Patent Application
20080179499
1. Field of the Invention
The present invention relates to a method of processing a digital signal of logarithmic response image sensor, and more particularly to a CMOS logarithmic response digital pixel sensor and a method of processing digital signals thereof.
2. Description of Related Art
Recently, CMOS image sensors have become commonly used in image detecting applications because they have high Quantum efficiency, low power dissipation, low readout noise, wide dynamic range, and random access ability. CMOS image sensors can also link to the CMOS process to reduce costs, so it has gradually replaced CCDs (Charge coupled devices). In all kinds of CMOS image sensors, a logarithmic response image sensor receives stronger illumination intensity, and has a wide dynamic range (around 120 dB) Compared with linear image sensors (with a dynamic range of around 60 dB) the logarithmic response CMOS image sensors are more suitable for being installed in a device that reads pictures or dynamic images via a photoelectric element, for example, a scanner, a cell phone with an image capturing apparatus, a mini-camera, or a monitor, etc.
Referring to
However, when reading out the voltage of the current amplification logarithmic response CMOS image sensor, the mismatch between pixels is easily influenced by a process of variation that causes the signals to become irregular because the NMOS transistor 101 must operate in the subthreshold operation area to output the logarithmic response signal, and, moreover, the process of manufacturing the current amplification logarithmic response CMOS image sensor makes the threshold voltage mismatched. Moreover, the current amplification logarithmic response CMOS image sensor lacks an effective method of removing fixed-pattern noise.
Although the current amplification logarithmic response CMOS image sensor is highly sensitive and can rapidly switch speeds, the photocurrent produced by the photodiode 104 receiving a usual range illumination is too small when it passes through the NMOS transistor 101. This causes the output voltage Vout to be limited to a very small voltage difference range (about 0.2 V˜0.5 V). When an analogy/digital converter recognizes a pixel signal, the input voltage range of the current amplification logarithmic response CMOS image sensor is 1V, if it is being converted by an 8 bit analogy/digital converter, the output voltage Vout of the current amplification logarithmic response CMOS image sensor must have 1/256V sensitivity. Therefore, the design of the analogy/digital converter in the back of the current amplification logarithmic response CMOS image sensor for recognizing an output voltage Vout is more complicated and difficult to implement.
In view of the foregoing shortcomings of the prior art, the present invention uses a linear analogy detecting circuit to detect light and compare an exponential reference signal with a linear reference signal to implement the function of an logarithmic response digital pixel sensor and solve the problem in the back of an A/D converter.
The present invention provides a logarithmic response digital pixel sensor comprising of a linear light detecting unit, an exponential producer, a counting unit, a comparator, and a memory unit. The logarithmic response digital pixel sensor first uses the linear light detecting unit to detect light and then produces a linear analogy detecting signal. The exponential producer provides an exponential reference signal. The comparator compares the linear analogy detecting signal with the exponential reference signal and outputs a comparison result to the memory unit. The memory unit produces a logarithmic response digital signal by processing the comparison result and a count reference signal provided by the counting unit.
Additionally, an encoder is used for encoding the logarithmic response digital signal, therefore the logarithmic response digital signal can be processed using a different encoding type and stored in the memory unit.
The present invention provides a processing signal method for a logarithmic response digital pixel sensor comprising the steps of: detecting light and producing a linear analogy detecting signal; providing a exponential reference signal and a count reference signal; comparing the linear analogy detecting signal with the exponential reference signal and outputting a comparison result; and finally receiving the comparison result and the count reference signal for producing a logarithmic response digital signal.
To make it easier for our examiner to understand the innovative features and technical content, we use a preferred embodiment together with the attached drawings for the detailed description of the invention, but it should be pointed out that the attached drawings are provided for reference and description but not for limiting the present invention.
Referring to
The encoder 26 is installed between the counting unit 25 and the memory unit 24, and encodes the logarithmic response digital signal Dout to a required code, and then stores the encoded logarithmic response digital signal Dout in the memory unit 24 for the application system 27 to read and process.
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In summation of the description above, the present invention using the linear analogy detecting unit to detect light improves upon the drawbacks of the prior art, and achieves the function of the logarithmic response image sensor by comparing the linear analogy detecting signal with the exponential reference signal, so that the design of the analogy/digital converter in the back of the image sensor will be uncomplicated and easily to implement. The signal process of the logarithmic response digital pixel sensor of the present invention has the advantage of low power consumption, fast operating speed and low production costs, so that the logarithmic response digital pixel sensor of the present invention can be integrated into a SOC system easily and with successful results.
Although the present invention has been described with reference to the preferred embodiments thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.
