Among the critical parameters that impact video quality, "bitrate" is undoubtedly a factor that cannot be overlooked. We all know that resolution represents the pixel value of an image, and frame rate affects the smoothness of a video. So, what does bitrate signify?

Bitrate, also known as Data Rate or Bitstream, refers to the number of data bits transmitted per unit of time, typically measured in bps (bits per second).

In the realm of computers, a bit (binary digit) is the smallest unit of measurement for information. If all data were simplified into 0s and 1s, one bit would represent either a "0" or a "1." Bitrate, or bitrate, signifies how many of these "0s" and "1s" a video file transfers within a given time period (usually a second). Therefore, a higher bitrate translates to more data transmitted per second, resulting in higher video precision but also larger file sizes.

However, this is merely a conceptual explanation. How does varying bitrate manifest in the viewing experience? Let's first examine two sets of video screenshots:

For the same video scene, when the resolution is 720p, the picture appears clear and smooth. However, upon upgrading to 1080p, the video becomes visibly blurry. The culprit here is bitrate. As resolution increases, so does the amount of data required to render the image. If the bitrate remains unchanged, there will be insufficient image data transmitted, leading to blurry and distorted visuals.

Yet, a higher bitrate is not always better. It necessitates higher bandwidth during transmission, resulting in larger files and increased resource consumption. Moreover, high bitrate videos require playback devices with robust decoding capabilities; otherwise, playback may become choppy. This is why high-resolution videos captured by professional cameras often struggle to play smoothly on office computers due to the limitations of their built-in players in handling such high bitstreams.

Finding the optimal bitrate—achieving minimal distortion with the lowest possible bitrate—is an ongoing pursuit in video encoding. Two primary methods exist: Constant Bitrate (CBR) and Variable Bitrate (VBR). CBR, as the name suggests, compresses the entire video using a consistent bitrate, offering simplicity, shorter encoding times, and stable video streams on streaming platforms.

However, since moving scenes consume more storage than static ones, a racing sequence and a presentation against a whiteboard require different amounts of data. This disparity justifies assigning varying bitrates, which is where VBR comes in. VBR employs three parameters: maximum bitrate, average bitrate, and minimum bitrate. During compression, the computer analyzes different frames, adjusting the bitrate higher for scenes with more movement and lower for static scenes, thus preserving video quality.

While VBR offers greater flexibility, it also complicates calculations, lengthens compression times, and imposes higher demands on decoders. Both CBR and VBR have their merits, and the choice between them ultimately depends on specific circumstances.