Median-based dark level acquisition for a frame rate clamp

1. A circuit for determining median dark level of an image sensor, the circuit comprising:

(a) a first circuit for comparing a current pixel value and a cumulative median;

(b) a storage element for storing the cumulative median level; and

(c) a second circuit which utilizes the first circuit for adjusting the cumulative median based on the current pixel level input for updating the cumulative median dark level.

2. The circuit as in claim 1 , wherein the storage element is a capacitor.

3. The circuit as in claim 2 , wherein the second circuit includes two current sources for adding or subtracting to the median dark level stored on the capacitor.

4. The circuit as in claim 3 , wherein the two current sources can be adjusted to provide coarse control for rapid determination of an approximate median level, or to provide fine control for more accurate determination of the median dark level.

5. The circuit as in claim 3 , wherein the second circuit includes logic circuitry for controlling the current sources.

6. The circuit as in claim 1 , wherein the first circuit is a comparator.

7. The circuit as in claim 1 , wherein the first circuit is either an operational amplifier or a linear circuit.

8. The circuit as in claim 1 , wherein the storage element is a counter or another digital equivalent.

9. The circuit as in claim 8 wherein the second circuit includes a logic circuit for controlling the counter or the digital equivalent for increasing or decreasing the median dark level stored on the counter or the digital equivalent.

10. The circuit as in claim 8 , wherein the counter or digital equivalent includes adjustable increments to provide coarse control for rapid determination of an approximate median level, or to provide fine control for more accurate determination of the median dark level.