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Constant bitrate (CBR) is an encoder rate-control mode that aims to keep a stream near a chosen bitrate or within a constrained rate range. Use it for live streaming when the platform recommends it or when a predictable data rate matters; for offline video, variable bitrate (VBR) or a constant-quality mode may suit your quality and file-size goals better.
What CBR controls—and what it does not
An encoder uses rate control to decide how many bits to spend representing audio or video over time. In CBR mode, it targets a set bitrate or stays within a constrained rate envelope. That does not mean every frame contains the same visual information: a still image and a fast-moving game scene can require different amounts of data to look equally clear.
Nor does a CBR setting guarantee that network traffic will be identical at every instant. Encoder implementation, buffering, protocol overhead, and transport behavior affect the throughput you observe. CBR describes the encoder’s rate-control behavior, not an assurance of perfectly flat network measurements.
CBR vs. VBR: which should you choose?
| Decision | CBR | VBR |
|---|---|---|
| Rate behavior | Aims for a chosen or constrained rate; exact behavior depends on the encoder. | Varies bitrate with content complexity. |
| Typical fit | Live streaming or bandwidth-constrained delivery where a predictable rate is useful. | Offline video where complex scenes can use more bits and strict rate predictability is less important. |
| Main compromise | A strict ceiling can reduce quality in complex scenes when the scene needs more bits. | Bitrate is less predictable, though allocating bits where they are useful can benefit video-on-demand encoding. |
Neither mode is universally higher quality or always right for streaming. Follow the destination platform’s requirements and the documentation for your encoder and codec. Google’s VP9 guidance recommends CBR for live streaming and VBR for high-motion video-on-demand encoding; it also explains that quality falls as a CBR ceiling is reached (Google for Developers: VP9 bitrate modes).
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When to use CBR for YouTube Live
YouTube’s current live encoder guidance recommends CBR for RTMP/RTMPS streams. Its bitrate recommendations depend on codec, resolution, and frame rate, so select a value from the relevant row rather than treating one bitrate as suitable for every stream. For example, YouTube’s published 1080p60 recommendations are 17 Mbps for H.264 and 12 Mbps for either H.265/HEVC or AV1. At 1080p30, they are 14 Mbps for H.264 and 10 Mbps for H.265/HEVC or AV1. These are YouTube’s recommendations, not universal targets for other destinations. See the full table, including minimums and other resolutions, in YouTube Help: Choose live encoder settings, bitrates, and resolutions.
The same YouTube guidance lists a two-second keyframe frequency as recommended and says not to exceed four seconds. Match the video codec and other encoder settings to the destination’s published requirements; the current page covers H.264, H.265/HEVC, and AV1.
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Fit the target to your upload connection
A recommended encoder bitrate is only useful if your connection can sustain the stream. Check upload capacity with a speed test, allowing for other network use rather than assuming the entire measured upload rate is available to the encoder. If the connection cannot reliably support the selected stream, lower the resolution, frame rate, or bitrate in line with YouTube’s table. CBR does not fix an undersized or unstable connection.
Test with the real workload
Before an important broadcast, test the complete setup with movement and audio similar to the actual stream. Check the destination’s stream-health indicators while testing and during the broadcast. A static test image may conceal bitrate or quality problems that appear in fast gameplay, camera movement, or detailed scenes.
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Set CBR in your encoder
CBR is not one universal switch with identical behavior across software. Confirm the selected encoder, codec, and destination before relying on a setting.
OBS and other graphical encoders
Choose CBR in the encoder’s rate-control setting, then set the bitrate to the value appropriate for the destination, codec, resolution, and frame rate. In OBS, the exact control labels and available options can vary with the selected encoder and version. For a YouTube Live stream, use YouTube’s current codec-specific recommendations, set keyframes to the recommended two seconds (and no more than four), and test the stream. Do not assume a CBR selection alone proves the output meets the platform’s requirements.
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FFmpeg and hardware encoders
FFmpeg options depend on the codec and encoder you invoke; an option for one encoder should not be copied blindly to another. Consult the relevant entry in the FFmpeg Codecs Documentation for its supported rate-control options and examples.
For NVIDIA’s documented NVENC workflow, the rate-control option is -rc cbr. NVIDIA describes -b:v as the target bitrate and says that setting -maxrate equal to -b:v enforces CBR behavior in that workflow. Setting -maxrate above the target permits VBR peaks, so it is not the same constraint. These details apply to the documented NVIDIA NVENC usage; other FFmpeg encoders may use different options. See NVIDIA’s FFmpeg and GPU acceleration guide.
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FFmpeg’s codec documentation includes an AAC CBR example at 64 kbps. That is an example of encoder options, not a universal audio bitrate recommendation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why a CBR stream may still look poor or have problems
- Blocky or soft detail during action: the chosen bitrate may be too low for the resolution, frame rate, codec, and scene complexity. Check the destination’s recommendations; test representative motion and consider reducing resolution or frame rate if upload capacity is limited.
- Dropped frames or unstable stream health: CBR does not eliminate congestion, insufficient upload capacity, encoder overload, or transport problems. Check upload performance and encoder load, then retest at a rate your connection can sustain.
- Measured bitrate fluctuates despite CBR: distinguish encoded rate control from instantaneous network throughput. Buffering, protocol overhead, transport behavior, and encoder implementation can make network measurements vary.
- Settings seem inconsistent across tools: rate-control modes and option names are encoder-specific. Verify the documentation for the exact codec and encoder in use instead of assuming that a CBR label or command option behaves identically everywhere.
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