B-frames are compressed video frames that can refer to other frames to save bitrate. Conventional bidirectional B-frames can add encoder latency because the encoder looks ahead and reorders frames. Use them for quality-focused streams only when your encoder and destination support them and your latency budget allows; disable them for very low-latency workflows or whenever the destination requires it.
What B-frames do
Video encoders reduce file size by predicting some frames from others. In a conventional bidirectional structure, a B-frame may use references both before and after it in display order. That can improve compression efficiency, but it may require the encoder to wait for future frames before emitting the current one.
The resulting lookahead and reordering add encoder-side delay. AWS Elemental describes latency increasing by a few frames for each additional B-frame in a GOP; removing B-frames avoids that particular cost but can require more bitrate to maintain similar quality. This is a qualitative explanation, not a universal benchmark: results vary with codec, encoder, preset, scene, bitrate, and the rest of the streaming chain. See AWS Elemental’s latency discussion.
Should you enable B-frames?
| Use case | Starting point | Reason and qualification |
|---|---|---|
| Quality-focused broadcast with ordinary viewer interaction | Enable a modest supported number, then test | NVIDIA recommends B-frames for game and studio broadcasting configurations. Its examples include a two-second GOP; this is vendor guidance, not a universal best setting. Confirm the platform accepts the stream. NVIDIA FFmpeg/NVENC guidance |
| Interactive, conferencing, cloud gaming, or very low-latency delivery | Disable conventional bidirectional B-frames, or evaluate a supported unidirectional mode | Unidirectional B-frames use past references and avoid reordering delay on supported NVENC configurations. NVIDIA reports 5–10% better compression than P-only encoding for this mode and setup; do not generalize that figure to other encoders or streams. NVIDIA’s NVENC examples and guidance |
| Amazon IVS Real-Time using its OBS/RTMP publishing workflow | Set B-frames to 0 | IVS says streams with B-frames will be automatically disconnected. Its guide recommends a 1- or 2-second keyframe interval. These instructions are specific to IVS Real-Time. Amazon IVS Real-Time RTMP guide |
| Limited video memory | Consider fewer or no B-frames | NVIDIA notes that B-frames require extra reordering buffers and recommends avoiding them when reducing video-memory footprint. NVIDIA NVENC programming guide |
How B-frames relate to total stream delay
B-frame reordering is one possible contributor to latency, not a measure of end-to-end delay. Glass-to-glass latency also depends on capture, encoding, network transport, platform ingest, transcoding, buffering, and playback. The cited guidance explains encoder and decoder buffering effects; it does not promise a particular total delay.
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Buffer settings matter too. AWS Elemental describes a workflow in which a default encoder buffer equal to twice the video bitrate produces a two-second decoder delay; reducing the buffer can affect quality. That example is product- and workflow-specific, not a universal decoder rule. Measure the complete path that matters to your audience rather than inferring it from the B-frame count alone.
Configure and test the setting
Check the destination first
Read the ingest requirements for the exact service and publishing mode you use. A destination-specific restriction takes precedence over a general quality recommendation. For example, Amazon IVS Real-Time’s OBS/RTMP guide requires B-frames disabled; do not assume the same rule applies to every IVS product or another platform.
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Match the encoder mode to the workflow
For conventional B-frames, use a supported modest count for a quality-oriented stream and compare it against a zero-B-frame encode at the same resolution, frame rate, bitrate, and content. For tight latency, begin with conventional B-frames disabled. If using NVIDIA NVENC, its documentation describes unidirectional B-frames as a low-latency option. NVIDIA’s FFmpeg example uses -bf 0 -unidir_b 1; it requires FFmpeg 7.1 or later and a compatible encoder setup. Options depend on GPU, codec, driver, and software version, so verify that your installed combination exposes and accepts them.
Do not confuse B-frame references with B-frame count
OBS’s Advanced NVENC Options page is specifically for OBS 31.0. It describes B-frame-as-reference as an HEVC/AV1 feature and says it can slightly improve quality with no performance impact. That option is distinct from enabling B-frames and is not a blanket latency recommendation. Check the OBS 31.0 NVENC options documentation for its scope.
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Treat example numbers as examples
NVIDIA’s 1080p30 unidirectional-B-frame example uses a 5 Mbps target, a 167K VBV buffer, and -bf 0 -unidir_b 1. NVIDIA explains a one-frame VBV buffer as bitrate divided by frame rate. These are example values for that configuration, not general bitrate or buffer recommendations; choose settings against your content, destination limits, and latency target.
Troubleshoot B-frame problems
- The platform disconnects the stream: Check its ingest documentation and set B-frames to zero if required. IVS Real-Time’s cited OBS/RTMP workflow disconnects streams that contain B-frames.
- Interaction feels delayed: Test with conventional B-frames disabled and compare encoder delay and measured end-to-end delay. Also inspect buffering and other stages; changing this setting alone may not address the dominant source of delay.
- Picture quality drops after disabling them: B-frames can improve compression efficiency, so a zero-B-frame encode may need more bitrate for similar quality. Test at a bitrate the destination accepts rather than treating the trade-off as a fixed percentage.
- Encoder memory is constrained: Reduce the B-frame count or disable it, then confirm the encoder remains stable under the actual workload.
- An FFmpeg option is rejected or has no effect: Verify FFmpeg is 7.1 or later for NVIDIA’s cited unidirectional example and confirm GPU, codec, driver, and encoder compatibility. The option is not supported universally.
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