We have just examined the 3 Gbps digital interface, which facilitates the carriage of 1920x1080 HD video at 60 fps, on a single interface. Currently, 1920x1080 is the HD video scanning format with the highest spatial resolution, but there are efforts underway to change this, of which we will hear more about later.
Before delving into "Super HD" resolution scanning formats, let's take a look at what resolutions are available to us today, and in what formats. To simplify things somewhat, for the purposes of this exercise, we will consider only static spatial resolution: the resolution of still pictures and objects. We will ignore dynamic spatial resolution (the spatial resolution of moving images or objects) and temporal resolution (resolution in time, i.e., frame rates). We will ignore the Kell factor, and, because we will not address interlaced scanning here, we will also ignore the interlace factor.
RESOLUTION OF FILM
HD resolution has historically been compared to the resolution of 35mm film, which has been a major acquisition format for television programming for the entire history of U.S. commercial television. What is the resolution of 35mm film? It of course depends on the specific film stock being considered, but we will use, as a typical example, Kodak 5293, a 35mm film stock widely used for television production. 5293 is color balanced for tungsten light, for which it has a published speed of ASA 200. The resolving power is stated as a single number, but of course things are more complicated than this.
Each of the three emulsion layers, red, green and blue, has its own resolution. The green layer, which is the topmost layer, has the highest resolution; and this is a good thing, as the human eye is most sensitive to light in the green portion of the spectrum. The red layer, the bottom layer, located next to the substrate, has the lowest resolution of the three. This contributes to the much-desired "soft look" of human skin realized with film, as all skin is fundamentally red in color. A more complete indication of the resolving capability of film (or video) is a set of modulation transfer function curves, but that is a subject for another column.
The resolution of film is typically expressed in line pairs per millimeter. This is measured by determining the number of sine-wave cycles from black to white to black again, usually represented by alternating black and white bars or lines, which may be discerned by an observer, per unit of distance. The achievable resolution for 5293 film stock is about 80 line pairs per millimeter. This figure is in the neighborhood of the best resolution that may be achieved with this particular stock. In practice, resolution is affected by a number of other factors including the lens used, the actual film speed used for shooting and processing, and the adequacy of the applied lighting. When we apply this resolving power to the full-frame aperture of some of the standard 35mm frame dimensions, we can get an idea of how it compares to HD video.
THE NYQUIST RULE
The grain structure of film is random, so its resolution is the same in all directions. 35mm Academy aperture is a frame that is 22mm wide by 16mm high. At 80 line pairs/mm, this yields a resolving power for the entire aperture of 1760 (h) x 1280 (v) line pairs. We are also mindful that some portion of the full frame will be cropped in post production, reducing the full-frame resolution by some amount. To determine the comparable resolution of the 1920x1080 HD sampling grid, we note that a horizontal line contains 1920 samples.
The Nyquist Rule tells us that in order to faithfully restore a sampled waveform, we must have at least two samples per cycle, and this brings us to the conclusion that the digital resolution of a 1920x1080 frame is about 960 cycles (line pairs) horizontally x 540 cycles (line pairs) vertically. If we apply this 1920x1080 sampling grid to the Academy aperture, we discover that the resolution in the horizontal direction, at 960 line pairs, is a little more than half the limiting resolution of the film, which is 1760 line pairs. In the vertical direction, the resolution of 1920x1080 HD is about 540 line pairs.
The vertical resolution of the entire Academy aperture frame is about 1280 line pairs, but the aspect ratio of the Academy aperture is 1.37/1, while the 16:9 HD frame has an aspect ratio of about 1.78:1. If we fill the Academy aperture frame width with the 16:9 HD frame, we will only fill part of the Academy aperture vertically, i.e., the HD frame will be letterboxed onto the Academy aperture frame. This effectively reduces the total vertical resolution of the film frame from 1280 to 989 line pairs, again a little more than twice the HD vertical resolution of about 540 line pairs.
If we step up to the frequently-used Super 35 camera aperture, we have several aspect ratios from which to choose, but the aspect ratio closest to the 1.78:1 HD aspect ratio is the cinematic "flat" aspect ratio of 1.85:1, which has the dimensions 24 x 13 mm. If we apply the same exercise to this aperture that we did to Academy aperture, we discover that the full-frame resolution in the horizontal direction is about 1920 line pairs, exactly twice the resolution of 1920x1080 HD. If we compensate for the aspect ratio loss, the vertical resolution of the film frame in the Super 35 flat aspect ratio cropped to a 1.78 :1 aspect ratio is about 1079 line pairs, again almost exactly twice the vertical resolution of the 1920x1080 HD frame. The conclusion that may be drawn from this is that 1920x1080 digital HD has a limiting spatial resolution about half that of 35mm film.
'SUPER' FILM RESOLUTION
Beyond 1920x1080 HD, higher spatial resolution may be achieved using 2K or 4K scanning. These are not video scanning formats (although as mentioned at the outset, there are efforts underway to change this); they are file-based formats. The 2K format is well-established as a data telecine format, and it can be produced by some current digital cinema cameras. It has a horizontal pixel count of 2048, about 107 percent of the horizontal pixel count of 1920x1080 video. It is capable of producing a horizontal resolution of 1024 line pairs, compared to the 960 line pairs achievable with 1920 horizontal pixels.
The next step up is 4K, which is achievable with some data telecines and scanners and some digital cinema cameras. There is a film scanner on the market that natively scans at 8K, with the scan being downsampled to a 4K output. 4K is 4096 pixels per horizontal line. If we scan the Super 35 Flat aperture at 4K, we will be scanning at a full-frame resolution of 2048 line pairs horizontally, twice that of 2K, and slightly more than the horizontal resolution of our 35mm film example at 1920 line pairs. To cut to the chase, scanning at 4K yields a resolution slightly better than that of our 35mm film example, making 4K a highly desirable archive format for 35 mm film.
As an example of "super" film resolution, let's take a quick look at 65mm film. As we know, "real" 70mm theatrical releases, as opposed to those shot on 35mm stock and blown up for projection, are shot on 65mm film stock, with the addition of another 5mm to the release print for sound tracks. 65 mm camera aperture is about 52.5mm x 23mm. At 80 line pairs/mm resolution, this yields a full-frame resolution of about 4200 line pairs horizontally by 1840 line pairs vertically.
To complete this picture, let us consider VistaVision, which is 35mm film run sideways. VistaVision was developed in the 1950's as a widescreen cinematic format. Because of its high rate of consumption of film stock and consequent high cost, it had but a short life in cinematic projection, but it was used for many years thereafter for the high-resolution capture of elements for insertion into theatrical films.
VistaVision aperture is about 37.7 mm x 25.2 mm, with an aspect ratio of about 1.5:1. If we apply our calculations, the VistaVision frame's resolution is about 4200 line pairs horizontally by 2016 line pairs vertically.
Resolution is a complex topic, and we know that an "adequate" amount of resolution in a given circumstance depends on many factors. This gives us a frame of reference for resolutions of film, HD, and resolutions beyond HD. In the future, we will look at some of the efforts to push resolution to the limit in video scanning formats.
Randy Hoffner is a veteran TV engineer. He can be reached through TV Technology.
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