ANSI SCTE 248 pdf free download

ANSI SCTE 248 pdf free download.Operational Practice on Multiple Audio Signaling.
6. Applicable Standards for Audio Language Signaling
6.1. Introduction
The two relevant standards for signaling the language of an audio stream are ATSC A153 Part 3 for television broadcast stations and SCTE 54 for cable television systems. These are transport layer specifications that tell how and where to carry the relevant signaling structures that describe the audio. The relevant signaling structures are the AC-3 audio descriptor (which is defined in ATSC A/52) and the ISO 639-2 language descriptor (which is defined in ISO/IEC 13818-1 Section 2.6.18).
6.2. ATSC A/53
ATSC A/53 Part 3 Section 5.8.1 requires that the AC-3 audio descriptor be carried in the ATSC broadcast stream and ‘nay optionally indicate language by including the ISO 639-2 language bytes within the AC-3 audio descriptor. Carriage of the ISO 639-2 language codes to indicate language is optional but recommended to support legacy devices such as cable set top boxes that may rely on it for language selection.
If the ISO 639-2 language descriptor is present for a given AC-3 audio elementary stream and, if the language code is present in the corresponding AC-3 audio descriptor, the language code in the ISO 639-2 language descriptor is set to the language code value present in the AC-3 audio descriptor.
The AC-3 audio descriptor contains a field named bsmod (Bit Stream Mode) that is used to indicate whether an audio stream has a service type of “main audio service: complete main” (bsmod=O), “associated service: visually impaired” (bsmod=2), or some other type.
6.3. SCTE54
SCTE 54 Section 5.9.3.1 requires the AC-3 audio descriptor be carried and the audio language is set using the language field of the AC-3 audio descriptor. The language may also be indicated using the ISO 639-2 language descriptor. However, some cable programming services do not carry the AC-3 audio descriptor and carry only the ISO 639-2 language descriptor.
Section 6 of SCTE 54 provides additional details regarding the signaling of VDS audio (called Video Description audio in SCTE 54). It permits using different language values in the ISO 639-2 and AC-3 descriptors. This approach recognizes that some “legacy” set top boxes use audio language information derived from ISO 639, while newer set top boxes use audio language information from the AC-3 audio descriptor. Ii provides for the ISO 639-2 language descriptor to continue to label the second audio stream as Spanish, even when it carries VDS audio. But the AC-3 audio descriptor would signal the VDS audio as English and as VDS (bsmod value 2). This approach would require the AC-3 audio descriptor to change throughout the day, depending on whether the current program’ s second audio stream was Spanish or VDS. The approach does not appear to have been implemented by MVPDs or programmers, or their respective technology vendors.
7. Audio distribution ecosystem From Studio to Consumer
This chapter is intended to help the reader track multiple audio channels from the studio to the consumer. It also attempts to identify areas where the appropriate metadata describing the audio feed is generated and how it is distributed. Some of the solutions for supporting multiple audio streams may require obtaining the content with a different set of audio tracks than is commonly obtained today.
ANSI SCTE 248  pdf download.