Highlight Handling
Thanks to Power HAD technology and 12-bit Advanced Digital Signal Processing, Digital Betacam camcorders cover a very wide dynamic range of up to 600%. Dynamic range is often defined as the capability of a camera to capture a very large difference between the brightest and darkest parts of a scene, and is very dependent on the knee circuits.
A well-known problem with handling highlights is their effect on skin tones. To overcome this problem, and improve overall picture quality, the new Digital Betacam camcorder range incorporates TruEyeª processing. Exactly like the human eye, this processes the dynamic range of the camera before gamma correction by using brightness, hue and saturation instead of individual red, green and blue signal processing. Knee correction is therefore effectively processed only on the brightness (luminance) signal content. As far as saturation is concerned, the video level of each of the red, green and blue channels can be compressed without causing hue rotation, although the saturation slowly decreases as the signal level
approaches clipping. The correct hue is maintained, and an image can naturally converge into white at the clip point.
Because the net result is faithful colour reproduction above the knee point, the knee point can be lowered from the conventional value of 98% to 85%, and also the knee slope is much larger. This gives a much wider visible dynamic range.
How TruEye Processing Functions
To understand the knee circuits used in the new camcorder, it is necessary to understand TruEye processing.
TruEye processing is one of the most innovative features that DSP allows, and makes it possible to reproduce a far more natural colour than a conventional analogue camera, even in severe shooting conditions. A knee circuit is necessary to compress the very wide dynamic range of the CCD sensor (600%) to the much more limited recording standard (109%). The knee circuit functions to compress the video output level when it exceeds a certain video input level, called the knee point. Thus, the dynamic range of the camera can be broadened, and images in bright areas that exceed the standard video level can be seen.
In cameras with conventional processing, this knee correction is performed individually in the red, green and blue channels. Since the knee correction is a non-linear process, and is located after gamma correction, the balance of hue, saturation and brightness are changed after processing. The point at which the knee circuit of each colour operates will depend entirely on the picture composition and colour balance. Therefore, one colour may be in the non-linear part of the transfer curve (above the knee point) while another is still quite linear (below the knee point). In this case, the hue is rotated in bright parts of the picture, and faithful colour reproduction is impossible. This is why human skin tends to look yellow when parts of the picture are even slightly overexposed. Remember that this is a normal situation where some areas in the picture tend to be more reflective than others Ð a shiny forehead is a typical example. The overall exposure will be correct.
Knee Saturation
Knee saturation acts together with TruEye to maintain the saturation in those picture areas compressed by the knee function. Normally, any highlight which is above the knee threshold will reduce the saturation until the final clip point, when that picture area will be perfectly white.
RGB Knee Compensation
Advanced TruEye
Conventional TruEye
