Image Processing Reference
In-Depth Information
Function
1.
Signal generation
2.
Amplification
and
differential
3.
Quantization
-
On
+
A⋅d (log
I
)
log
I
M
fb
V
p
Reconstructed signal
V
p
-
Off
+
(b)
Time
Reset
V
diff
Effect
Dynamic range
improvement by
logarithmic output
Accuracy
improvement
Judgment
Reset level
Off threshold
On threshold
(a)
(c)
Time
FIGURE 7.12
Event-driven sensor: (a) pixel schematic and the role of each block; (b) log
I
and reconstructed log signals;
(c) temporal transition of A·d(log
I
). (Reprinted with permission from Lichtsteiner, P., Posch, C., Delbruck, T.,
Journal of Solid-State Circuits
, 43, 566-576, 2008.)
7.8 k fps (780 Mpixels/s) of 10
5
pixel numbers with analog parallel output. When additional
measures are included to achieve high speeds, it is an impressive imager filled with ideas.
7.3.2 Event-Driven Sensor
The sensor described here is the second example that does not belong to “almost all image
sensors”
15
in this topic.
It is not a frame-based type of sensor in which signal charges are
integrated for a prefixed exposure period. Its principle of operation is completely different
except for photoelectron conversion.
As shown in Figure 7.12a, a pixel is composed of three blocks.
16
In (1), the signal generation
block, a photocurrent through a PD is not integrated but monitored constantly. The voltage at
the node, which is connected to the cathode of the PD and the source of the feedback transis-
tor,
M
fb
, is amplified by an inverting amplifier, and the output is connected with the gate input
of
M
fb
to form an amplified and log-transformed voltage output of photocurrent
I
. The output
is transferred to (2) the amplification and differential block, and is temporally differentiated;
however, only the component that varies with time is amplified. The output is transmitted to
(3), the quantization block, and is monitored by an increase/decrease checking comparator,
which emits a pulse signal of ON or OFF in accordance with an increase or decrease in input
change, when a predefined amount is observed. The signal is generated by pulses indicating
an increase or decrease in the predefined amount of input change only at the time of change
and only at the changed pixel, there is no signal showing light intensity directly. In that con-
text, this sensor has communality* with the pulse output sensor described in Section 5.3.3.2.3.
Therefore, light intensity or the change of light intensity information is quantized in these
sensors differently from “almost all sensors,” while time information is not quantized.
Thus, the obtained information is only the pixel address and time, and the increase or
decrease at which the predefined amount of change occurred. That signals are generated
* Neither of these sensors belongs to the frame-based integration mode type, but both emit a pulse signal and
reset monitoring integration to restart, when the light intensity information changes to a predefined amount.
