From Lattice Semiconductor: Everything You Ever Needed to Know About Video Quality
Everything You Ever Needed to Know About Video Quality
When it comes to viewing a video, how do you decide which quality is best? Thanks to relentless marketing, most people would probably think that 8K is the way to go. After all, when it comes to video quality, the higher the resolution the better quality, right? Actually, that is not always the case. While resolution certainly does matter, video quality is actually the combination of a number of factors. Everything from the refresh rate of the screen to the compression applied, all will affect the final quality of the video.
At Lattice, we think about all of the factors that could impact video quality when we are creating our products. Our extensive selection of wired and wireless video solutions are some of the best on the market and have been designed to provide the optimum viewing experience. The 4K wireless video solution we introduced recently is a testament of Lattice’s commitment to innovate in video connectivity.
While we consider hundreds of factors when building the latest silicon, here are a few guiding principles that we think about when designing the best video connectivity solution. Understanding these principles is the key to unlocking the highest quality video experience.
Principle # 1:
Once quality is lost, it doesn’t come back
Your home theater is a chain of devices, each with its own maximum input and output video quality. The cables or wireless connections linking the devices also support their own maximum video quality. If any of those connections along the way do not support the video quality being output, the video quality is automatically scaled to the lowest supported format. This is called “downsampling.”
Sometimes you may also hear about “upsampling.” Upsampling tries to recover lost video quality by interpolating missing video data. Although viewing upsampled content might be better than viewing at the lower resolution, it may result in an artificial experience and is never as good as the original video. Upsampling can never give you back the quality you have lost.
Principle # 2:
Quality is more than just resolution
Video data is comprised of 4 main components.
- Resolution – The total number of pixels in one image. Often this is indicated by a shorthand such as 1080p or 4K. More resolution generally allows more image detail.
- Refresh Rate – The number of times that a pixel will refresh in a second. This is measured in hertz (Hz). So 60 Hz refreshes 60 times a second, and 120 Hz refreshes at 120 times a second. Video sequence that involves motion, such as sports, benefits from a higher refresh rate.
- Color Depth – The amount of information given about the color of each pixel. 8-bit color allows for 16.77 million colors to be shown, while 12-bit color allows for 68.7 billion distinct colors.
- Chroma Subsampling – This is a technique used to reduce the amount of color data by encoding less resolution for Chrominance (two color component) than Luminance (black and white). This is normally indicated with 3 numbers as 4:2:0, where the first number indicates the amount of luminance data, and the second and third number indicate the amount of chrominance data.
As a video image passes from one device to the next on its way to the screen, all four components matter to the final video quality. Often, the total bandwidth supported is a critical piece to determining what video quality is supported, and which video standard is used.
Principle # 3:
You can compress…carefully
Wireless video technologies always have lower bandwidth compared to wired technologies. To close that gap, wireless technologies use compression to maximize the use of available bandwidth. It is also used in wired video technologies such as Display Stream Compression (DSC) in DisplayPort and HDMI.
Lossless compression refers to video compression that does not lose any video data in the process. This is excellent if you want to maintain the full quality of the original source material. All video editing for example, should be done in a lossless environment. However, it does little to help wireless video transmission because the bandwidth reduction is small.
Visually lossless compression does lose some data, but because the visual difference falls below the threshold of human perception, we cannot tell it apart from the original. This is a fine solution for transmitting a video image to its final source, either over the internet, or from a Blu-ray player to the DTV. Example includes H.264 and DSC.
Lossy compression refers to compression that does lose significant visual data in a way that can be observed by human eyes. This can result in a greatly reduced bandwidth requirement for the video image. A well-known format is JPEG.
One more thing to think about when considering compression is how much latency it adds. The higher the latency, the greater the delay between the video source and display. Latency is not so much of an issue if you are watching a movie as long as there is no intermittent buffering, but can be irritating if you are trying to navigate a menu on screen or play a video game where response time defines the user experience.
Principle # 4:
It’s all about the standards
Understanding the complexity of video quality explains why standards are so critical in the video space. Video standards do two main things – they help define the set of video formats that can be supported by a given standard. When two devices support different maximum video qualities, standards define methods for devices to gracefully negotiate down to the best common format possible.
So where does this leave you? First, before you buy a new high-end component for your home theater system, make sure you audit your existing components to understand how they are going to affect your video quality. Second, remember to look beyond resolution. If you are considering using a wireless technology, make sure you know how much it is compressing and what that effect will be on quality and latency. Finally, start looking into different video standards that are scalable to take in more complex concepts in video connectivity in the future.
Video Technology Comparison Table
To help you understand the video quality you can expect from a given standard, we’ve created the following chart as a handy reference guide. The table is not meant to be an exhaustive list, but should serve as an entry point for those interested in the relationship between video quality and bandwidth. Video bandwidth is calculated based on the active pixel area (those that your eyes can see) only. Enjoy!
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