Pixel Size vs Sensor Efficiency

Started May 18, 2012 | Discussions thread
DSPographer Senior Member • Posts: 2,464
Re: You have ignored read-out speed

bobn2 wrote:

DSPographer wrote:

bobn2 wrote:

DSPographer wrote:


To simultaneously read out more than two rows of pixels through the first stage pixel source follower, we also need to route more column output wires in parallel. This is why more than two row read-out can be difficult when using front illumination.

You only read out one pixel through the first stage pixel source follower, except for the very rare incidence of a shared source follower (shared between 2 or 4 pixels). Even then the SF feeds into just one column line.

Did you understand my comment?

I did. I was trying to be diplomatic rather than suggest that you didn't understand the architecture of a typical sensor, because there you seem to be taking about an arrangement that doesn't happen. I find it difficult to interpret a statement such as 'simultaneously read out more than two rows of pixels through the first stage pixel source follower' as anything other than you thought two rows would be read through the first stage source follower. this never happens, since the first stage source follower is one per pixel, only one pixel is ever read through a first stage source follower, be it simultaneously or not - apart from the few shared SF examples.

Your problem is that you start by assuming I don't understand what I am talking about. This started with how to increase the amount of time available to read each pixel since that determines the bandwidth and thus the noise of the first amplifier stage- the pixel SF. Looking at one column of the bottom half of the array, each row is read in turn onto the same column line. To increase the number of rows without slowing down the frame rate or decreasing the output capacitance that the source follower drives, it would be necessary to read two rows of the same column simultaneously. This can be done by running parallel output column lines for a single column to separate banks of row buffers.

What you wrote is consistant with what I am saying. To read two rows simultaneously to two row buffers at the bottom of the array, you would need two separate output lines for each column.

Certainly, so you do not ever read two rows simultaneously (in each half of the array). What you do is subdivide the rows, sometimes in blocks, but more usually interleaved (because it gives less objectionable video) into as many banks as you need. Each bank leads to its own analog output channel and ADC. Because each bank contains fewer pixels than does a full row, it can be read slower. There is not a need for multiple column lines.

We can already have just one A-D per column with an Exmor type sensor. That keeps the A-D rate nice and slow but doesn't change the reading rate of the first stage amplifier: the pixel SF.

To slow down the reading rate of the pixel SF, you need the rows to output on seperate column lines. This is easy to do between the top and bottom half of the array, but to have two rows of pixel SF active simultaneusly on the bottom half of the array, you need to run parallel column lines for their outputs. Of course it is simple to slow down the second stage amplifiers and A-D converters etc without adding column lines but that doesn't help the noise from the pixel SF.

So to read four rows at once you would need parallel output wires for each column: two lines in parallel per column going to the top, and two going to the bottom.

Wrong idea, see above.

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