视频 (Video) - 属性 (Properties) - 比特率 (Bitrate)

Bitrate is the number of bits per second. The symbol is bit/s. It generally determines the size and quality of video and audio files: the higher the bitrate, the better the quality and the larger the file size because File size = bitrate (kilobits per second) x duration. In most cases, 1 byte per second (1 B/s) corresponds to 8 bit/s.
它通常决定视频和音频文件的大小和质量：比特率越高，质量越好，文件大小也越大，因为 File size = bitrate (kilobits per second) x duration。在大多数情况下，1 byte per second (1 B/s) 对应于 8 bit/s。
码率影响视频文件的大小。码率越大，视频文件越大；码率越小，视频文件越小。动态编码方式，把较高的采样率用于复杂的动态画面，而把较低的采样率用于静态画面，合理利用资源，达到画质与文件大小可兼得的效果。

码率 (kbps) = 文件大小 (MB) / 时间 (秒)

Video bitrate affects video quality in several ways. First, it is the key measure of any video file size. Secondly, high video bitrate results in high video quality and low bitrates result in poor video quality. However, using extreme high bitrate is just waste bandwidth.
视频比特率以多种方式影响视频质量。首先，它是衡量任何视频文件大小的关键指标。其次，高视频比特率导致高视频质量，而低比特率导致差的视频质量。但是，使用极高的比特率只是浪费带宽。

帧率与画面的流畅度成正比。帧率越大，画面越流畅；帧率越小，画面有卡顿感。可变码率编码中，帧率也会影响文件的大小，帧率越高，每秒钟采集的画面越多，需要的码率越高，体积也越大。画面的帧率高于 16 时，人眼就会认为是连贯的，当帧率达到一定数值后，再增长的话，人眼也不容易察觉到有明显的流畅度提升了。

如果限定码率，那么帧率越高，编码器就必须加大对单帧画面的压缩比，就是通过降低画质来承载足够多的帧数。一般 fps = 20 就已经可以达到很好的用户体验了。

In general, a higher bitrate will accommodate higher image quality in the video output, only when comparing the same video with the same resolution. Bitrates should be expected to go up whenever the resolution goes up, as more data is being processed.
通常，只有在比较具有相同分辨率的相同视频时，更高的比特率才能在视频输出中容纳更高的图像质量。随着分辨率的提高，随着处理更多数据，比特率应会上升。

Therefore, high video bitrate may provide excellent quality, but it can also place a major strain on your hardware which can result in stutters.
因此，高视频比特率可以提供出色的质量，但同时也可能给您的硬件造成很大的压力，从而导致卡顿。

A bit is defined as the smallest amount of data on a computer. And bitrate is the rate (per second) at which bits are transmitted to an output. So, the video bitrate calculator refers to the software used to determine the amount of data processed in a video clipper specified time.
位定义为计算机上最小的数据量。比特率是将比特传输到输出的速率 (每秒)。因此，视频比特率计算器是指用于确定视频剪辑器指定时间中处理的数据量的软件。

For your videos to stream steadily, the video resolutions have to match with the right video bitrate.
为了使视频稳定地流传输，视频分辨率必须与正确的视频比特率匹配。

• For full HD videos with a standard resolution, set bitrate between 3,500 to 5,000 kbps.
• For regular HD videos with a standard resolution, set bitrate between 2,500 to 4,000 kbps.
• For full HD videos with high resolution, set bitrate between 4,500 to 6,000 kbps.
• For regular HD videos with high resolution, set bitrate between 3,500 to 5,000 kbps.

Another consideration is that variable bitrate (VBR) encoding will produce better image quality than constant bitrate (CBR) in most applications.
另一个考虑因素是，在大多数应用中，可变比特率 (VBR) 编码将比恒定比特率 (CBR) 产生更好的图像质量。

A video bitrate is the number of bits that are processed in a unit of time.Remember that 1 byte consists of 8 bits. Video data rates are given in bits per second. The data rate for a video file is the bitrate. So a data rate specification for video content that runs at 1 megabyte per second would be given as a bitrate of 8 megabits per second (8 mbps). The bitrate for an HD Blu-ray video is typically in the range of 20 mbps, standard-definition DVD is usually 6 mbps, high-quality web video often runs at about 2 mbps, and video for phones is typically given in the kilobits (kbps).
视频比特率是单位时间内处理的位数，请记住 1 字节由 8 位组成。视频数据速率以每秒比特数给出。视频文件的数据速率是比特率。因此，将以 1 megabyte per second 的速度运行的视频内容的数据速率规范指定为 8 megabits per second (8 mbps)。高清蓝光视频的比特率通常在 20 mbps 的范围内，标准清晰度 DVD 通常在 6 mbps 的范围内，高质量的网络视频通常以 2 mbps 的速度运行，而电话视频通常以千位为单位 (kbps)。

HD 720p @ H.264 high profile 2500 kbps (20 MB/minute)
HD 1080p @ H.264 high profile 5000 kbps (35 MB/minute)

0. OBS (Open Broadcasting Software)

OBS is free software that allows video-makers to capture and record videos as well as live streaming. Video viewers usually choose video resolutions according to their device’s compatibilities and data affordability.
OBS 是免费软件，可让视频制作者捕获和记录视频以及实时流媒体。视频观众通常根据设备的兼容性和数据承受能力来选择视频分辨率。

1. 属性 (Properties) -> 视频 (Video)

尺寸 (Dimensions): 1920 x 1080
编解码器 (Codec): H.264 (High Profile)
帧率 (Framerate): 15 帧/秒 (frames per second)
比特率 (Bitrate): 3205 kbps

2. VLC -> 工具 (Tools) -> 媒体信息 (Media Information) -> 统计 (Statistics)

Bit rate：码率
Maximum bit rate：最大码率
bits per second，bps
kilobits per second，Kbps
megabits per second，Mbps

3. I 帧和 P 帧

在视频压缩过程中，I 帧是帧内图像数据压缩，I 帧是独立帧。P 帧则是参考 I 帧进行帧间图像数据压缩，P 帧不是独立帧。在压缩后的视频中绝大多数都是 P 帧，视频质量主要由 P 帧表现出来。由于 P 帧不是独立帧，只是保存了与邻近的 I 帧的差值，实际上并不存在分辨率的概念，应该看成一个二进制差值序列。而该二进制序列在使用熵编码压缩技术时会使用量化参数进行有损压缩，视频的质量直接由量化参数决定，而量化参数会直接影响到压缩比和码率。

分辨率的变化称为重新采样。由高分辨率变成低分辨率称为下采样，由于采样前数据充足，只需要尽量保留更多的信息量，一般可以获得相对较好的结果。由低分辨率变成高分辨率称为上采样，由于需要插值等方法来补充缺少的像素点，会带有失真。

4. GOP (group of pictures)

在视频编码序列中，GOP (group of pictures，图像组) 指两个 I 帧之间的距离。

References

Bitrate Viewer
http://www.digital-digest.com/software/Bitrate_Viewer.html