Technical document

What is the difference between CBR and VBR?

Posted on by CCTVAPP.NET
05
Oct

In the surveillance camera system, especially the IP camera system, the settings on image compression standards, network bandwidth control mode, etc. are extremely important, directly affecting the image quality. camera and storage time of the whole system.

This article presents VBR, CBR, Smart Codec concepts commonly found in the setup menu of IP cameras or DVRs.

1. Difference between VBR and CBR?

  • VBR: Variable Bitrate = bandwidth that changes according to the density of motion in the camera’s field of view in the direction of maintaining the same image quality.
  • CBR: Constant Bitrate = constant bandwidth fixed
  • In each camera, it is possible to choose whether to use VBR or CBR mode. With VBR, the set bandwidth value is “maximum limit”, while with CBR, the set bandwidth value does not change all the time.
  • The setting of the bandwidth directly affects the image quality and recording capacity. Set too low, the image is blurred, set too high, the recording capacity is large.
  • With CBR mode, because the bandwidth is always fixed, it is easy to estimate the recording capacity. For example, 1 camera, set bandwidth = 2 Mbps(2 Megabit / sec) (2048 Kbps = 2048 Kilobit / sec), the capacity of continuous recording in 1 day is: 24 (hours) x 60 ( minutes) x 60 (seconds) x 2 (Mbps) / 8 (1 byte = 8 bits) = 21,600 (Megabytes) ~ 21.6 GB
  • With VBR mode, because bandwidth is the value maximum, only when there is a lot of movement, the camera will increase the bandwidth to that maximum value, so the recording capacity cannot be calculated correctly. However, based on motion density estimation it is also possible to calculate. For example, with the same camera as above, installed at a place with a movement density of 70% (70% of the day there are people, vehicles, etc.), the estimated recording capacity is:  21.6 x 0.7 = 15.1 GB
  • The above calculations are true for all camera types, all compression standards (H264, H265,…), all resolutions, frame rate,…

2. Set the maximum bandwidth value as appropriate ?

  • Usually when the camera is shipped, the manufacturer has set the default parameters for the best image quality. However, depending on the actual use such as: viewport scene, requirements for storage days, resolution, frame rate, compression standard, number of accesss, network viewing requirements (as well as response requirements) response of the internet connection),… which can reset the bandwidth parameters accordingly.
  • The following table recommends setting bandwidth values ​​for resolution cameras from CIF ( 320×240) to FullHD (1920×1080)

3. Compare the recording capacity between VBR and CBR modes

  • On the same camera, H264 compression standard, resolution 2688 x 1520, frame rate 25 fps, maximum bandwidth/bandwidth 8192 Kbps, room observation, recording in time 2 minutes

4. Smart  What is the code?

  • Currently, camera manufacturers, in addition to introducing cameras using the H264 compression standard, H265 also introduces H264+, H265+, H264/H265 Smart Coding (Panasonic), Smart Codec ( DAHUA, HIKVISION), Zipstream (Axis), Wisestream (Hanwa Techwin), Smart Stream II (Vivotek),…
  • These compression standards are compression standards that have been improved by manufacturers compared to other standards. standard compression H264, H265, based on a few principles such as:
    • Apply different levels of compression to each region of the image (area with motion is less compressed, a region with no motion is much compression)
    • Increase (decrease frequency) appearance time of I-FRAME (complete frame, containing enough information, only when there is movement, will it be included in I-Frame, when there is no motion takes time to appear I-Frame)
    • Denoise
    • …
  • With applied compression standards Using Smart Codecs, storage capacity is reduced (through bandwidth reduction) by 40-70%.
  • Smart Codecs only works with VBR mode (Nearly) n here Panasonic says their Smart Coding mode can work with CBR mode)
  • Actual test with 1 camera, H265 compression standard, resolution 2688 x 1520, frame rate 25 fps, VBR, maximum bandwidth of 8192 Kbps, observing the office, recording in 2 minutes

Source: https://silversea.vn/