every.channel/evolution/proposals/ECP-0077-explicit-aac-transcode-profile.md

ECP-0077: Explicit AAC-LC Live Audio Profile In wt-publish

Context

Live OTA inputs expose multiple AC-3 audio tracks (5.1 + stereo language variants). Browser watcher behavior is more stable when the published relay stream has a single explicit AAC-LC stereo track shape.

Decision

In ec-node wt-publish transcode mode, force explicit stream mapping and AAC profile:

• -map 0:v:0
• -map 0:a:0?
• -c:a aac
• -profile:a aac_low
• -b:a 160k
• -ac 2
• -ar 48000

Rationale

• Removes ambiguity from ffmpeg auto stream selection when multiple audio tracks exist.
• Keeps audio encoding browser-friendly and deterministic.
• Preserves optional audio behavior (0:a:0?) for edge cases where input temporarily lacks audio.

Reversibility

• Revert to ffmpeg auto mapping/profile by removing explicit -map and -profile:a flags.