processing, this does not mean
new noise is added.
