HD cameras & formats
by Richard Wood
on
Sep 16, 2007 at 8:32:22 am
Help me understand; The SOny PDW-F350 shoots 1080 x 1440. Isn't this a 4:3 aspect ration? So I don't understand why it is in the HD 16:9 format? Also, what is different, other than the obvious media difference, between the blue laser PDW-F350 and the HD tape camera F900? Why would someone use the F900 over the laser camera? Is HD 1080 x 1920 or 1080 x 1440 ??
Re: HD cameras & formats by Don Greening on Sep 17, 2007 at 8:48:01 am
[Richard Wood]"Isn't this a 4:3 aspect ration?"
That's correct. However, the pixel aspect ratio, or PAR of 1440x1080 is different in that the pixels are wider than they are tall. The PAR is, in fact, 1.333 times wider than they are tall, sometimes referred to as anamorphic PAR. This is apparently done to reduce the size of the video data stream. So, if you multiply 1440 by 1.333 you get (almost) 1920. When you capture this type of video you set your NLE to capture as an anamorphic picture, which will result in a corrected aspect ratio of 16:9. @ 1920x1080.
For more info on this topic you might want to read all the posts at the link below:
Re: HD cameras & formats by Don Greening on Sep 17, 2007 at 9:11:19 am
[Don Greening]"which will result in a corrected aspect ratio of 16:9. @ 1920x1080.
"
Actually, that's not true. The picture is still 1440x1080, but because it's anamorphic it just LOOKS like 1920X1080. That's what I meant but I thought I'd better clarify this, otherwise it's confusing. My bad.
Re: HD cameras & formats by Tim Kolb on Sep 17, 2007 at 5:04:21 pm
Unfortunately this seems like a simple question until you get all the qualifiers and exceptions in there and then you see why content producers are going out of their minds trying to understand all these technical nuances and compromises...
First, we have to specify that the sort of image that is STORED on media is a separate issue from what the standards say needs to be PLAYED BACK AND DISPLAYED (emphasis, not yelling)...
The problem with HD is that it creates huge pools of data. The problem with creating...and then handling huge pools of data is that it is not cheap. Unfortunately for manufacturers "cheap" is a very popular feature...so some compromises need to be made.
Just like DV and DigiBeta were technically creating pictures that complied with the same standard, there was an obvious quality...and cost difference. Why? Because in order to make smaller data sets to handle in DV, more info was thrown out and what was left was compressed more aggressively. DV technically played back a picture that could fill the same monitor that Dbeta could, but the image quality was noticeably degraded...not a problem for corporate and news guys, but higher end production would typically aim a bit higher...it's all in what's right for your situation.
Now...in comes HD. 5X the data and editing stations and recording media did not take an instantaneous leap forward in speed/capacity and cost-effectiveness to coincide with the adoption of HD. The answer to the dilemma is to agressively throw out info and compress similar to the days of DV, but in many ways, even more aggressive than that.
In terms of DV, the format had a similar luma pixel count to DigiBeta...but DV threw out more color information, forcing those discarded color values to be calculated by simply "filling gaps" when the video was decompressed. This did result in some perceived image "vibrance" loss to the eye when compared to Dbeta or DVC Pro50, which had twice the color values stored and far less "gap" to recalculate and fill.
Now, with HD we have the added feature of tossing out actual luma samples. In HDcam's case, Sony had the foresight to see that studio infrastructure was not going to be upgraded across the board the day the first HD piece of equipment was unpacked...so they decided to create an HD format (HDcam) with an SD data rate (135 Mb/s) so that existing SDI (SDTI) infrastructure could be utilized to ship the stuff around. n order to get what would be a 1.2 Gb/s data rate at 1920x1080 uncompressed down to nearly a tenth of it's original size, the camera drops the picture resolution down to 1440x1080 from 1920x1080, then throws out 2/3rds of the color information BEFORE they even start to "compress" the data set. So effectively, HDcam is 1440x1080 with a color difference subsample of 3:1:1. HOWEVER (emphasis again) when this plays out of the deck through HDSDI, the signal that the deck reconstructs from that stored signal is actually 1920x1080 4:2:2. Yes, all that data was not stored and the difference in resolution needs to be "manufactured" on playback, but your NLE will see 1920x1080 4:2:2 coming down the pipe on HDSDI.
