Let's not confuse resolution with definition in the UHD signal

José Manuel Menéndez, professor of the Polytechnic University of Madrid and director of the RTVE Chair – UPM, provides the keys to two concepts especially discussed in the world of UHD: resolution and definition.

If we ask anyone about the difference between two TV signals, one SD and the other HD, surely one of the first answers they will provide is that “the HD one usually looks better.” Correct. If we insist and ask why it looks better, it is very likely that it will be argued, among other things, that “HD has more dots in the image than SD”. TRUE. So, is definition the same as resolution? The answer is no. Both concepts are related, but they are not the same.

As mentioned in the chapter 1 of the UHD Spain White Paper, in its first edition, the concepts of SD and HD in television are not new. The first TV systems were based on mechanical elements and presented only 30 image lines. In parallel, several electronic systems were proposed that already offered more than 400 image lines. These electronic systems were considered as High Definition (High Definition – HD), compared to the reduced quality offered by mechanical systems. Subsequently, starting in the 1950s, the standards that remained active in the world for decades were proposed: NTSC, PAL and SECAM.

In the world of analog television, the concepts of definition y resolution They were linked. The vertical resolution or definition was the number of active or visible lines in the scene, while the horizontal resolution or definition was the fineness of detail with which an image is reproduced, and was closely related to the working bandwidth, since the greater this was, the greater the capacity of the analog signal to represent abrupt transitions (edges) in the scene, that is, the high spatial frequencies. The horizontal resolution or definition was, therefore, a subjective concept, since it depended on the visual acuity of the eye.

The first “HD systems”

With all these precedents, it is normal that analog TV systems proposed mechanisms to improve the subjective perception of the quality of the images, and these systems were called HD systems, since they improved the "level of detail with which the elements that made up the scene were appreciated."”. Over time, the audiovisual signal offered by the NTSC, PAL and SECAM standards were considered standard definition systems (Standard Definition – SD), characterized by:

• A video signal with between 484 and 576 active lines, with an aspect ratio (ratio between the width of the screen and its height) of 4/3, interlaced scanning and a frame refresh rate between 50 and 60 fields/sec.
• A monaural audio signal until the end of the 80s, when the NICAM 728 system was deployed and stereo and digital audio was available.

In the mid-80s, several companies and consortia launched a race to develop an analog system that offered greater quality and detail in the scene, and the name HD re-emerged. In Europe, the proposal is called HD-MAC (High-Definition – Multiplexed Analogue Component). This system presents:

• Una señal de vídeo analógico con 1152 líneas visibles, manejando alta frecuencia para poder representar mejor las altas frecuencias espaciales en la imagen, relación de aspecto 16/9, barrido entrelazado y frecuencia de refresco de cuadro de 50 campos/seg.
• Una señal de audio digital estereofónico de alta calidad.

Se hicieron pruebas de emisión de dicho sistema HD-MAC en las olimpiadas de Barcelona 92 y en la EXPO de Sevilla del mismo año.

La digitalización de la señal, tanto SD como HD, supone la separación completa entre los conceptos de resolución y definición:

• The concept of resolution, in a digital signal, is associated with the number of columns and rows of pixels that the image presents. The horizontal resolution, therefore, is the number of columns, and the vertical resolution is the number of rows.
• The concept of definition is linked to the idea of fineness of detail with which one is able to reproduce the elements of the scene, and it will depend on the capacity of the capture system, how the signal is processed in production, how it is encoded, compressed and transmitted, how it is represented on the screen, and the visual capacity of the observer, that is, all the elements of the chain.

Definition to make an impact

If the set of elements with which the definition was increased in the jump from SD to HD is analyzed, it is observed that it is small, although sufficient to achieve a very important impact on subjective perception. The importance of the audio signal in the final subjective quality of the observer (as demonstrated by the BBC) should not be forgotten, so aspects of both the video and audio signal must be considered.

With all of the above, the elements introduced by the TV with the HD signal in relation to the previous SD signal to improve definition are:

• It increases the resolution of the images in a very important way (222.2% in the case of 1280 columns × 720 lines, and 500% in the case of 1920 columns × 1080 rows, the latter known in the industry as full high definition (Full High Definition – FHD), providing much more image information.
• The aspect ratio is changed from 4/3 to 16/9, offering a more panoramic view close to the aspect ratio of the cinematographic signal.
• Both the use of interlaced signal as progressive signal, which makes it easier to start the first progressive signal emissions that can offer greater capacity in the representation of the movement of objects in the scene.
• In relation to the audio signal, the use of 5.1 multichannel systems, thereby offering a greater capacity to laterally locate sound sources, and to generate low-frequency, high-level sound effects, similar to what is offered in movie theaters.

