Selecting the Right Bit Rate for CBR CamerasBy John Honovich, Published Mar 12, 2012, 08:00pm EDT
Some of the most popular cameras on the market, like Panasonic and Sony, are especially tricky to configure the right bit rate. In our experience, because of these camera's use of Constant Bit Rate, most deployments significantly waste bandwidth in real world usage. This note explains the issues involved and how to overcome this problem.
Many camera manufacturers only support Constant Bit Rate (CBR) streaming, generally done to ensure quality video level. With CBR, the camera will always stream at the same bandwidth level regardless of the scene or conditions. For HD cameras, Panasonic and Sony typically set their default CBR rates at either 2Mb/s or 4Mb/s (depending on model). Equally important, this bit rate assumes the use of maximum frame rate of 30fps.
[UPDATE: Panasonic has added VBR streaming with new firmware. They call it 'Best Effort'. If you do not find it in your camera's configuration menu, upgrade to the most recent firmware.]
Most users, however, will set significantly lower frame rates. The average frame rate in real world usage is between 6 - 10 fps - 60-80% less than the max frame rate. Bandwidth consumption scales rather linearly - as the frame rate drops, bit rate needed drops at a proportionate level (see our bandwidth vs frame rate results).
Problem and Solution
The practical problem is that many users will drop the frame rate but not the bit rate. As a result, you can have a camera set to 8fps at 2Mb/s. This is highly inefficient for almost every scene possible. The plus side is that your video quality should look good (assuming camera is in focus, etc.). However, a lot of bandwidth is being wasted. You could drop the bit rate to 1Mb/s or even 750Kb/s and get the same visual quality.
When people talk about HD video requiring 2Mb/s - 4Mb/s, this is premised on running full frame rate video and typically assumes a moderate amount of motion (people walking, cars driving through, etc.). However, if one is not using full frame rate video (like most people), 2Mb/s - 4Mb/s is almost certainly too high.
The 'Exact' Right Bit Rate
The one challenging element remaining with CBR is setting the 'exact' or the precise right bit rate. Returning to our HD camera at 8fps, 2Mb/s is definitely too high but it is hard to say whether 500Kb/s, 750Kb/s or 1Mb/s is right. As we demonstrated in the bandwidth vs image quality shootout, the 'best' level depends on the motion in the scene - a hallway might be fine with 500Kb/s but a parking lot might need 1Mb/s.
Getting the exact right setting is fundamentally hard with CBR and is a big reason why we recommend VBR and VBR plus a cap as superior alternatives. CBR locks the bit rate yet bandwidth needs vary with the scene over time. Nonetheless, a number of the most respected cameras, like Panasonic and Sony, only use CBR. If you are going to use these cameras, it is imperative that you properly appreciate and optimize the use of CBR.
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