Bitrate is a measurement of the amount of data used to encode a single second of video. It mainly affects the size and quality of your video. Bitrate is measured in bits per second (bps). While kilobits per second is used for measuring audio files, the same value isn’t used for video files as they are significantly larger in size. Video files are measured using megabits per second.

Two important terms that are often confused are Mbps and MBps. Megabits (Mbps) per second refers to the uploading and downloading speed, while megabytes (MBps) per second defines the amount of info that is transferred.

A higher bitrate improves the quality of a video, while lower bitrate results in a lower quality. Size of a 1 hour 1500 kbps video will be 1500 * 60 * 60 kilobits = 1500 * 60 * 60 / (1000 * 8) MBs of data = 675 MB per hour of video data.

Video Quality

Bitrate, Resolution and Frame rate impact the video quality.

  • Resolution – Video resolution refers to the number of picture elements (pixels) spread across a display unit. A 1080p video means – the height of video is 1080 pixels. If the aspect ratio of video is 16:9, the width of video becomes 1080*16/9 – 1920 pixels. So, in common convention 1080p means 1080 pixels as the height of video and 1920 as the width of video. A higher concentration of pixels leads to higher visual quality.
  • Frame Rate – A high frame rate is also important when attempting to reach a desirable video quality for one’s stream. A high frame rate yields smooth playback.

Higher resolution yields a clearer picture. On the other hand, higher bitrate yields smoother playback. Because of the lower resolution, a lower bitrate can usually still provide a desirable resolution.

Even with the same resolution, optimum bitrate might vary depending on the video frame rate required for the video. For video lectures with about 30fps it can go up to 2000kbps, while with video content with a higher framerate like 60fps or more it can go up to 5000kbps.

Bitrate Modes

Commonly used video encoder bitrate modes are

  • Constant bitrate (CBR) encoding maintains a set bitrate over the entire video clip, but limits the image quality in most cases—especially for complex video segments. CBR is often not an optimal choice for streaming since it does not allocate enough data for the complex segments. This results in lower quality overall.

    In CBR encoding, the maximum bit rate is capped to a user-specified value. Encoder dynamically adapts the bit rate of each frame according to previously coded frames and limits the variations of the bit rate in subsequent frames so that perceptible disruptions in the video quality are minimized. However, in some situations, CBR encoding may result in temporary degradation in the video quality. For instance, frames with a large size (due to a scene change or high motion) may be encoded using a higher QP value in order to maintain a constant coded bit rate at the output.
  • Variable bitrate(VBR) typically produce significantly higher quality at similar bitrates. The primary benefit of VBR encoding is that it allocates a higher bitrate to the more complex segments of media files and lower bitrates to the simple segments. Adding up the bitrates and dividing by the duration (in seconds) gives the average bitrate for the file.

    VBR encoding is an open loop scheme that maintains the video quality at a constant level because each video frame is coded using a fixed QP. This simplifies the codec design, and minimizes encoding and decoding time. Because there are no rate caps, the video bit rate can be bursty and content dependent. For example, scene changes or high motion tend to generate more bits per frame than video conferencing applications.

Variable Bitrate

VBR works as follows

  • A VBR system does a first pass of the video file. During this, it analyzes the complexity of the video file.
  • The VBR system then categorizes the segments of the video file by their complexity.
  • VBR systems transmit data at a variable rate.
  • They use one bitrate for the complex segments of the video file, and they use another bitrate for the simpler segments.
  • Since they adjust bitrates based on the complexity, VBR systems produce a better quality of videos.
  • VBR encoding systems produce output data at different rates for different segments.

VBR systems make two passes in this process. Naturally, this process takes longer. This is a key difference with CBR systems. It is not suitable for live streaming of video, you can’t wait for VBR systems to make two passes since the encoding must be in real-time.

Coding efficiency, video content, frame rate, and quantization all impact the video bit rate. If coarse quantization is used, the quantization parameter (QP) value becomes larger. A larger QP value leads to a higher degree of lossy encoding, which reduces video quality and bandwidth requirements. Many sports programs typically require higher frame rates to capture fast movements and this further increases the video bit rate. For example, a 60 Hz video requires twice the bit rate of the same video with a frame rate of 30 Hz.