Regarding video coding, the use of 8 or 10 bits/pixel In both cases, the colorimetric space is practically the same, and both systems make use of a similar gamma pre-correction and based on the response of the TRCs. Technological evolution and the arrangement of receivers in homes has meant that the SD video signal is usually compressed and transmitted with MPEG-2, and the HD signal with H.264. With both compressors, high signal quality can be achieved depending on the degree of compression (with higher or lower bit rate), therefore, the compression carried out should not be considered for definition purposes.

What UHD brings

However, the jump from HD to Ultra High Definition (Ultra High Definition – UHD) represents a broader set of elements that facilitate greater definition in the image:

• The resolution of the images is increased by multiples of FHD resolution: it is proposed to work with 4K signal (which involves twice as many rows and columns as the FHD signal, i.e. 3840 columns × 2160 lines), format also known as UHD-1; or with signal 8K (cuatro veces más filas y columnas que FHD, es decir, 7680 columnas × 4320 líneas), formato conocido como UHD-2, manteniendo siempre la relación de aspecto de 16/9. Con ello, se aporta mayor nivel de detalle en los objetos capturados en la escena.
• Se abandona, por completo, el uso de barrido entrelazado, usándose, exclusivamente, formato progresivo que permite una mejor descripción del movimiento de los objetos en la escena.
• Se contempla el uso de frecuencias de refresco de cuadro de imagen más altas HFR, lo cual, unido al formato progresivo, facilita que, con presentación en grandes pantallas y a distancias cortas de las misma, la percepción subjetiva de movimiento de los objetos se mejore.
• Se codifica con un 10 bit minimum, also allowing work at 12. This greater bit depth/pixel (called high bit/pixel depth – High Bit Depth – HBD) supposes a better ability to describe textures and smooth variation surfaces on the objects in the scene.
• It contemplates the use of high dynamic range (High Dynamic Range – HDR), trying to offer images with contrasts from light to dark that are more similar to what the human eye perceives in reality, and avoiding the problems known as banding when one does not work according to the Barten model. This aspect is facilitated by the possibility of current screens to offer greater contrasts.
  (with purer blacks) and higher peak brightness (above the traditional 150 nits of CRTs), even reaching several thousand nits.
• He colorimetric space defined by the recommendation for UHD increases that used by the traditional HD signal by 111.1%, which implies a greater ability to represent the original colors of the elements of the WCG scene.
• In relation to the audio signal, UHD brings with it multiple improvements, with a set of elements that have given rise to the concept of next-generation audio (Next Generation Audio – NGA) in the UHD signal, and which includes:
  • Facilitate the font location
  • three-dimensional sound, that is, not only with lateral panoramic control, but also vertical, causing a greater sensation of immersion in the scene.
  • Allow the optimal reproduction of three-dimensional sound fields with the speaker infrastructure available to the user.
  • Provide mechanisms to personalize the sound experience, allowing the selection of languages, control of levels between sound sources (dialogue versus ambient noise, for example) to facilitate intelligibility, selection of optional channels (such as, for example, audio-description), etc., all under the control that is facilitated in the production of the audiovisual material.

Improvements in favor of subjectivity

UHD nos ha traído, de momento, el descubrimiento de un problema del que no éramos muy conscientes hasta la fecha: hacer que los sensores mantengan su tamaño aún incluyendo una muy alta densidad de píxeles nos produce un problema de difracción en los bordes del diafragma de la cámara de captación, tal como también se refleja en el capítulo 3 del Libro Blanco de UHD Spain, lo cual plantea problemas a la hora de usar números-f altos (iris cerrado) en los bloques ópticos.

Con todo lo anterior, queda patente que las evoluciones de SD a HD, y especialmente de HD a UHD, siempre han tratado de incorporar varias herramientas para generarnos una mejor sensación subjetiva “natural vision” and, with it, a “greater fineness of detail in observing the scenes” on the screen. Increasing the resolution is one of them, very important, but the jump to UHD offers other very important elements as well, with a strong impact on subjective perception when consuming audiovisual content.

When my family and friends, who know that I move in this world, they ask me for advice on buying a new UHD screen, the truth is that I always have the same dilemma: How the hell do I explain to them, in two words, NOT to buy a screen that only has more pixels?

Jose Manuel Menendez

Professor of the Polytechnic University of Madrid y director of the RTVE Chair – UPM