[opendtv] Re: The curse of Bayer pattern sensors

  • From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sun, 09 Feb 2014 09:28:10 -0500

On 2/9/2014 8:27 AM, John Shutt wrote:
Not being a camera designer, I can't say for certain, but most high end cameras use a prism to split the incoming light into three primary color RGB paths, and have three light sensors, CCD, CMOS, or whatever devices they use these days.
You are correct as far as traditional video cameras are concerned, with image sizes (these days) no larger than 11 mm diagonal ("2/3-inch" size).

It would seem to me to be trivial to either physically or mathmathetically selectively filter each color appropriately.
And that IS what they do.

But what are being discussed in this thread are single-sensor "digital-cinematography" cameras with on-chip multi-color filtering, typically with an image diagonal in the 30 mm range (35 mm to S35 movie-frame size). In the old days, there were certainly three-tube color cameras with larger than 11 mm image diagonals, but they had longer back-focal distances, too, to provide room for the larger prism.

The digital-cinematography cameras typically use the ARRI PL lens mount, with has a "flange depth" (distance from the mount flange to the image plane) of 52 mm. The common B4 video-camera lens mount (BTA S-1005-A) has a seemingly similar flange distance of 48 mm, providing room for the prism. Larger single-color sensors would require larger prisms, making it impossible to use existing PL-mount lenses without quality-reducing adaptors.

Also, many video systems (downstream from the imaging element) don't work in the RGB space, but in the Y, R-Y, B-Y component space. I know from the good old analog (Betacam) days and the early digital (BetaSX) days that the Y channel was allocated twice the bandwidth (or number of bits) as either the R-Y or B-Y channels (a.k.a 4:2:2 sampling rate.) This too would stand in for a form of low pas filtering of the R and B channels, no?
Unfortunately, the low-pass filtering needs to occur before aliases are generated.


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