VBR and CBR are most common video stream bitrate control technologies. Generally, VBR can retain the best image quality during video streaming, thus it’s better than CBR. However, if you want to transmit video stream over constrained network bandwidth or want to reduce the storage, you may opt for CBR. CBR is most commonly used for streaming video content using the Flash Media Server (RTMP). Lastly, VBR is the default method for bitrate controlling of the IP cameras. in addition to VBR and CBR, today some IP cameras can support CVBR and AVBR. AVBR stands for adaptive variable bitrate, and CVBR stands for constrained variable bitrate. They are better bitrate technology over general VBR and CBR.
Figure 1 illustrate the PSNR, bitrate and file size performance for the CVBR compared against other rate control algorithms including VBR, AVBR and CBR. In this example, a video sequence comprising of segments with varying motion intensity is encoded using these algorithms. While encoding a low motion segment like at the beginning of the video, the CVBR naturally achieves low bitrate similar to that of VBR and AVBR, and hence saved some bits compared with CBR. When encoding a segment with intensive motion like at the end of the sequence, the CVBR would use some of the previously saved bit budget to allow instant bitrate go reasonably higher than the target average bitrate while still meeting the constraint of the overall bitrate or filesize. Therefore, compared with other rate control algorithms, the CVBR can reduce video quality fluctuation and hence achieve better video quality for the highly intensive motion segments. Meanwhile, it is worth noting that average bitrate from the CVBR is often lower than the target long-term bitrate which is the one from the CBR.
Constrained Variable Bit Rate (CVBR) is a rate control algorithm that minimizes the video quality fluctuation while conforming the target average bitrate and several instant bitrate constraints. In particular, these instant peak bitrate constraint and long-term average bitrate constraint guarantee the encoded bitstreams to be transmitted through a channel with a given bandwidth and/or stored in a device with specified volume or write speed. Under these constraints, an underlying Variable Bit Rate (VBR) control mechanism is employed to maximize picture quality over varying contents.