AV1 Codec: Main Tier vs High Tier Differences

This article provides an overview of how the AV1 video codec differentiates between Main Tier and High Tier applications. In the AV1 specification, tiers specify the maximum bitrates allowed for a given operating level, dividing decoding requirements into standard consumer playback and resource-intensive, high-throughput scenarios. Understanding this distinction is essential for content distributors, hardware manufacturers, and developers aiming to optimize video delivery without exceeding decoder hardware constraints.

The Role of Tiers in AV1

In modern video codecs like AV1, a Level dictates the maximum processing requirements, such as resolution, frame rate, and maximum luma sample rate. Within these levels, AV1 introduces Tiers to set the upper boundary on maximum bitrates (MaxBitrate).

While the level ensures a decoder can process a specific amount of pixel data per second, the tier determines how much compressed data the decoder must ingest and buffer in real time.

Key Differences Between Main Tier and High Tier

1. Maximum Bitrate Capabilities

The fundamental distinction between the two tiers is throughput:

• Main Tier: Establishes lower maximum bitrate thresholds suitable for everyday streaming scenarios.
• High Tier: Increases the maximum bitrate limit significantly—typically by a factor of four (4×) compared to the Main Tier at the same level.

2. Level Availability

Tiers do not exist across all levels:

• Levels 2.0 to 3.3: Defined solely for Main Tier. Lower resolutions (SD up to 1080p at moderate frame rates) do not require high-tier specifications because their bitrates remain within standard streaming limits.
• Levels 4.0 and Above: Support both Main Tier and High Tier. These levels encompass high-resolution, high-frame-rate content such as 4K and 8K.

3. Application and Use Cases

The tier selection depends directly on the target delivery environment:

• Main Tier Applications:

  • Consumer Video on Demand (VOD): Platforms like YouTube, Netflix, and Vimeo use Main Tier to balance high visual fidelity with manageable bandwidth.
  • Mobile Devices and Smart TVs: Tailored for hardware with constrained memory bandwidth and thermal dissipation limits.
  • Standard Broadcast and Web Video: Designed for typical residential internet connections where extreme bitrates cause buffering issues.

• High Tier Applications:

  • Professional Video Production and Post-Production: Intermediate delivery and mastering workflows requiring visually lossless compression.
  • High-Fidelity Cloud Gaming: Scenarios where low latency and complex scene dynamics necessitate very high bitrates to avoid compression artifacts.
  • Premium Live Broadcast: Feeds and contribution links between production trucks and broadcast centers.

Hardware and Decoder Compliance

AV1 decoder compliance rules mandate backward compatibility between tiers:

• A High Tier decoder at a specific level must be fully capable of decoding Main Tier bitstreams at or below that same level.
• A Main Tier decoder is not required to handle High Tier streams, even at the same level, because it lacks the necessary buffer sizes and processing throughput.

By categorizing profiles and levels into Main and High Tiers, the AV1 standard allows low-power consumer devices to implement cost-effective decoders, while reserving High Tier profiles for performance-critical, high-bandwidth applications.