Rolling and Global Shutter Mechanisms

In Rolling Shutter mode, charge transfer happens on a per row basis whilst in global shutter charge transfer happens for the whole sensor or globally. To read out a pixel in Rolling Shutter mode, the following occurs within the analog circuitry:

  1. The read out node is reset

  2. The node level (reference level) is measured

  3. Charge is transferred from photodiode to node

  4. The node level (signal level) is measured

  5. Reference level (step 2) is subtracted from signal level (Step 4) to get real signal

This process is commonly referred to as CDS (Correlated Double Sampling) and is done in the analog domain before digitization. The reason it is required is due to what is known as reset noise, this arises because every time the node is reset it does not settle at exactly the same level and hence the actual level must be measured (Step 2.) and subtracted from the signal level (Step 4) to get the real signal.

Rolling Shutter Mode: charge transfer happens on a per row basis; therefore each row follows steps 1 – 5, until the entire sensor is read out. The disadvantage of this is that the start and end exposure time moves by the row read out time for each subsequent row. So whilst each row of pixels is exposed for exactly the same length of time they do not all start and end at exactly the same time.

Global Shutter Mode: the start and end of the exposure do occur at exactly the same time for every pixel (not just for pixels in the same row); therefore Step 3. has to occur for all the pixels at the same time. Because of this, the reset and reference read occur before this global transfer for every row. Since the same read out circuitry is used for every row there is nowhere to store the measured reference level for every pixel and so a reference frame is actually digitized and read out from the sensor and then the signal is digitized and read out from the sensor. The two are subtracted to get the ‘real signal’. Reading two frames to get a real signal frame effectively halves the Cycle Time when compared to Rolling Shutter.