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 a predictable rate matters—YouTube recommends CBR for RTMP/RTMPS live streams. CBR is not a guarantee of identical network traffic every instant, and it cannot compensate for inadequate upload capacity or a poorly chosen bitrate.
How CBR works
An encoder turns video and audio into a data stream. Its rate-control mode governs how it allocates bits over time. With CBR, the encoder targets a set rate or constrained rate envelope. The image itself does not contain the same amount of detail in every frame: a still shot is simpler to encode than fast movement, but CBR aims to keep the data rate around its target by adjusting how those scenes are represented.
“Constant” describes the encoder’s rate-control aim, not necessarily the amount of data observed on the network at every instant. Codec and encoder behavior, buffering, protocol overhead, and transport can affect measured throughput.
CBR vs. VBR: which should you choose?
| Decision | CBR | VBR |
|---|---|---|
| Rate behavior | Aims for a chosen or constrained rate; implementation details vary. | Varies the rate with content complexity. |
| Typical fit | Live streaming or bandwidth-constrained delivery where predictability matters. | Offline encoding, especially when complex scenes can use more bits and strict rate predictability matters less. |
| Main trade-off | If the rate ceiling is too low for a complex scene, quality may fall as the encoder reaches it. | Rate is less predictable, but bits can be allocated differently across simple and complex scenes. |
Neither mode is universally better. Google recommends CBR for live streaming with VP9 and VBR for high-motion VP9 video-on-demand. For offline work, also consider constant-quality modes if available in your encoder, and weigh image quality, file size, and delivery constraints. Follow the receiving platform’s requirements and the documentation for your specific encoder.
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When to use CBR for YouTube Live
YouTube’s current live-encoder guidance recommends CBR for RTMP/RTMPS ingest. Its published bitrate recommendations vary by codec, resolution, and frame rate; they are YouTube recommendations, not universal targets for other destinations. At 1080p30, YouTube lists 10 Mbps for AV1 or H.265/HEVC and 14 Mbps for H.264. At 1080p60, it lists 12 Mbps for AV1 or H.265/HEVC and 17 Mbps for H.264. Consult YouTube’s encoder settings, bitrates, and resolutions for the full table and current requirements.
YouTube’s guidance also specifies a two-second keyframe frequency, not exceeding four seconds. It lists H.264, H.265/HEVC, and AV1 as video codec options. Match the codec, resolution, frame rate, keyframe interval, and bitrate to the settings supported by your encoder and destination rather than assuming one combination fits every stream.
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Check the whole upload path
- Run an upload speed test and make sure the connection can sustain the stream’s bitrate with capacity to spare for normal network variation and other household or office traffic.
- Test before the event using movement and audio similar to what you will actually stream. A static test screen will not reveal how demanding a fast-moving scene may be.
- During the stream, monitor YouTube’s stream health and encoder status. A configured target alone does not confirm the connection is delivering reliably.
These checks follow YouTube’s operational guidance; they reduce guesswork but do not guarantee a problem-free stream.
CBR settings depend on the encoder
CBR is a mode, not a single universal switch with identical behavior in every application. Check the documentation for the encoder and codec you use, and distinguish the target rate from any minimum or maximum rate limits.
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FFmpeg
FFmpeg’s codec documentation includes encoder-specific CBR options and examples, including an AAC example at 64 kbps. That figure is an example, not a general audio recommendation. The available flags and their meanings depend on the selected encoder and codec; use the relevant entry in the FFmpeg Codecs Documentation rather than copying options across encoders.
NVIDIA NVENC
In NVIDIA’s documented FFmpeg workflow, -rc cbr selects CBR. The guide describes -b:v as the target bitrate and says setting -maxrate equal to -b:v enforces CBR behavior in that workflow. A higher -maxrate allows peaks above the target, so it is not the same constraint. See NVIDIA’s FFmpeg with NVIDIA GPU Hardware Acceleration guide for the documented options.
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VP9
Google’s VP9 CBR example sets target, minimum, and maximum rates to the same value and uses realtime settings. The rate ceiling limits how much data is sent; as a complex scene reaches that ceiling, quality can fall. See Google’s VP9 bitrate modes guidance for the codec-specific explanation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why a CBR stream may still show a changing bitrate
A CBR setting does not promise perfectly level network throughput at every measurement interval. The encoder’s implementation, buffering, protocol overhead, and transport behavior all influence what a network monitor reports. Separate the configured encoder target from the traffic measured by your connection or streaming dashboard.
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- The measured rate moves slightly: Short-term variation can reflect buffering, transport, and measurement intervals; check the encoder’s configuration and longer-term stream health.
- The stream drops frames or reports poor health: Check upload capacity, network congestion, encoder load, and whether another application is using the connection. A nominal CBR target does not rule these out.
- The picture breaks down during motion: The selected rate may be too low for the chosen resolution, frame rate, codec, and scene complexity. Compare it with the destination’s recommendation and test representative movement.
- The rate exceeds the intended target: Check whether the encoder permits peaks above its target. In NVIDIA’s documented NVENC workflow, a maximum rate higher than the target allows VBR peaks rather than the tighter CBR behavior.
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