Saturday, December 31, 2011

Canon Super-35mm Cinema Sensor Explained

Canon published few whitepapers talking about design considerations of its Super-35mm sized CMOS sensor for recently announced EOS C300 video camera. The first one "New 35mm CMOS Image Sensor for Digital Cine Motion Imaging" gives the sensor spec:

The color filter array is the classic Bayer. Canon explains the resolution choice:

Illustrating the separate CFA array and the CMOS imager while also
showing the CFA separated into its component color filters to better expose
the structure of their respective sparsely sampled lattices

"The image sensor readout strategy radically departs from the customary “De-Bayer” deployment of quincunx sampling of the green photosites to maximize the green video resolution (and hence the matriced Luma resolution). The design strategy of this new sensor is to not to seek any form of “4K” resolution — but rather to specifically confine the reconstruction of each of the R,G, and B video components to a full digital sampling structure of 1920 (H) x 1080 (V) — according to the SMPTE 274M HDTV Production Standard."

Showing the concept of structuring the final Green video component within the
pre-processing LSI from the two dual video readouts from the CMOS image sensor

"The dual Green process offers the following significant technical advantages:

  1. Doubles the effective saturation level of the summed Green output video
  2. Increases the noise of the final Green output only by a factor of square root of two
  3. Combination of 1) and 2) increases the effective dynamic range of the green signal — and as a consequence, that of the matriced Luma signal
  4. Increases the effective output green video bit depth
  5. The half-pixel offset between the two separate green sampling lattices — both horizontally and vertically — virtually eliminates the first order sideband spectra associated with the sensor sampling process. This eliminates green aliasing.
  6. Creates an effective FIR* filter within the readout process that aids the optimization of the horizontal MTF and the progressive vertical MTF and associated aliasing."
The summation of two greens is said to increase the DR from 70dB to 73.5dB in green (in fact, from 70.5 to 73.5) or to 72dB in luma (12 stops).

Although the camera frame rate is 24p fps, the readout speed is 1/60s to reduce rolling shutter effects:

In 60i mode each half-frame is read at 1/120s - same per-row speed as in 24p mode.

The low read noise is achieved by limiting the readout amplifier bandwidth, as shown below:

Another Canon whitepaper "RGB Resolution Considerations in a New CMOS Sensor for Cine Motion Imaging" shows advantages of the proposed green processing in resolution extension:

Showing the two separate green 1920 (H) x 1080 (V) photosite lattices
and the horizontal and vertical timing offsets between each of the
two “diagonal” pixels that are summed during the readout process

The resulting horizontal and vertical MTFs of the whole system are improved:

The summary says: "A new CMOS image sensor has been described. It represents a definitive decision by Canon to enter the global field of digital cinematic motion imaging. It is anticipated that there will be many progressive advances in the years ahead. Accordingly, a priority was assigned to taking a first step into this important field of imaging by placing an initial focus on originating a very high quality RGB video component set specifically intended for high-performance High definition video production."

Another whitepaper is titled "Sensitometric Characteristics of EOS C300 Digital Cine Camera" and mainly focused on system processing of the video signal, introduces "Canon-Log" response.

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