Encoding · Storage · Networking
CBR vs VBR: which bitrate type to set on a security camera
Constant bitrate (CBR) holds the stream at one rate whatever the camera sees. Variable bitrate (VBR) lets the rate follow the scene, up to a maximum you set. CBR makes the network and the disks predictable; VBR spends the bits where the picture needs them.
How each one behaves
| CBR (constant) | VBR (variable) | |
|---|---|---|
| Quiet scene | Stays at the set rate. The encoder pads the stream with filler data. | Drops well below the maximum. |
| Busy scene, rain, night noise | Stays at the set rate. Quality drops if the rate is too low for the detail. | Rises toward the maximum to keep the quality. |
| Network load | Flat and known in advance. | Varies; plan links for the maximum. |
| Storage | Exactly bitrate × time. | Usually less than the maximum, never more. |
What we measured
On our own encoder test bench (a 3D scene streamed as a camera would, read back with ffprobe on the RTSP side, October 2026):
- CBR 4096 kbps, H.264 High, 1920×1080 at 25 fps: 4,098 kbps measured, steady second to second.
- VBR with a 2048 kbps maximum, H.265, 1280×720 at 15 fps, a quiet scene with one moving object: 957 kbps measured, under half the cap.
That gap is the whole argument for VBR on quiet scenes, and the reason a CBR stream that never drops is easy to plan around.
Which one to pick
- CBR on a link with a fixed capacity you must not exceed: a wireless bridge, a 4G or LTE uplink, a VPN to a remote recorder. Also when the storage budget has to be exact.
- VBR for most recording on a local network. It saves disk on the long quiet hours and keeps detail when something happens.
- With VBR, set the maximum high enough for the busiest moment you care about. A cap that is too low turns VBR into a CBR that gives up quality at the worst time.
Sizing storage for each
Storage per day is bitrate × 86,400 seconds ÷ 8. A 4 Mbps stream fills 43.2 GB a day. For CBR that is the real figure. For VBR, use the maximum to size the disks and expect to use less. Our storage calculator does this per camera and for the whole recorder, and the bandwidth calculator checks the switches and uplinks.
Questions
Does VBR save storage?
On scenes that are quiet most of the time, yes. A variable bitrate stream drops well below its maximum when little moves, so a car park at night or an empty corridor records far less than the cap. Size the disks for the maximum anyway, because a busy scene or rain can hold the stream near it for hours.
Why does CBR use the full bitrate on a still image?
Because that is the promise it makes. When the scene needs less than the set rate, the encoder fills the rest of the budget with padding data that decoders discard, so the network and the recorder always see the same number.
Which is better for video analytics?
Analytics care about picture quality, not the bitrate type. CBR set too low loses detail exactly when a lot moves, which is when detection matters. VBR with an adequate maximum keeps the detail in busy moments.
What do camera menus call these settings?
Usually Bitrate Type, with Constant and Variable. Variable often comes with a quality or level setting next to the maximum bitrate.
See the bitrate for the whole system
Plan the cameras on the real site, then get bandwidth and storage for every camera in one report.