AJA KONA3X User Manual
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Whitepaper Author:
Jon Thorn, Product Manager, Mac Desktop Products—AJA Video Systems
2K Overview
Understanding 2K Workflows in Today’s
Post-production Evironments
The Image Size of 2K: Traditional Cinema and Digital Cinema
2K is a term, like SD and HD, used in today’s post-production environment to describe a particular image size and quality of data.
2K data exceeds our pre-existing television broadcast standards for both SD and HD and is therefore most commonly associated
with traditional cinema and the emerging digital cinema initiative.
When working with data for eventual cinematic projection, FX work or digital intermediate purposes, 2K is usually defined as
2048x1556 pixels. This size represents the “full” size of the 35mm film between the sprockets. Therefore the result, 2048x1556
pixels, appears as a 4x3 image when compared to an HD image which is typically 16x9. In 2K, other image sizes can be derived
from this 2048x1556 source by taking a cropped portion of the image for use. For a traditional cinematic projection scenario, the
final delivery of this 2048x1556 data is onto 35mm film. The film undergoes photochemical and mechanical processes before the
image reaches the screen.
The other common size attributed to 2K is 2048x1080; this is the standard to which digital cinema currently adheres. Most digital
cinema projectors have this 2048x1080 image size as a supported resolution and in many cases, as a maximum resolution. Here
the data at 2048x1080 need not undergo a photochemical process; it can stay data for its path to projection.
So the first obvious advantage of working with 2K images as opposed to HD is the size of the image that can be generated,
manipulated, and ultimately projected. Some will argue that a better choice for cinematic work is a 4K resolution. Certainly this
is true, but working with 4K would be exponentially more of a burden than working with 2K’s already large data requirements;
4K is simply beyond the scope of many pieces of equipment be it scanners, storage or projectors. To be fair, some 4K and beyond
resolution is already being performed for select feature films—and even then for select shots in many cases. Perhaps in the future
4K will supplant 2K as HD is replacing SD, but that day seems to be quite distant, so for now a large body of the motion picture
community has settled on the high quality of 2K.
2K Color: More Like Film than Video
2K images, like SD and HD images, can come in 8 bit, 10 bit, 12 bit, 16 bit etc. But most commonly, 2K files are in written in a 10 bit
Log RGB or RGB format. This provides for 1024 gradations of a given color in three equivalent colors of red, green and blue. By
using RGB, 2K data can emulate, to some extent, film which achieves its color reproduction via red, green and blue layers of emulsion.
The actual 2K image sizes of 2048x1556 and 2048x1080 are usually written in two similar, but slightly differing, file formats; Cineon
or DPX (Digital Picture Exchange format). The Cineon file format traces it’s roots back to one of the earliest “film as digital” devices,
the Kodak Cineon. The Kodak Cineon, introduced in 1992, was a scanner that took film images and translated them into digital
data. Today many devices from a number of manufacturers perform such a process.
Copyright ©2006 AJA Video Systems, Inc
Since files bearing the .cin extension were always related to film, they tend to always be in Log RGB. Log RGB is a color scheme
designed to approximate the characteristics of film emulsion in a digital environment. An easy analogy is this: Log RGB is like a
“digital film negative” while linear RGB (usually just referred to as RGB) is like a “digital film positive.” To transform a log RGB image
into a “positive”, Look Up Tables (LUTs) can be applied to the image so that this raw image can be seen as it would be in its finished
form.
As already mentioned, Cineon files are not the only file format that can be used to house the 2K data. DPX (Digital Picture Exchange) is quickly becoming the standard since being defined by SMPTE. Like files bearing the .cin extension, files bearing the
.dpx extension can be Log RGB, but they can also be linear RGB.
Cineon and DPX files at full size 2048x1556 and 10 bit quality tend to exist as individual frames that occupy 12.2 MB of data. At
12.2 MB/frame, data rates for a second of video climb to roughly 300MB/sec. By comparison, the highest quality HD video images
rarely exceed 200MB/sec and most HD formats use only a little over 100MB/sec. Furthermore, most HD material is usually somewhat compressed in order to be recorded onto tape formats; 2K data can achieve an uncompressed status by going straight to a
disk array.
If the first major advantage of working with 2K images is their size, then the second advantage of working with digital images at
2K is their handling of color. 2K color data in Log RGB can more closely emulate film properties than video properties. Furthermore, this color information need not be compressed due to the limitations of tape recording, but rather the data can be dealt
with as uncompressed when recorded directly to disk.
The Source of 2K Data: Scanning, Telecine and Digital Cameras
Until recently, the only way to acquire a 2K image was to shoot on 35mm film and then scan the original camera negative (OCN).
To be fair, most 2K data is still generated in this method since film is still seen as the de facto medium for recording moving images for projection at the highest possible resolution. 2K scanning persisted as the solitary method of creating 2K data until a
handful of telecine machines came on the market that could move data at 2K resolution.
Now we are seeing the first generation of what can truly be referred to as “digital cinema” cameras; they lack video recording
devices, specifically tape recording capability, in their design and instead concentrate on producing electronic data that aims to
emulate, or exceed, the image quality of traditional photochemical film processes.
Transporting 2K Data: HSDL Defined
Moving 2K data is no small task; remember that at 12.2MB/frame, data moves at roughly 300MB/sec. Storage devices, such as
RAIDs (Redundant Array of Independent Disks), must have very high bandwidth capabilities to handle recording or playing back
the data at full speed. The storage devices must also have adequate space to hold 2K data, as an hour of 2K just exceeds 1 Terabyte.
But storage devices are for storing the data; transporting the data is something different. HSDL (High Speed Data Link) is easiest for video professionals to understand in this way: HSDL is like Dual Link HD for the transmission of 2K data. Where Dual Link
HD moves across two SDI cables as video, HSDL moves 2K data over the two SDI cables. Because of the amount of data previously mentioned, full size 2048x1556 images cannot be transmitted over the dual SDI lines at full frame rate. Instead of moving
the data at 23.98 (24) frames per second, the HSDL transport stream often adheres to 14.98 frames per second. Because this is
transmitted data, not video, the data can be recorded at the 14.98 frames per second rate to a high speed storage device and then
played back from this high speed storage device at full speed, typically 23.98 frames per second. In contrast to 2048x1556 images,
2048x1080 digital cinema 2K can move across HSDL at the full frame rate of 23.98 frames per second.
Facilities with two lines of HD-SDI rated cable, in a limited sense, are already prepared for the transmission of 2K data when attached to HSDL devices. If a facility already had storage that could exceed 300MB per second, a facility might also be ready to
record and play back 2K data at full frame rate. Again, this transition to a “digital film” environment is far less cumbersome than a
4K digital environment where such numbers more than double those of 2K.
So a third advantage beyond image size and color reproduction is the ability of 2K data to use pre-existing elements of HD infrastructure when adopting 2K. For many facilities, be they large full-service or boutique, this makes the financial transition from HD
to 2K less of a burden than a transition to 4K.
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