Bitrate - themactep/thingino-firmware GitHub Wiki

Video Encoder Bitrate Modes

This page documents common video encoder bitrate and quality control modes: CBR, VBR, Capped VBR, Fixed QP (FixQP), and Capped Quality. Each mode represents a different strategy for controlling compression, quality, and bandwidth usage.

Overview Table

Mode Controls Output Size Predictability Quality Consistency Typical Use Cases
CBR Bitrate Very High Low–Medium Live streaming, broadcast, strict bandwidth limits
VBR Quality (indirect) Low High File encoding, VOD, archival
Capped VBR Quality + Max Bitrate Medium High Streaming with bandwidth limits
FixQP Quantizer Very Low Very High Testing, low-latency, analysis
Capped Quality Quality + Bitrate Cap Medium–High High Modern streaming, adaptive delivery

Constant Bitrate (CBR)

Description

CBR enforces a constant output bitrate over time. The encoder adjusts quality dynamically to ensure the bitrate stays at (or very near) the target.

Pros

  • Highly predictable bandwidth usage
  • Compatible with legacy decoders and delivery systems
  • Ideal for real-time transmission
  • Simplifies network planning

Cons

  • Quality fluctuates with scene complexity
  • Inefficient compression (wastes bits on simple scenes)
  • Complex scenes may show blocking or banding

Appropriate Use

  • Live streaming (RTMP, SRT, broadcast)
  • Satellite, cable, or radio links
  • Environments with strict bandwidth ceilings
  • Older hardware encoders or platforms

Variable Bitrate (VBR)

Description

VBR allows the encoder to vary bitrate freely based on scene complexity, targeting a desired quality level rather than a fixed bitrate.

Pros

  • Better overall visual quality
  • More efficient compression
  • Allocates bits where they matter most
  • Ideal for offline encoding

Cons

  • Unpredictable instantaneous and average bitrate
  • Potential delivery issues for constrained networks
  • File size not precisely known ahead of time

Appropriate Use

  • Video-on-demand (VOD)
  • Local playback
  • Archival and mezzanine files
  • Two-pass encoding workflows

Capped VBR

Description

Capped VBR is VBR with an enforced maximum bitrate. The encoder prioritizes quality but will reduce quality if the bitrate ceiling is reached.

Pros

  • Better quality than CBR
  • Prevents bitrate spikes
  • Safer for streaming than pure VBR
  • Good balance of efficiency and predictability

Cons

  • Still less predictable than CBR
  • Quality may degrade during complex scenes
  • Requires careful tuning of cap values

Appropriate Use

  • Streaming platforms with max bitrate limits
  • CDN-friendly uploads
  • Cloud transcoding pipelines
  • Adaptive bitrate ladder generation

Fixed Quantization Parameter (FixQP)

Description

FixQP locks the quantization parameter (QP) to a fixed value. Bitrate is entirely unconstrained and varies based on content complexity.

Pros

  • Extremely consistent visual quality
  • Minimal encoder decision overhead
  • Useful for benchmarking and debugging
  • Very low latency

Cons

  • No bitrate control whatsoever
  • Can produce extremely large files
  • Unsuitable for network delivery
  • Poor storage efficiency

Appropriate Use

  • Encoder testing and research
  • Quality comparison experiments
  • Low-latency internal pipelines
  • Intermediate analysis encodes

Capped Quality (Quality-Based with Bitrate Cap)

Description

Capped Quality targets a quality metric (CRF, CQ, ICQ, etc.) while enforcing a maximum bitrate. It is conceptually similar to capped VBR but is explicitly quality-driven.

Pros

  • Consistent perceived quality
  • Prevents excessive bitrate spikes
  • Efficient compression
  • Widely supported in modern encoders (x264, x265, AV1)

Cons

  • Average bitrate still varies
  • Requires tuning quality and cap values
  • Not as deterministic as CBR

Appropriate Use

  • Modern streaming workflows
  • Adaptive bitrate streaming (HLS, DASH)
  • Cloud and SaaS video platforms
  • High-quality VOD delivery

Practical Guidance

Choose CBR if:

  • Bandwidth is fixed or guaranteed
  • You are doing live or real-time delivery
  • Infrastructure expects constant rates

Choose VBR or Capped Quality if:

  • You want the best visual quality per bit
  • You are encoding files or VOD
  • Some bitrate variability is acceptable

Choose Capped VBR if:

  • You need quality prioritization with safety limits
  • You are targeting CDNs or streaming services

Choose FixQP if:

  • You are testing, benchmarking, or analyzing encoders
  • Bitrate and file size do not matter

Notes

  • Many encoders expose these modes under different names (CRF, CQ, ICQ, QP, VBV, HRD).
  • Hardware encoders often approximate these behaviors rather than implementing them strictly.
  • Modern workflows increasingly favor quality-based modes with caps over pure CBR.