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CONCEPT Cited by 1 source

RTC codec rate control

RTC codec rate control is the discipline of making a video encoder emit a stable, accurate bitrate — without significant overshoot or undershoot — under the constraints of real-time communication: sub-300 ms end-to-end latency, frequent target-bitrate changes from the network congestion controller, and frequent resolution changes driven by bandwidth fluctuations.

Why RTC rate control differs from VOD

In VOD encoding, multi-pass techniques can smooth bitrate allocation across scenes. In RTC:

  • No multi-pass — frames must be encoded in a single pass with bounded latency.
  • Instantaneous overshoot causes freezes — the peer's jitter buffer is small; any spike can overflow it.
  • Undershoot is equally harmful — it misleads the bandwidth estimation module, slows bitrate ramp-up, and degrades video quality.
  • Frequent target-bitrate changes — the client may update the encoder's target rapidly as network conditions shift.
  • Frequent resolution changes — the client adjusts resolution during a call; the rate-control algorithm must remain stable across transitions.

Meta's approach (AV1 RTC)

Per sources/2026-06-22-meta-adopting-av1-for-real-time-communication-rtc-at-scale:

  1. Track concepts/vbv-delay frame-by-frame; use buffer status to guide bitrate allocation.
  2. When overshoot occurs, reduce rate of subsequent frames to keep VBV delay under control.
  3. Strictly limit key-frame bitrate — no "boost" for reference quality in RTC.
  4. Leverage concepts/reference-picture-resampling (AV1 feature) to change resolution without generating a key frame, avoiding bitrate spikes on resolution transitions.
  5. Balance overshoot prevention against undershoot — revised algorithm to maintain accurate bitrate in both directions.

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