CMOS Tech Question.

[Matthew Greene]

It was brought up by someone to me that most expert explanations of how a CMOS work don't make sense and I had no answer.

(assuming a 1/60 shutter speed) Most people say the CMOS sensors scan from top to bottom line by line in a 60th of a second which doesn't make sense to me because in order for each pixel to be exposed for 1/60th of a second with that methodology on lets say a 1080P frame, with that math it would take several seconds to expose all the horizontal pixel lines in that single frame.

Can someone explain to me how quickly the readout from top to bottom actually happens? Does it differ among chips and how is the readout speed specified/measured (in numbers),

 

[Tim Sutherland]

if it scans top to bottom in 1/60th of a second, that means you divide 1/60th by the number of lines for the read time per line, not multiply it.

the read/reset speeds of the red sensors are some of the fastest if not the fastest in the world. that's why you don't see too much skew on it. for more answers than that, i yield to someone else.

 

[Mitch Gross]

All the pixels get switches on in a rolling fashion. Start on the top line and they start switching on one after another, in microseconds. The second row begins the process shortly after the first -- it does not wait until it is complete. The same with the third, fourth, etc., in a cascading action. This read cycle can take a matter of milliseconds if not microseconds. It depends on many aspects of the camera design, and not really with the sensor design itself. It is about drivers and computing power behind the sensor.

There is a large portion of time that the entire sensor is exposing. Once this cycle is complete (set by your shutter speed, aka shutter angle) then the reset cycle begins, which scans off the sensor in the same fashion. There is a short step where the black point is set here in the cycle as the levels are all set to off and then the cycle can repeat. The length of time for the sensor to be off is a function of the amount of time you wished the cycle to be open during exposure. The read/reset part of the time is extremely small.

I don't know the cycle time in the RED One but I do understand that it is pretty small. The Phantom HD has an incredibly small read/reset cycle time but then it has to because it is designed to be such a high speed camera that this is a necessity of function. It's just a few microseconds and it is impossible to ever see skew.

 

[Matthew Greene]

Thanks Mitch, that makes all the sense in the world to me.

Most descriptions I've read lead one to believe that the whole sensor scans line per line totalling 1/60th of a sec for the full scan. That did not mathematically compute.