Re: HD cameras & formats

[BKM] "Would you care to elaborate on that? And any info on P2 DVCPROHD would be great. I know that most versions of the Panasonic Cameras only shoot 960x720 and alot of processing is done in camera to get it to 1080."

Of course, this all depends on which Panasonic camera you are referring to as they just introduced a camera with a 1080 sensor...

The Varicam has a 1280x720 sensor and only records 720p. As you stated, DVC ProHD 720p is 960x720 in the file, and when the data is transfered directly into FCP, the preset is for a 960x720 non-square pixel frame size. At 960x720, the stored file is 4:2:2 color subsampling.

DVC ProHD decks and some newer cameras including the HVX200 can do 1080p/i in DVC ProHD. These frame sizes in-file are 1280x1080 for 29.97 fps and 1440x1080 for 25 fps.

The HVX200 is a unique piece as it has sensors that are physically 960x540...an image is created by "pixel-shifting" some values to create more discrete image samples. I've worked with HVX200 files and frankly, they look better than they have a right to for that price. What I've never understood is what the flow chart looks like inside the camera.

DVC ProHD is, on PLAYOUT, a 1280x720 square pixel 4:2:2 format (or on the appropriate equipment, a 1920x1080 square pixel 4:2:2 format)...remember as when it first came out it was principally an HDSDI in/out tape format so the image had to be re-constituted at the full size for serial interface, just like HDcam does. On the HVX200, my question on how one gets from the sensor to the recorded size is whether or not the 960x540 image is then interpolated up to 1280x720, and THEN subsampled back to 960x720...or if the sensor is oversampled or "pixel shifted" in both directions, you would come up with 1920 (960x2) x1080 (540x2)...then would it scale that down to 1280x720?...or 960x720?...this isn't clear to me.

One interesting area of DVC ProHD tech specs is the data rate itself when the framerate is adjusted...and much of this didn't come to the surface generally until the P2 cards came out and Panasonic was trying to really present as sensible an economic picture as they could... DVC ProHD is 100 Mb/s. this is a fact that they've frankly been bludgeoning HDV with since HDV hit the market. What they don't mention when they're making those comparisons is that normally DVC ProHD 720p is a 60p file...it doesn't matter what frame cadence you choose, it gets packed in a 60p stream. Most Varicam users who shoot with and love that camera (I think it makes wonderful images myself) shoot 24p (23.97) with it for a "filmic" aesthetic, which that camera does really really well. However, when you are shooting 24 frames/s into a 60 fps stream, each frame needs to be duplicated, one for two frames, the next for three, the next for two, then three, and so on...

When you handle this material and cull out the redundant frames to get down to the 24 you need, you've discarded 36 redundant frames, which, as Panasonic points out every chance it gets, are all I-frame compressed (they aren't long-GOP like MPEG2 based HDV). So you've tossed 60% of your frames...and therefore 60% of your data. DVC ProHD at 24p is a 40 Mb/s file...not a 100 Mb/s file.

So...next to a 35 Mb/s variable data rate XDcam recording, a 40 Mb/s constant bitrate recording doesn't have that much of a margin...though I think the difference in camera heads would be evident in the aesthetic of the pictures.

So...once again, image quality relies on a collection of factors...and we haven't even mentioned lenses which might be the most impactful factor...then the camera head, the processing, luma and chroma subsampling, then compression and what data rate is used.

...very few clean and neat answers out there and even fewer completely compatible comparisons.





TimK,
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Kolb Productions,

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