With HDV and XDcam, these formats are actually shooting 1440x1080 (or, with JVC's HDV1 format, 1280x720p, square pixel). HDV does not pretend to have the 1920 horizontal resolution either on the camera sensors (Canon's is full res 1440x1080) or from tape playback with FW output. The only time when an XDcam or HDV2 (Sony/Canon) device needs to "create" a 1920x1080 image from it's 1440x1080 information is when it outputs full raster HD via HDSDI, which only takes square pixels. (again, JVC's HDV1 is already square pixel so no problem) Also, the MPEG compression utilized by HDV and XDcam has a color subsampling rate of 4:2:0, which does affect quality even when you consider that HDcam's 4:2:2 is interpolated from the 3:1:1 information it has stored...
When comparing the two cameras however (F900 vs the XDcam cameras), the camera heads are simply night and day different. The F900 can be bought for something slightly less than 100,000 dollars...maybe the cost is coming down somewhat in recent times...and the 350 XDcam camera can be purchased for something around 20,000 dollars. This should be the first indicator that there is a significant difference in the capabilities of the two cameras.
When it comes to the recording formats, HDcam simply carries more data...even at it's agrresively compressed data rate of 135 Mb/s, it's storing 4X the data that XDcam's top data rate of 35 Mb/s. Also, as much color data as the HDcam recording format discards, XDcam tosses out even more. More discarded information and higher compression ratios affect image quality.
Hopefully some of that helps clarify the situation. i purposely tried to stay away from any more numbers than I absolutely needed (yes, really...) to make the point to try to reduce the confusion...for anyone who feels this is a little vague. I purposely left out DVC ProHD's compression/sub sample scheme as the question was focused on HDcam vs XDcam.
Re: HD cameras & formats by Don Greening on Sep 17, 2007 at 6:13:21 pm
Exellent post, TIm. You made the "confusing" much less so, and in a way that kept my attention. Your price comparison between the F900 and the XDCAM 350 had me wondering if perhaps a rather large portion of the price difference is because of high def. recording/playback heads and tape transport, which are of course not present in the XDCAM line. Correct me if I'm wrong, but don't the Sony HDCAM decks start at around 60 grand USD? Would I be correct then, to assume that this would have more of an impact on the price rather than a large difference in CCD, signal processing, lens quality etc. between the two cameras?
Re: HD cameras & formats by Tim Kolb on Sep 17, 2007 at 8:12:40 pm
[Don Greening]"Your price comparison between the F900 and the XDCAM 350 had me wondering if perhaps a rather large portion of the price difference is because of high def. recording/playback heads and tape transport, which are of course not present in the XDCAM line. Correct me if I'm wrong, but don't the Sony HDCAM decks start at around 60 grand USD? Would I be correct then, to assume that this would have more of an impact on the price rather than a large difference in CCD, signal processing, lens quality etc. between the two cameras?"
well...first, I wasn't including lenses in my price listings...second, i think the JH3 HDcam VTR lists for under 30K USD. I'm sure the heads have something to do with it (as in the case of the HVX200 vs the Varicam), but the 2/3" CCDs in the F900 are significantly better performers than the current 1/2" sensors in the existing XDcam line (there is a 2/3" sensor camera head on the way for XDcam...).
All around, the overall price on the camera and recording system is less because they've made more compromises in XDcam equipment than in HDcam equipment...as far as what accounts for 51% of the difference vs 49%...I don't think either one of us have the clout to have Sony's accountants spell that out for us...
Re: HD cameras & formats by BKM on Sep 17, 2007 at 7:07:04 pm
"I purposely left out DVC ProHD's compression/sub sample scheme."
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.
Re: HD cameras & formats by Tim Kolb on Sep 17, 2007 at 8:46:17 pm
[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.
