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Beyond the Resolution: How to Achieve 4K Standards

12 Jun 2017

The following article is inspired by the training delivered by Adriano D’Alessio of the Lightware—a leading manufacturer of DVI, HDMI, and DisplayPort matrix switchers, signal extenders, and accessories for the professional AV market based in Hungary.

Screen resolution is one of the primary display characteristics both professionals and consumers are after when making a purchase decision. Surprisingly, most overlook or sometimes are unaware that the panel resolution is just one part of the equation. Whether your 4K UHD screen is going to outshine your neighbor’s standard Full HD TV will depend on a range of factors. The whole system—panel, connecting standards, switches, media players, and other accessories— needs to meet a set of requirements to deliver the required quality of the output. 

In this article, we cut through the noise of numbers surrounding resolutions to help you:

  • understand the requirements for building true 4K resolution systems;
  • choose the right solution for a professional AV installation;
  • know the difference between various resolutions;
  • provide clarity around the bandwidth requirements and interface options for reproducing higher video resolutions and frame rates.

Resolutions, defined

Resolution is the number of pixels that comprise the image on the screen in each dimension.

  • High definition (HD) is a term used to describe an image resolution that contains 1,280 lines of pixels along the horizontal axis and 720 lines of pixels along the vertical axis. 
  • Full HD (or FHD) is defined as 1,920 by 1,080 pixels.
  • Ultra-high definition (UHD) is the term introduced by the Consumer Electronics Association to describe image resolutions of at least 3,840 by 2,160 pixels. This standard is often referred to as a ‘consumer 4K’, or 4K UHD by TV manufacturers as they must maintain 16:9 aspect ratio.

True 4K resolution is defined by the Digital Cinema Initiative and has 4,096 by 2,160 pixels format. While UHD is not the standard 4K resolution, 4K resolution is UHD. The highest UHD resolution is 8K, defined by 7,680 by 4,320 pixels image dimensions.

Screen Resolutions Compared

Viewing angle and distance

The perception of image quality is affected by the distance of the observer from the screen. This is because the human eye can see about 60 pixels per 1-degree of viewing angle. Below this limit, we can detect the individual pixels, and the image will appear grainy. Above that count, we are unable to distinguish separate pixels. The higher the resolution, the sharper the picture quality.

Pixel Pitch and Distance from the Screen

For example, a display with 4K resolution has significantly higher pixel density than a screen of the same size in FHD. As 4K panels pack substantially more pixels in a 1-degree angle of view, we can get closer to the display without being able to see the separate pixels even with a larger screen size.

4K is currently the most sought after standard in professional AV industry, digital cinema and post-production, broadcast, and consumer electronics.

Recommended Viewing Distance per Screen Size

The system approach to delivering 4K

Given the ever-increasing resolution of displays, the screens now need substantially more data delivered considerably faster in order to output visual images at their full capacity. As video content frame rates increase, the bandwidth required to transfer data to a device increases too.

When it comes to understanding the bandwidth requirements and infrastructure needs, resolution is an important, but not the only parameter that drives the trade-offs and design considerations.

Let’s define the bandwidth and the components that determine it.

Video Processing for Different Resolutions

The bandwidth requirements are based on the resolution, frame rate, chroma subsampling, and bit depth.

Frame rate

Frame rate is the frequency, at which a processing unit produces unique sequential images (referred to as frames) to achieve the illusion of natural motion. It is measured in frames per second (FPS).

Refresh rate is the number of times per second an image on the screen is re-drawn and is a characteristic of the display, expressed in hertz (Hz).

The greater the frame rate, the smoother the video motion appears (up to a point). Refresh rate sets a cap as to how many frames a screen can effectively display. Modern display solutions support 30Hz, 60Hz, and even 120Hz for some UHD models.

Chroma subsampling

There are two types of cells in the human eye—cone cells and rod cells. Cone cells are responsible for our trichromatic color vision, and they work best in bright environments. Rod cells are in charge of brightness sensing and are utilized in dark viewing conditions. As we have more rod cells, human visual system is more sensitive to luminance than the difference in color.

We have discussed the concept of color perception and color spaces in detail in our Color and Displays whitepaper. From the bandwidth requirement perspective, it’s important to understand how chroma subsampling works.

In video systems, the chroma component represents the color information, while brightness is described by the luma component.  As humans have sharper perception to luminance, video systems designers introduced chroma subsampling, allowing to devote more bandwidth to luma component, while transmitting chromatic information at a lower resolution, maximizing perceived image quality under specific bandwidth requirements.

Chroma Subsampling Explained

A variety of sampling methods are used in the professional AV industry:

  • 4:4:4—luminance and chrominance components are sampled for each pixel, so no subsampling occurs. This scheme is equivalent to R’G’B color scheme and amounts to the highest bandwidth value.
  • 4:2:2—two neighboring pixels have the same chrominance samples, but each pixel has its own luminance sample. This scheme reduces the bandwidth of the video signal by one-third with low visual performance impact.
  • 4:2:0—four neighboring pixels have the same chrominance samples. The speed of the chroma channels is reduced to 25%, resulting in 50% of bandwidth sampling.
  • 4:1:1—four pixels have the same chrominance samples, but four in a row. Allows to half the bandwidth as well.

Subsampling Methods

Sampling Methods for Pro-Av Industry

Image Quality Comparison with Different Sampling Methods

4K interface options

4K standard uses 4:2:0 subsampling to reach 60 frames per second but to still fit within the 10 gigabits per second (Gb/s) limit of today’s commonly available interfaces.

Need for Updated Signage Infrastructure

Most common interface options include the following:

  • HDBaseT is a global connectivity standard for transmission of uncompressed high-definition video and audio, Ethernet, controls, and power over a single standard long-distance cable. It can support 4K at 30Hz, but not 60Hz.
  • HDMI (stands for High-Definition Multimedia Interface) is a standard for creating high-bandwidth connections between digital devices. This proprietary digital interface is suitable for transmitting uncompressed video and audio, but requires device compatibility.
  • DisplayPort is a digital display interface for connecting video, audio, USB, and other data sources to a display. Can deliver the highest possible resolution at a full 60Hz frame rate.

Interface Options

Interfaces of the past—limitations

Bandwidth Limitations per Interface

HDMI 1.4 and DisplayPort 1.1 have a limitation of 8 GB/s, so they can carry 4K at 30 Hz.

The only two interfaces that can transmit a 4K 60Hz signal over one cable are DisplayPort 1.2 and HDMI 2.0.

While we will not be covering the evolution of these common interfaces in detail, it’s useful to outline their past limitations as it pertains to 4K bandwidth requirements.

  • Single cable interfaces, such as HDBaseT and HDMI 1.4 can only carry 4K at 30Hz
  • Dual cable interfaces, for example, dual HDMI 1.4, could transmit 4K at 30Hz, 4:2:2 schema, 30 bit
  • Quad-cable interfaces, for example, quad HDMI 1.3, 4K at 60Hz, 4:4:4 schema, 24 bit

Single-cable interfaces of today

Modern interfaces, such as HDMI 2.0 and DisplayPort 1.2 can transmit 4K 60Hz signal over a single cable:

  • DisplayPort 1.2 can carry 4K at 60Hz, 4:4:4 schema, 30 bit
  • HDMI2.0 can carry 4K at 60Hz, 4:4:4 or 4:2:2 schemas, 30 or 36 bit

We hope you find this information useful when configuring your systems and making business decisions about the infrastructure to achieve 4K standards. If you have other questions regarding signage, please let us know here.