Configure av1_nvenc Rate Control and Quality in FFmpeg

This article provides a practical guide on how to configure the rate control and quality settings for NVIDIA's hardware-accelerated AV1 encoder (av1_nvenc) in FFmpeg. You will learn how to utilize different rate control modes—such as Constant Quality (CQ), Variable Bitrate (VBR), and Constant Bitrate (CBR)—along with NVENC-specific presets and tuning options to achieve the optimal balance between file size, encoding speed, and visual fidelity.

Rate Control Modes in av1_nvenc

Rate control determines how the encoder allocates bits to different parts of the video. The av1_nvenc encoder supports several rate control methods via the -rc private flag, which overrides standard FFmpeg rate control behavior.

1. Constant Quality (CQ / Target Quality)

For local recordings and archival purposes where file size is secondary to visual quality, the target-quality VBR mode is recommended. This mode maintains a consistent level of quality throughout the video.

  • Command options: Set -rc to vbr and define the quality level using -cq.

  • Example:

    ffmpeg -i input.mp4 -c:v av1_nvenc -rc vbr -cq 25 -b:v 0 output.mkv

    Note: Setting -b:v 0 is required when using -cq to prevent the encoder from targetting a default bitrate limit.

2. Variable Bitrate (VBR)

VBR is ideal when you want to target a specific average file size but allow the bitrate to spike during complex, high-motion scenes.

  • Command options: Specify the target bitrate with -b:v and a maximum limit with -maxrate.

  • Example:

    ffmpeg -i input.mp4 -c:v av1_nvenc -rc vbr -b:v 5M -maxrate 10M -bufsize 20M output.mkv

3. Constant Bitrate (CBR)

CBR is crucial for live streaming, where a predictable and steady stream of data is required to prevent buffering.

  • Command options: Set -rc to cbr and match -b:v, -maxrate, and -bufsize.

  • Example:

    ffmpeg -i input.mp4 -c:v av1_nvenc -rc cbr -b:v 6M -maxrate 6M -bufsize 6M output.mkv

Configuring Quality Presets

NVIDIA uses a standardized preset system ranging from p1 (fastest, lowest quality) to p7 (slowest, highest quality). The default preset is p4.

To set the preset, use the -preset flag:

ffmpeg -i input.mp4 -c:v av1_nvenc -preset p6 -rc vbr -cq 24 -b:v 0 output.mkv
  • Use p1 to p3 for low-latency, real-time applications where encoding speed is critical.
  • Use p4 to p5 for balanced daily encoding.
  • Use p6 to p7 for high-quality archival encoding where you want to squeeze out maximum efficiency.

Advanced Fine-Tuning Options

To maximize the efficiency of the AV1 encoder, you can enable several NVENC-specific hardware features in your FFmpeg command.

Multipass Encoding (-multipass)

Multipass encoding allows the encoder to analyze the video frames before compressing them. This significantly improves quality at a given bitrate.

  • 0 or disabled: Single-pass encoding.
  • 1 or qres: Two-pass full resolution (slower, highest quality).
  • 2 or fullres: Two-pass quarter resolution (good balance of speed and quality).
ffmpeg -i input.mp4 -c:v av1_nvenc -multipass fullres -b:v 5M output.mkv

Adaptive Quantization (-spatial_aq and -temporal_aq)

Adaptive Quantization (AQ) optimizes bit distribution across the frame based on human visual perception.

  • Spatial AQ (-spatial_aq 1): Allocates more bits to flat areas (like skies or walls) to prevent color banding and blockiness.
  • Temporal AQ (-temporal_aq 1): Reduces blockiness in fast-moving scenes by tracking motion over time.
ffmpeg -i input.mp4 -c:v av1_nvenc -spatial_aq 1 -temporal_aq 1 output.mkv

Comprehensive Example Command

For a high-quality, efficient AV1 encode suitable for offline viewing or uploading, use a command that combines two-pass VBR, spatial and temporal AQ, and a high-quality preset:

ffmpeg -i input.mp4 -c:v av1_nvenc -preset p6 -rc vbr -cq 24 -b:v 0 -multipass fullres -spatial_aq 1 -temporal_aq 1 output.mkv