OK, so then what difference in oversampling requirements are there
between 1080@24p and 1080@60p acquisition?
On 18-Aug-06, at 10:11 AM, Craig Birkmaier wrote:
At 2:13 PM -0400 8/17/06, John Golitsis wrote:What kind of oversampling are you talking about here?
On 17-Aug-06, at 10:06 AM, Craig Birkmaier wrote:
To explain, the problem with cameras is that you need to oversample by a factor of 1.5 to 2, to produce images that can take full advantage of the available spectra in a 1920 x 1080 format. We are not there yet even for 1080@24P. Doing this at 1080@60P is even further out in the future.
We have been through this MANY times over the years.
What we are talking about here is primarily related to the MTF - modulation transfer function - of the acquisition device. If you sample at the same resolution as the emission format it is not possible to fully saturate the spectra available within that raster. You have issues with sampling accuracy, dealing with detail in the gaps between the individual samples in the sensor, noise, and other signal processing errors. When we over sample by a factor of 1.5X to 2X the target sample raster, we can eliminate most of these issues when we resample down to the the target raster resolution. In the process we eliminate entropy (a big help for compression systems that must deliver these images), and we improve the MTF.
We have been oversampling in the horizontal domain in SDTV cameras for many years. Oversampling in the vertical domain has little benefit for interlaced systems and adds significant cost, although a 1080@30i camera will produce better quality SDTV. Most SDTV cameras have between 900 and 1300 samples per line.
You might recall some information posted by Mark Schubin a while back, describing an issue he encountered with an HDTV camera that can be switched between 1080@30i and 720@60P. I believe the show was being shot in 1080@30i, but one of the cameras had been left in 720P mode. He indicated that on a studio monitor it was nearly impossible to detect the difference, although the 720P source was being upconverted to 1080i by equipment downstream of the camera.
The 720P source was benefiting from oversampling, as the camera sensor operates at 1920 x 1080. Couple this with the reduced stress on a downstream emission encoder, and you can begin to understand why it is possible to deliver a superior 720P picture with less bits than its 1080i equivalent.
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