We have made progress with our R&D and standardization activities related to the 8K Super Hi-Vision (SHV) video system.
■ Wide color gamut
The SHV video format employs wide-gamut system colorimetry that covers a wider range of colors than for Hi-Vision and can reproduce most real object colors. We designed the spectral sensitivity characteristics that would be needed for a camera supporting the wide-color-gamut and used the results to develop prisms for full-resolution SHV cameras with three 33-megapixel image sensors (1). We built a wide-color-gamut SHV system consisting of this wide-color-gamut camera and a wide-color-gamut SHV projector (see 1.1.3). At the NHK STRL Open House, we used the system to capture and display images of objects with highly saturated colors and demonstrated the improvement in accuracy of the reproduced colors including highly saturated ones.
In order to convert the colors of the SHV wide-gamut system colorimetry into those of the Hi-Vision color space, we are studying a method that decreases only chroma without altering lightness and hue to maintain image quality. Since sufficient data had not been reported on the characteristics of perceptual hues in the color space outside the Hi-Vision color-gamut, we performed subjective evaluation tests using the wide-color-gamut SHV projector. The results showed that some hues have different characteristics from the ones predicted by the conventional uniform color space.
■ High resolution
We are studying technologies to convert SHV video captured by 8-megapixel four-sensor SHV cameras or 33-megapixel single-sensor SHV cameras, which has a Bayer pattern pixel structure, into full-resolution SHV video with high quality. In the Bayer pattern, two green, one red and one blue pixels are arrayed in a square grid. We devised a method suitable for real-time processing to interpolate color signals precisely (2) and confirmed that it can generate color images that look natural and have better resolution and minimal artificial colors.
To better understand the resolution characteristics of SHV cameras, we developed a new method to measure not only horizontal or vertical but also multi-dimensional resolution characteristics precisely by analyzing an image of a slightly-slanted knife-edge target captured by the cameras (3).
To clarify the relation between spatial video resolution and preferred viewing distance during TV viewing, we measured the preferred viewing distance when displaying a program consisting of still images at different levels of spatial resolution. The results showed that a shorter viewing distance tends to be preferred for higher spatial resolution (4).
We are standardizing an interface for connecting video devices in the studio to transmit various UHDTV signals over 10-Gbps multilinks at the Association of Radio Industries and Businesses (ARIB). The UHDTV signals include full-specification SHV signals (7680 × 4320 pixels, 120-Hz frame frequency, 12 bit), 4K signals (3840×2160 pixels), 60-Hz frame frequency, and the 4:4:4, 4:2:2 and 4:2:0 formats. The ARIB Standard STD-B58 (5) was established in March 2014, and it includes specifications of the data mapping and serialization, transmitter and receiver characteristics, and optical connector characteristics.
■ Subjective evaluation of image quality
We studied methods for subjectively evaluating the image quality that would be suitable for SHV video, which has high resolution and a wide field of view. Through subjective evaluation tests of coded video, we found that subjective evaluation scores depend on the viewing distance and horizontal viewing position. Based on this result, we proposed that for a subjective evaluation of SHV video using a display of about 80 inches, three evaluators should observe horizontally trisected parts of an image at a viewing distance of 0.75 times the screen height. This research was conducted as part of a project commissioned by the Ministry of Internal Affairs and Communications, titled “The research and development project for the expansion of radio spectrum resources”.
■ Broadcasting format standardization
We contributed to the standardization of ultra-high definition television (UHDTV) broadcasting systems at the Information and Communications Council and ARIB. In March 2014, the Council issued specifications for video formats including SHV with a 120-Hz frame frequency and High Efficiency Video Coding (HEVC).
(1) T. Soeno, K. Masaoka, T. Yamashita, T. Nakamura, R. Funatsu, Y. Nishida, M. Sugawara, A. Saita: “Wide-color-gamut Super Hi-Vision Camera,” ITE Annual Convention 2013, 10-5 (2013) (in Japanese)
(2) S. Tajima, R. Funatsu, Y. Nishida: “Chromatic interpolation based on anisotropy-scale-mixture statistics,” Signal Processing, Vol. 97, pp. 262–268 (2014)
(3) K. Masaoka, T. Yamashita, Y. Nishida, M. Sugawara: “Modified slanted-edge method and multidirectional modulation transfer function estimation,” Optics Express, Vol. 22, No. 5, pp. 6040–6046 (2014)
(4) M. Emoto, M. Sugawara: “Still Images of High Spatial Resolution Enable Short TV Viewing Distances,” ITE Transactions on Media Technology and Applications, Vol. 2, No. 2, pp. 185-191 (2014)
(5) ARIB Standard STD-B58 (ver. 1.0), “Interface for UHDTV Production Systems” (2014) (in Japanese)