Programmable digital black level calibration with feedback control

• US 8,115,835 B2
• Filed: 10/27/2008
• Issued: 02/14/2012
• Est. Priority Date: 05/28/2008
• Status: Expired due to Fees

• Current Assignee: MediaTek Singapore Pte. Ltd. (MediaTek, Inc.)
• Original Assignee: MediaTek Singapore Pte. Ltd. (MediaTek, Inc.)
• Inventors: Noguchi, Yasu, Sasaki, Kazuya
• Primary Examiner(s): Hasan, Mohammed
• Application Number:
  US12/290,158
• Publication Number:
  US 20090059012A1
• Time in Patent Office: 1,205 Days
• Field of Search: 348241-243, 348/207.99, 348/220.1, 348/231.99, 348/691, 348/692, 348/255, 348/308
• US Class Current: 348/243
• CPC Class Codes: H03M 1/1019 by storing a corrected or correction value in a digital look-up table
• CPC Class Codes: H03M 1/12 Analogue/digital converters H03M1/001 – H03M1/10 take precedence
• CPC Class Codes: H04N 25/63 applied to dark current
• CPC Class Codes: H04N 25/71 Charge-coupled device [CCD] sensors; Charge-transfer registers specially adapted for CCD sensors

Abstract

A programmable digital black level calibration circuit comprises a combining circuit, a digital programmable gain amplifier (PGA), and a black level feedback circuit. The combining circuit combines a digital image signal for optical black (OB) pixels and a feedback signal and outputs a digital PGA input signal. The PGA amplifies the digital PGA input signal by a PGA gain value and outputs a digital PGA output signal. The black level feedback circuit receives the digital PGA output signal and a target black level and in response outputs the feedback signal such that a black level of the OB pixels is calibrated with respect to the target black level. The programmable digital black level calibration circuit calibrates the black level in pure digital domain using signed data buses. The target black level is adjustable to a desired positive or negative value independent from the PGA gain value.

Claims

1. A device, comprising:

a combining circuit that combines a digital image signal and a feedback signal and outputs a digital programmable gain amplifier (PGA) input signal;

a digital programmable gain amplifier (PGA) that amplifies the digital PGA input signal by a PGA gain value and outputs a digital PGA output signal; and

a feedback circuit that receives the digital PGA output signal and a target black level and in response outputs the feedback signal such that a black level of the digital image signal is calibrated with respect to the target black level.

2. The device of claim 1, wherein the device is part of an analog front end (AFE) integrated circuit of a digital camera, and wherein the digital image signal represents a sequence of optical black (OB) pixel values from an image sensor of the digital camera.

3. The device of claim 1, wherein the target black level is adjustable to have a value between a negative number and a positive number.

4. The device of claim 1, wherein the combining circuit subtracts the feedback signal from the digital image signal.

5. The device of claim 1, wherein the feedback circuit comprises:

a comparator that compares the digital PGA output signal to a target black level and in response outputs an error signal;

an integrator that receives the error signal and in response outputs an integrator output signal;

a feedback gain control amplifier that amplifies the integrator output signal by a feedback gain value and outputs the feedback signal; and

an OB register that stores an OB feedback value of the feedback signal.

6. The device of claim 5, wherein the integrator accumulates the error signal with the integrator output signal.

7. The device of claim 1, wherein the combining circuit, the PGA and the feedback circuit form a calibration feedback loop, and wherein the loop gain value of the calibration feedback loop is programmable to obtain a desired calibration response.

8. The device of claim 1, wherein the digital PGA input signal and the feedback signal are carried by signed data buses.

9. The device of claim 1, further comprising:

a black area generator that generates a black area indication signal for OB pixels, wherein an OB feedback value of the feedback signal is stored in an OB register, and wherein the feedback circuit is deactivated such that a final OB feedback value stored in the OB register remains unchanged when the black area indication signal is unasserted.

10. The device of claim 9, further comprising:

a low pass filter that filters the digital image signal when the black area indication signal is asserted.

11. The device of claim 1, further comprising:

a serial interface, wherein the target black level is loaded into a target black register via the serial interface, and wherein the PGA gain value is also loaded into a gain register via the serial interface.

12. A method, comprising:

(a) receiving a digital image signal, wherein the digital image signal represents a sequence of digitized optical black (OB) pixels from an image sensor;

(b) combining the digital image signal and a feedback signal and outputting a digital programmable gain amplifier (PGA) input signal;

(c) amplifying the digital PGA input signal by a PGA gain value and outputting a digital PGA output signal; and

(d) calibrating a black level of the digital image signal with respect to a target black level by receiving the digital PGA output signal and the target black level and in response outputting the feedback signal.

13. The method of claim 12, wherein the target black level is adjustable to have a value between a negative number and a positive number.

14. The method of claim 12, wherein the combining in (b) involves subtracting the feedback signal from the digital image signal.

15. The method of claim 12, wherein the calibrating in (d) comprises:

comparing the digital PGA output signal to a target black level and in response outputting an error signal;

receiving the error signal and in response outputs an integrator output signal;

amplifying the integrator output signal by a feedback gain value and outputs the feedback signal; and

storing an OB feedback value of the feedback signal.

16. The method of claim 15, wherein both the PGA gain value and the feedback gain value are programmable to obtain a desired calibration response.

17. The method of claim 12, further comprising:

(e) receiving a black area indication signal for the sequence of OB pixels, wherein an OB feedback value of the feedback signal is stored in an OB register, and wherein the calibrating in (d) is deactivated such that a final OB feedback value stored in the OB register remains unchanged when the black area indication signal is unasserted.

18. The method of claim 17, further comprising:

(f) filtering the digital image signal using a low pass filter when the black area indication signal is asserted.

19. The method of claim 12, wherein the digital PGA input signal and the feedback signal are carried by signed data buses.

20. A device, comprising:

an analog to digital converter (ADC) that converts an analog signal to a digital ADC output signal, wherein the analog signal represents a black level of a sequence of optical black (OB) pixels; and

means for calibrating the black level with respect to a target black level, wherein the digital ADC output signal is combined with a feedback signal and amplified to output an amplified signal, and wherein the amplified signal and the target black level are used to generate the feedback signal in calibrating the black level.

21. The device of claim 20, wherein the target black level is adjustable to have a value between a negative number and a positive number.

22. The device of claim 20, wherein the feedback signal is carried by a signed bus.

23. The device of claim 20, wherein the means includes a digital programmable gain amplifier (PGA).