|
XDCAM-EX
- 1920 x 1080 full raster
- 4:2:0 color sampling
- long-Group of Pictures (GoP) encoding
- 8-bit quantization
- MPEG-2 encoding technology
- 35 megabits/sec average data rate
AVCCAM
- 1920 x 1080 full raster
- 4:2:0 color sampling
- long-Group of Pictures (GoP) encoding
- 8-bit quantization
- H.264 MPEG-4 Layer 10 encoding technology (recorded as AVCHD)
- 21 megabits/sec average data rate
As you can see, they're pretty much the same across the board, the same basic specifications, with the difference being the encoding technology used. AVC is later/newer technology, a more advanced and more efficient encoder. AVC is able to squeeze more picture into the same amount of bandwidth (think of it as miles per gallon in a car; MPEG-2 might get 20 miles per gallon, AVC gets about 45 miles per gallon in comparison).
One of the major differences between how AVC works, versus MPEG-2, is in the block sizes that they use. MPEG-2 uses 8x8-pixel blocks, breaking the entire picture up into 8x8-pixel areas. When things are going smooth, you'll never know they're there, but when things get really hairy in the compression, you'll see that blocky texture appear; you may have noticed this effect on HDTV programming or youtube/vimeo when net congestion hits; the image can start turning blocky. Where MPEG-2 uses a fixed 8x8 block size, AVC can vary its block size, using as small as 2x2 pixel blocks. Because of this, AVC can have a lot smoother degradation of the image; it won't turn to discrete digital blocks, and the result is usually a more organic/natural looking image, rather than a blocky digital-looking image. This is a key factor in what I was looking for: if I could get the EX1 to go blocky, what would happen to the AVCCAM?
So, armed with an EX1 and an HMR10 and an SxS card, I set out to see if I could break some codecs. I put the camera in the most compromising, challenging situations I could think of -- billowing banners against a backdrop of highly detailed trees and bushes, with the wind blowing, handheld. Driving and panning past fields of reeds and pine trees with their billions of needles and individual reeds all blowing in the wind. Snap zooms onto flags with a tree background. Handheld shots of a palm tree as 45-mph traffic zoomed through the frame. Rotating, spinning, bouncing, whatever I could do to challenge the codecs as hard as I could. I also pushed them to the limit by using 1080/60i mode (interlaced is much harder on a codec than progressive encoding is). I also boosted up the in-camera sharpness, because that really makes codecs unhappy!
The great thing about this is, because the AVCCAM recorder was taking the uncompressed 4:2:2 HD-SDI feed off the EX1, and also recording to its internal SxS memory card, it meant that I had absolutely pixel-for-pixel identical frames going to both codecs. No complications from trying to match two cameras here, and there were no other factors to consider -- both codecs were being fed the identical same image from the same camera head. So I made sure to snap my fingers at the beginning of each take to create a "sync mark", and then I could line all the footage up in the NLE and see exactly what the differences were, frame by frame. Any differences would be purely attributable to the codec and nothing else.
I then sifted through the footage, and found that by and large, most of it looked pretty much the same. As I said before, XDCAM-EX is a reasonably robust codec and it handles most situations pretty well. Through most of the scenes, there wasn't much of a difference between both codecs (except, of course, for AVCCAM taking up around half the space to do the same job, a not-insubstantial consideration). I toggled back and forth and saw that while there were minor differences here and there, for the most part there wasn't much to show them as being all that different, and they both looked very good.
But every once in a while, I struck gold (er, meaning, I pushed the codec so hard it shattered into a mess of blocky pixels!) Sometimes I pushed the camera so hard the codec would break down. Those were the moments I was looking for. And I'm surprised to say -- in those cases, AVCCAM mopped the floor with XDCAM EX. XDCAM went mega-blocky, but AVCCAM stayed smooth, even when it lost detail. In every case that I examined, every single time, the AVCCAM looked better. Its "more organic" degradation isn't a myth or marketing speak, it's for real. And, the MPEG-2 nature of XDCAM caused some frequent spurious pixels popping up all over the image when it was too stressed, but AVCCAM kept it together much more efficiently.
So -- conclusions? Easy -- AVCCAM's 21mbps PH mode is a clearly superior codec over XDCAM EX. No question, no argument, no doubt. While XDCAM kept up with AVCCAM in almost all the testing, it took nearly 70% more space to do it. And then there were the codec-stress times when AVCCAM was just significantly superior. Any way you slice it, the recorded image of the AVCCAM was as good or better, on cheaper media, in much smaller file sizes for easier archiving and storage (and not to mention the metadata that you can use in AVCCAM, helpful in locating your archived clips).
All long-GoP codecs are not created equal (even within the h.264 family!) but AVCCAM's PH mode is more than a match for XDCAM EX, and records on commodity memory, and takes up less space. After filling the XDCAM's 8GB SxS card I had, the AVCCAM's 8GB SD memory card was less than half full. I am not what you might call a fan of long-GoP codecs (I prefer intraframe, thankyouverymuch) but I will say that I am pretty impressed with how well AVCCAM worked here.
For those who want to see the kind of examples I pulled from my footage, here are animated .GIF files with particularly illustrative differences. The following are pixel-for-pixel extractions out of the original uncompressed 1920x1080 HD frames, as recorded by each recorder and then frame-grabbed from my NLE. They were then stacked and animated in .GIF format for use here. And, keep in mind, these are frames from the most-stressed situations, so none of these frames are going to look actually "good" -- but the question is, which one is better?
|
