When 4K TVs replaced HD TVs in your home, manufacturers revealed some interesting terms like “Ultra HD upscaling”. But upscale is not a unique feature, it only allows 4K TVs to work with lower resolution video formats like 1080p and 720p.
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All TVs can be upscale
Upscale makes low-resolution content that can fill the entire TV screen. Without it, a low-resolution video will take up less than half the screen space. This is a typical feature on all TVs. Even 1080p TVs are available, they can upscale 720p content and display it in full screen mode on 1080p screens.
Upscale UHD will make 4K TV work like any other TV. It can display lower resolution content on the entire 4K screen.
Upscale 1080p content on a 4K screen looks better when projecting 1080p content on a normal 1080p screen. But upscale is not “magic”, you cannot get sharp images like real 4K content.
Compare physical resolution and optical resolution
Before going into more upscale, we need to understand the concept of image resolution. Pictures or videos with high resolution will look better than pictures or videos with lower resolution.
However, we often forget some important factors are mainly the difference between physical resolution and optical resolution. These elements work together to create a good image and they are the basis for understanding upscale.
- Physical resolution: On a technical specification table of the TV, the physical resolution is simply called the resolution. This is the number of pixels on the screen. 4K TVs have more pixels than 1080p TVs and 4K images are 4x the size of 1080p images. All 4K monitors, regardless of their size, contain the same number of pixels. Although high-resolution TVs can use extra pixels to provide additional details, they don’t always work that way.
- Optical resolution: This is why older disposable camera photos look better than digital camera photos. When an image looks sharp and has a wide dynamic range, it will have a high optical resolution. Sometimes TV “wastes” its high physical resolution when displaying videos with low optical resolution. This makes the image blurry and the contrast unclear.
- Pixel density: The number of pixels per inch on the screen. All 4K displays contain the same number of pixels, but on smaller 4K screens, the pixels are closer together, so they have a higher density. For example, an iPhone 4K has a higher pixel density than a 70-inch 4K TV. Screen size is not the same as physical resolution and pixel density. The screen’s pixel density does not determine its physical resolution.
Now that we’ve all seen the difference between physical and optical resolutions, it’s time to explore the process of upscale TV.
Upscale makes the image bigger
All TVs contain a bunch of interpolation algorithms, which are used to upscale low-resolution images. These algorithms add pixels to an image to increase their resolution. But why do you need to increase image resolution?
One thing to remember is that the physical resolution is determined by the number of pixels on the screen. It is not related to the actual size of the TV. A 1080p TV screen consists of 2,073,600 pixels while a 4K screen has 8,294,400 pixels. If you display 1080p video on a 4K TV without performing upscale, that video will only take up a quarter of the screen.
For 1080p images to fit a 4K screen, it needs to reach 6 million pixels through the upscale process (at this point, it will become a 4K image). However, the upscale depends on a process called interpolation.
Upscale reduces optical resolution
There are several ways to interpolate an image. The most basic way is called interpolation of the nearest pixels (Nearest Neighbor Interpolation). To do this, the algorithm adds an empty pixel grid to the image and then guesses the color value of each of those empty pixels by looking at the four adjacent pixels.
For example, a blank pixel surrounded by white pixels will turn white, while an empty pixel surrounded by white and blue pixels will become light blue. This is a simple process but leaves a lot of blurred and rugged digital traces in the image. More easily understood, the interpolated image has poor optical resolution.
Left: The image has not been edited. Right: Image after using the nearest pixel interpolation method.
Compare the two images above: one unmodified on the left and one on the right as a victim of the interpolation of the nearest pixels. The image on the right does not look clear although it has the same physical resolution as the left. This happens when a 4K TV uses the method of interpolating the nearest pixels to upscale images.
Many people will wonder why their 4K TV does not occur this phenomenon? Yes, the answer is because 4K TV not only uses this method but it also works with other interpolation methods to upscale images.
Upscale solves the problem of optical resolution
The method of interpolating the nearest pixels is a powerful method to increase image resolution without taking into account the optical resolution. That is why the TV uses two different forms of interpolation together with the nearest pixel interpolation. These are bicubic interpolation and bilinear interpolation.
Left: An example of a bilinear interpolation. Right: An example of duplex interpolation.
With the duplex interpolation method, each pixel added to the image will look at 16 neighboring pixels to get color. This makes the image look softer. On the other hand, linear parallel interpolation only looks at the two closest pixels and produces a sharp image. By combining these methods and applying a number of filters to the contrast and color modes, your TV can create images that do not degrade optical quality or have them but are negligible.
However, even using proper interpolation, some videos may add “ghosts” after being upscale, especially if your low-cost TV crashes when upscale. The extra details also become clearer when super low-quality images (under 720p) are upscale to 4K resolution or images are upscale on extremely large TVs with low pixel density.
Although all TVs offer an upscale feature, the use of different algorithms results in different picture quality.