SCTE 242-2 2022

Next Generation Audio Coding Constraints for Cable Systems: Part 2 – AC-4 Audio Coding Constraints

Executive Summary

This document is part of a suite documenting coding constraints of Next Generation Audio (NGA) systems for cable television. It is intended to be used in conjunction with the specific audio technologies described in subsequent Parts of this standard [SCTE 242-1].


This document is part two of a three-part standard that specifies the coding constraints of Next Generation Audio system for cable television. In conjunction with [SCTE 242-1], this document defines the coding constraints on AC-4 for cable television.

The carriage of the streams described in this specification is defined in [SCTE 243-2] in conjunction with [SCTE 243-1].


The Next Generation Audio (NGA) system audio system provides immersive and personalizable sound for television. It is not compatible with the audio system used in [SCTE 54]-era service.

AC-4 is a highly efficient audio Codec that supports legacy channel-based content, object-based and channel-based immersive content, personalized content, and advanced metadata as defined in ETSI [TS 103 190-2].

The AC-4 audio system has several features specific to AC-4 that are utilized in the ATSC 3.0 audio system.

The A/V frame alignment feature of AC-4 avoids complex problems that can occur when trying to keep content in sync at segment boundaries, without compromising the audio at switching points. When enabled, this feature simplifies splicing workflows. It also simplifies transcoding from or to formats that use video-based data frame alignment, such as HD-SDI.

AC-4 supports the generation of coded audio frames that represent the same time interval as the associated video frame.