AV1 Level Constraints with Zond 265

Introduction

The AV1 codec, developed by the Alliance for Open Media, is widely used by modern streaming platforms such as YouTube and Netflix.

When analyzing AV1 bitstreams in Zond 265, you may encounter the table “Level Constraint check”. This article explains what it means and how to interpret it.

What Is an AV1 Level and Tier?

AV1 Level is a set of constraints that defines the maximum complexity requirements for decoding a bitstream. It guarantees that a compliant decoder can correctly decode any AV1 bitstream that complies with all the constraints of the specified level. Level is responsible for the computational complexity of the stream (resolution, fps, throughput, tiles, headers). Restrictions are presented in subsection A.3. Levels of Annex A: Profiles and levels in the AV1 specification.

AV1 Tier is an additional class of constraints within the same Level that determines the permissible bitrate and compression ratio. AV1 has two Tiers: Main and High. Main is the primary mode, used for general purposes. High is for streams requiring a higher bitrate and compression ratio.

Level limitations can be divided into 4 groups:

1. Frame Size

• MaxPicSize – Maximum number of luma samples in a single frame. Luma samples correspond to the number of frame pixels.
• MaxHSize – Maximum frame width in luma samples.
• MaxVSize – Maximum frame height in luma samples.

2. Decoding Rate

• MaxDisplayRate – Maximum frequency of pixel output to the screen in samples/sec.
• MaxDecodeRate – Maximum frequency at which the decoder should process pixels.
• MaxHeaderRate – Maximum number of decoded OBU headers per second.

3. Bitrate

• MainMbps – Maximum bitrate for Main Tier. HighMbps is used for High Tier.
• MainCR – Maximum compression ratio for Main Tier. HighCR is used for High Tier.

4. Tiles

• MaxTiles – Maximum number of tiles in a single frame.
• MaxTileCols – Maximum number of tile columns.

What Does “Level Constraint” Mean?

The Level Constraint parameter indicates whether the encoded bitstream conforms to the constraints of the declared level. The encoder sets a specific Level, Zond 265 calculates the actual stream parameters and compares them with the constraints in Annex A. If at least one constraint is violated, the stream does not match the declared Level.

The table below shows typical examples of video stream parameters for some levels:

How to check Level constraints in Zond 265

Starting with version 5.7 Zond 265 added the ability to check Level constraints for AV1 codec. The following parameters are checked:

• MaxPicSize
• MaxHSize
• MaxVSize
• MaxDisplayRate
• MainMbps
• Tiles
• TileCols

To check level limits, open the av1 bitstream in the Zond 265. After doing this, the Stream Stats tab will display bitstream parameters such as level and tier and a table with a level constraints check, which will show the current values ​​for the open file and the maximum allowable values ​​for the current level, as shown in the image below.

Also, if some parameter does not match the level constraint, it will be marked with a red mark and an error will be shown in the Errors tab. For example, let’s take a file in which the number of tiles exceeds the level constraint. The errors tab will display a description of the error:

Level Constraint checking helps you ensure playback compatibility, find encoder configuration errors, identify incorrect bitstreams, verify standard compliance, select optimal parameters for specific devices.