Zareen Tasnim Khaled | June 27, 2019 | 505 views | 4 People said helpful.
Video quality is a subjective terminology- it’s from the perspective of the end-user. To what extent a particular video meets the viewer’s expectation is Video Quality. It is nothing but one of the features of a video being transmitted in a system.
Even if different observers watch the same video in the same condition, videos will be scrutinized in contrasting manners because here comes the question of expectation versus being accustomed to. Video quality can be assessed either objectively by mathematics or subjectively by human observers and their involvement by classification or ranking. Today, video quality is essential to many content providers, service providers, and network operators.
Few key concepts to familiarize ourselves with the video quality parameters and finding possible answers to the aforementioned questions are:
A pixel is the minuscule fragment of an image or display that a computer can print or display. If you zoom in an image, it will appear like a mosaic pattern by small tiles, and this is a pixel. This concept of the pixel is true in digital imaging so a pixel (also known as Picture Element or Pel) is the smallest addressable element of a picture. A display with a resolution of 1280 x 768 will yield a maximum of 98,3040 pixels on the screen. As we know, an atom is the smallest unit of matter, in the same way, a pixel is the smallest unit of an image.
Now a question may arise whether or not pixel corresponds directly to size.
Surprisingly there is no fixed answer to that. If pixel quality is fixed, then the number of pixels in a video is fixed.
E.g a 720p will have 720*1080 = 777600 pixels. But how much of data is captured in each pixel can vary and is determined by the streaming provider. Thus, Different Video Provider can provide different size of videos for the same length and same pixel quality. It is even possible that say 1 hour, 1080p video is lower size than 720p of Video. To think of what determines the video size, the correct term is bitrate. It usually represents by kbps (like 1000 kbps). We will be discussing bitrate, another quality index, in the latter part of this article.
Now since we have defined a pixel, let’s talk about resolution.
Resolution is the number of distinct pixels in an image. A high-resolution image will have more dots, which allows it to display more detail. Resolution generally refers to monitors, printers, and bit-mapped graphic images. It is usually denoted as width × height and the units in pixels. For instance, “1024 × 768″, 1024 indicates the width and 768 indicates the height that the display could be determined in pixels. The pixel resolution determines the quality-more pixels per inch of the monitor screen gives better image results.
Our monitor has a resolution, as well as dot-matrix and laser printers where the resolution denotes the number of dots per inch. A digital camera has a resolution too, quoted as the number of megapixels (millions of pixels it can capture and save as image). A higher megapixel count in a digital camera will mean that larger pictures can be taken and printed.
The common numbers we see are 720p, 1080p and 2160p, 4K or 8K. The “p” and “i” denotes whether it is a progressive-scan or interlaced-scan display.
(We will discuss more in the last part of this article). Here is a quick overview of the common resolution.
A video bitrate is the number of bits in a unit of time. We know that 1 byte is 8 bits. Video data rates are expressed in bits per second (bit/s). Bitrate determines the size and quality of the video. It is proportionate which means that the higher the bitrate, the better the quality. This also portrays the larger the file size because the file size is equal to bitrate (kilobits per second) multiplied with duration.
Bitrate is ‘data rate’ for a video file. A data rate specification for video content that goes at 1 megabyte per second will have a bitrate of 8 megabits per second (8 Mbps).
As intended to discuss earlier, let’s see how bitrate is linked to the size of a video. Bitrate, represented by kbps where kb is kilobytes of data per second. Hence, the size of a 1 hour, 1200 kbps video will be:
= 1200*60*60/8000 MBs of data
= 540 MB per hour of video data.
In the same way, 1 hour 1000 kbps video will be 450 MB in size whereas 600 kbps will be 270 MB in size.
No, pixels and bitrate are not technically related in any way. The higher the pixels, the higher the bitrate, provided that the streaming provider is same and vice-versa. However, it is to be noted that different service providers may offer different pixels at the same bitrate.
The two types of bit rate encoding are:
1. Constant Bit Rate: CBR is an encoding method that keeps a fixed bit rate throughout the whole video, for easy playback and quick loading. Its perfect use is in cloud-based streaming services where the video is continuously being downloaded and sometimes lags with sharp increase in bit rate. But on the downside, CBR produces a bigger file size, so it hampers some output formats.
2. Variable Bit Rate: VBR is an encoding method that allows the bitrate of a video file to dynamically change, that is, increase or decrease. It’s based on the extent of details required in a given moment of motion. Although it’s less invariably easy to playback, it produces a remarkable higher video quality than CBR with much lower file size. VBR only manipulates its bit rate when required and used mostly for progressive and direct downloads.
We have now come upon another vital parameter related to video quality.
A video is, in fact, thousands of individual images in a sequence and such individual images are frames. The frequency of those independent frames appearing consecutively on the screen in a given time period is the frame rate.
When only a few frames per second are captured and displayed, some details of movements are missed. So the video will be choppy. For a smoother action, there should be a higher frame rate.
Frame rate is measured by frames per second (FPS). The three main FPS standards are 24p, 25p and 30p (Here “p” is frame progressive). For example, at 25 fps, 25 distinct images would appear consecutively within one second. If the fps is too low, the movement will look broken.
Each image represents a frame and frame rate is the speed at which those images are shown.
As we have seen, video pixel, resolution, bitrate and framerate have their own individual significance when it comes to video quality. There are, however, other parameters as well related to video quality like encoding, decoding, compression, file format, interlaced, progressive, etc. which we will discuss in other topics.