Adaptive gain and offset control in a digital video decoder

1. A converter in a digital video decoder, the converter comprising:

an analog-to-digital converter (ADC) comprising

an input data port to accept an analog composite video baseband signal (CVBS);

an input control port to accept a gain indicator and an offset indicator; and

an output data port to provide a digital CVBS signal; and

automatic gain and offset control (AGOC) logic comprising

an input port coupled to the output data port of the ADC; and

an output port coupled to the input control port of the ADC and providing the gain indicator and the offset indicator;

wherein the offset indicator and gain indicator are based on a maximum signal level and a blanking level and wherein the offset indicator is based on a current phase of a frequency of oscillation.

2. The converter of claim 1 , wherein the blanking level comprises a filtered sequence of measured blanking levels.

3. The converter of claim 1 , wherein the gain indicator is based on both the maximum level and the blanking level.

4. The converter of claim 1 , wherein the maximum level comprises a filtered sequence of measured maximum levels.

5. A method for adjusting an output signal of an analog to digital converter (ADC) in a digital video decoder, the method comprising:

setting an offset indicator to an initial offset value;

setting a gain indicator to an initial gain value;

converting an analog composite video baseband signal (CVBS) into a digital composite video baseband signal based on the offset indicator and the gain indicator;

measuring the digital composite video baseband signal to determine a maximum level and a blanking level;

increasing the offset indicator, if the blanking level is below a blanking target range;

decreasing the offset indicator, if the blanking level is above the blanking target range;

determining a frequency of oscillation;

compensating the offset indicator based on a current phase of the frequency of oscillation;

measuring the digital composite video baseband signal to determine a maximum level;

increasing the gain indicator, if the blanking level is below the blanking target range and the maximum level is below a maximum target range;

decreasing the gain indicator, if the blanking level is above the blanking target range; and

decreasing the gain indicator, if the maximum level is above the maximum target range.

6. The method of claim 5 , wherein the act of measuring the minimum level comprises:

measuring a sequence of minimum levels; and

filtering the sequence of minimum levels to determine the minimum level.

7. The method of claim 5 , wherein the act of measuring the blanking level comprises:

measuring a sequence of blanking levels; and

filtering the sequence of blanking levels to determine the blanking level.

8. The method of claim 5 , wherein the act of measuring the maximum level comprises:

measuring a sequence of maximum levels; and

filtering the sequence of maximum levels to determine the maximum level.

9. A converter in a digital video decoder, the converter comprising:

means for measuring a blanking level of a CVBS signal;

means for measuring a maximum level of the CVBS signal;

means for determining a difference between the maximum level and the blanking level;

means for adjusting a gain indicator if the difference is outside a first target range;

means for determining if the blanking level is within a second target range; and

means for adjusting an offset indicator if the blanking level is outside the second target range;

wherein the means for adjusting the offset indicator comprises

means for increasing the offset indicator, if the first difference is below the second target range; and

means for decreasing the offset indicator, if the first difference is above the second target range.

10. A method for shifting blanking voltage towards a reference voltage in an output signal of an analog to digital converter (ADC) in a digital video decoder, the method comprising:

measuring a blanking level of a CVBS signal;

measuring a maximum level of the CVBS signal;

determining a difference between the maximum level and the blanking level;

adjusting a gain indicator if the difference is outside a first target range;

determining if the blanking level is within a second target range; and

adjusting an offset indicator if the blanking level is outside the second target range;

wherein the act of adjusting the offset indicator comprises

increasing the offset indicator, if the first difference is below the second target range; and

decreasing the offset indicator, if the first difference is above the second target range.

11. The method of claim 10 , wherein the first target range represents voltages within a predetermined error from 0.7 V.

12. The method of claim 10 , wherein the first target range represents voltages between 0.65 V and 0.75 V.

13. The method of claim 10 , wherein the act of adjusting the gain indicator comprises:

increasing the gain indicator, if the difference is below the first target range.

14. The method of claim 10 , wherein the act of adjusting the gain indicator comprises:

decreasing the gain indicator, if the difference is above the first target range.

15. The method of claim 10 , wherein the second target range represents voltages within a predetermined error from 0.3 V.