Can anyone explain why rolling shutter is still necessary? Wouldn't current electronics have the memory bandwidth needed to just sample the entire cmos sensor at once?
Sampling the entire CMOS sensor at once is not really possible when you stop to consider the sheer magnitude of bits required to represent a single still image. The CMOS sensor only has a finite number of pins connecting it to the logic board responsible for polling its state, so it makes sense to use an addressing system to read the sensor data. This addressing system uses a small number of addressing bits (each pin is usually just one bit) to move a sliding window over a much larger amount of data. Regular computer memory works the same way. This enables that data to be passed through to the camera in small pieces. While this can be made rather fast with good engineering, it is not instant, and the non-instantaneous nature produces the rolling shutter effect.
It's the physical limitation on the number of pins that creates the need for the rolling shutter in modern CMOS design. Creating speedier memory access routines can reduce the effect of the shutter, but not eliminate it.
A possible alternative approach would be to build some memory into the image sensor, so that you can ask it to "lock" all the bits in place, and then read those bits out at your logic board's leisure. That should effectively eliminate the rolling shutter effect, (More or less, assuming your "freeze" signal arrives at all the pixels more or less at the same time) at the engineering expense of needing to add per-pixel memory to your image sensor, and additional electronics to perform the lock. I'm no expert on the subject, but I would not be surprised if more modern image sensors have some feature for this purpose, especially those used in high speed cameras.
Numerous CMOS, CCD, and other types of focal plane / 2D detectors have global shutters where all pixels 'integrate photons or energetic particles' at the same time for a set amount of time.
Read-out is performed in a rolling fashion... the data generally has to be serialized for analog to digital conversion (although not always) and/or transmission over a communication protocol or write to file.
A global shutter detector at the hardware level has settings for framerate and light integration time (among other things). Readout time is more or less constant for a particular camera sensor. So if integration requires say 20ms, and readout takes 30 ms... you're looking at about 20 FPS image acquisition speed.
Good overview: http://www.red.com/learn/red-101/global-rolling-shutter
Not so sure. Phone cameras are big selling points for phones, and have some hardware innovations (and access to much faster processors than the average "high end" DSLR/mirrorless), that , if we ignore the smaller by necessity optics, are in the same or even better league tech-wise to the average Sony/Canon/etc. Talking of course for flagship iOS/Android phones, not the average phone.
Canon, for example, announced a global shutter CMOS sensor last year. The problem here is that those extra components come at a cost. Either they crowd out existing components on the sensor, which would dramatically reduce light sensitivity. Or, they require additional layers in the CMOS die, which significantly increases fabrication cost due to design costs, fabrication time, lower yield, etc. Additionally, the extra step in digitization reduces effective frame rate.
Only a few use cases would benefit, so the there's no economic incentive to change how it works.
Edit: technical details based on dpreview claims it's actually not a global shutter, just a very quick rolling one. "...this capability stems from a stacked CMOS image sensor, which includes processing circuitry nearer the pixels and features built-in memory to deliver all this data to the off-board processors at a rate they can cope with. It's this structure that enables the camera to shoot at 20 frames per second and do so with an electronic shutter that's fast enough to minimize the rolling shutter effect."
Don't CMOS sensors increase in pixel density (from a few megapixels to 30 and even 50 mp in some Sony full frame cameras today)?
If the camera sensors were made as just 4K resolution sensors it might be easier to scan it all at once in fast enough time (though we also increasingly ask for more slow-motion capabilities, which requires even faster frame rates).