Varicam or HD

["Victor Wolansky" <victorw(at)NATMEDIA.com>]

I found this searching on this topic, I think is very useful information to share..

 

 

Riddle#2: Does the Varicam have more Color Space than the F900? Is the Varicam better for green and blue screen matting?

What I'm discovering is a bit of an eye opener in terms of Color Space and Image Sampling between the Sony CineAlta and the Panasonic Varicam.

Have you heard the common misconception, prevalent across the industry, that the Varicam records a 4:2:2 color space to tape? The argument goes like this ...... while the Varicam has less resolution in luma, it has more resolution in chroma compared to the Sony F900 CineAlta. So this means the colors are sharper, therefore the over all image will tend to look relatively sharper as well, despite there being less pixels in total compared to the F900. Consequently, this helps the Varicam produce an image that looks just as sharp as the F900. While the images are indeed amazingly similar, this reasoning is bogus.

And if this were true, then the Varicam might turn out to be the better tool for doing green and blue screen work. On the face of things, it might seem the Varicam will do a better job with pulling mattes with double the color resolution. After all you are keying to a color, and if the colors are sharper, then it should work better - right? Again, this reasoning is bogus. The real story is a little more involved, as I will try to explain.

It is amazing how entrenched the misconception is that the Varicam has approximately double the color space of the F900. But in a sense this is true ..... it all depends on how you look at it! Lets start by looking at precisely how each HD camcorder system processes and records its images to tape. But in so doing, we can't lose sight of the fact that the starting point for each camcorder is different. The Sony CineAlta has approximately twice the number of pixels per CCD compared to the Panasonic Varicam (2.2 megapixels for the F900 verses 1 million pixels for the Varicam).

The first step in both cases is pre-filtering in luma. Both systems use the same reduction factor, namely 1.33^* (recurring). The F900 starts with 1920 pixels per line, and filters down to 1440 in luma (1920 divided by 1.33 = 1440). The Varicam starts with 1280 pixels per line, and filters down to 960 in luma (1280 divided by 1.33 = 960). The original "1920", and the original "1280", are defined by their respective format specifications.

Next, chroma is sampled at 3:1:1 in the case of the Sony camcorder, and at 4:2:2 in the case of the Varicam camcorder. So the CineAlta has 1440 in luma and (1440/3)= 480 in chroma, while the Varicam has 960 in luma and (960/2) = 480 in chroma. Note that both systems now have precisely the same number of pixels per line, namely 480, representing color information!!!

Just to be clear about the mathematics I used here, the divisors 3 and 2 respectively, come from the how each camcorder subsamples its color space. The CineAlta is 3:1:1, so it subsamples its chroma channels by a factor of 3 in relation to its luma channel. Specifically, it samples chroma at 1/3 (one third) that of the luma channel. So this is why I divided by 3. The luma is sampled at 1440 /3 = 480 which is the chroma sampling. Similarly, the Varicam is 4:2:2, so it subsamples its chroma channels by a factor of 2 in relation to its luma channel. Specifically, it samples chroma at 1/2 (one half) that of the luma channel. So this is why I divided by 2. The luma is sampled at 960 /2 = 480 which is the chroma sampling.

While both systems (coincidentally) have equal chroma resolution, the Sony HDCAM has 50% more luma resolution. This is not surprising considering it started out with twice the number of pixels in the first place.

Another way to look at this is as follows:

HDCAM = 1440 in luma, and (1440 /3) = 480 in chroma = 1440:480:480 = 3:1:1

HD Varicam = 960 in luma, and (960 /2) = 480 in chroma = 960:480:480 = 4:2:2

In an absolute sense, the CineAlta and the Varicam have identical color space i.e. 480 pixels per line!!!

But relatively speaking, the Varicam samples a slightly lower resolution image at a slightly higher rate for its colors. "A-little-more-of-a-little-less" means there is no net gain in terms of color space compared to the CineAlta. However, the CineAlta has a comparative advantage in terms of resolution in luma (or the black and white component of the image) by a factor of a half i.e. 50% more resolution in luma. Or 1440 pixels compared to 960 pixels (mathematically speaking: 960 x 50% = 480, then 480 + 960 = 1440) (or 960 x 150% = 1440) (or 1440/150% = 960) etc.

Sony's 3:1:1 exactly equals Panasonic's 4:2:2 in terms of color space in an absolute sense. But in respect to the Varicam's own inherent line resolution, it has twice the rate of color sampling compared to the CineAlta. This leads to the conundrum that it is true to say the CineAlta and the Varicam have identical color space, but at the same time, it is also true to say the Varicam has twice the color space of the CineAlta. This is of course is contradictory.

The truth of the matter can only be resolved when you look at the total number of pixels per line that are sampled for color information.

3:1:1 color sampling of 50% more line resolution = 4:2:2 color sampling of 50% less line resolution.

To complete the story, both camcorders sample at 10 bit, pre-filter, compress, and record to tape at 8 bit. In other words, both systems work in a very similar way, and indeed, they both produce very similar looking images.

Many people believe that the Varicam performs better for green and blue screen matting processes, compared to the Sony F900. I think the reality is that they are about the same in their blue and green-screen matting capabilities.

Just as an aside, the Sony HDCAM (D-11) compression scheme is based upon a 1440 x 1080 image format and 3:1:1 chroma subsampling, as described above. Downsampling is required upon compression, and upsampling is required upon decompression. The downsampling inherent in HDCAM compression of 1080i or 1080p means that luma sample aspect ratio is effectively 4/3 (and chroma subsampling is equivalent to 4:1:1) relative to the original 1920x1080 image array. So luma is multiplied by 4/3 or 1.33^* (recurring) i.e. 1440 multiplied by 1.33^* = 1920. Hence it is 3:1:1 color space in respect to 1440 x 1080, but really only 4:1:1 color space in respect to the originally captured 1920 x 1080, and also the 1920 x 1080 produced after decompression of the signal and upsampling. In some ways it is hokus pokus with numbers to say HDCAM is 3:1:1, as 4:1:1 is perhaps more accurate in its practical application. Same with the Varicam, it is 4:2:2 color space in respect to 960 x 720. This leaves us with the somewhat awkward ratio of 4:1.5:1.5, which is the color space in respect to the originally captured 1280 x 720, and also the 1280 x 720 produced after decompression of the signal and upsampling.

 

 

Victor Wolansky

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