CTA-CEB21-A recommends the actions ATSC receivers with a single audio decoder should take in the presence or absence of AC-3 audio metadata carried in ATSC standards-based broadcasts.

These services are defined in the ATSC references shown in the References section. CTA-CEB21-A includes recommendations and suggestions on how these receiver actions should interact with user selections via different implementation choices. CTA-CEB21-A is in no way intended to prohibit consumer device manufacturers from including additional features, and CTA-CEB21-A should not be interpreted as recommending an upper limit to device performance.

CTA-CEB21-A provides recommendations to manufacturers to facilitate user setup of audio features in the receiver without professional assistance. This bulletin provides guidance in the following areas for the reasons listed below:

• One purpose of the bulletin is to describe recommended receiver behavior in the presence of the minimum required audio metadata described by ATSC A/53.
• Another purpose of the bulletin is to describe recommended receiver behavior in the presence of additional optional audio metadata described by ATSC A/53.
• This document is organized by first providing descriptions of the key metadata elements and the structural locations in the MPEG-2 Transport Stream (table sections, descriptors, etc.). The recommendations for using this information along with user settings to control audio stream selection are then provided.