AAC: Low Overhead Audio Transport Multiplex (LATM)
LATM packages MPEG-4 audio and configuration information in a multiplex structure that can be carried over several delivery transports.
A source described as AAC/LATM combines an audio coding format with a multiplex structure. The receiver first interprets LATM's configuration and payload arrangement, then supplies the encoded audio to the appropriate decoder.
Low Overhead Audio Transport Multiplex groups audio data and the information needed to interpret it. LOAS, Low Overhead Audio Stream, can provide synchronization and length framing around that multiplex. They are different layers, not two names for one AAC codec.
| Layer | Reader's operation |
|---|---|
| LOAS, when used | Find the framed LATM data in a continuing byte sequence |
| LATM | Interpret multiplex setup, payload boundaries, and configuration reuse |
| AAC coding | Reconstruct the sound from encoded audio units |
| TS or RTP carriage | Obtain this component through the outer delivery structure |
Configuration reuse and receiver state
RFC 6416 describes a LATM audioMuxElement carrying payload and, in its in-band configuration mode, a useSameStreamMux indication. A StreamMuxConfig describes how the audio and its configuration are arranged.
An initial element can supply configuration A and its audio. A following element can indicate that the previous setup is reused and carry the next payload without a complete repeat of A. The receiver retains A and applies it while parsing and decoding that next payload.
This saves repeated descriptions, but makes retained setup part of the receiving state. If the reader joins after A and receives only reuse indications, it needs the applicable configuration before it can interpret those payloads. Repeating configuration periodically helps a receiver joining or recovering from loss.
If the stream changes to configuration B, the reader must follow the new description rather than continue using A. A change near a programme transition is therefore meaningful evidence when sound begins failing at that point.
Configuration can arrive elsewhere
LATM over RTP can use out-of-band setup supplied through session description. That route does not need to repeat the same configuration within every audioMuxElement. The delivery convention establishes how the receiver obtains it.
This is why stripping a supposed fixed header from every packet is not a general LATM-to-ADTS conversion. A converter needs to parse the multiplex, recover setup and encoded units, and write the destination's actual conventions. Some audio configurations cannot be represented by the same simple ADTS fields as another stream.
For an Emby recording, compare the coding variant, multiplex/configuration handling, and output package. Reading AAC from one wrapper and reading it from LATM are different input paths to potentially the same audio decoder. The configuration trace explains a startup or transition failure more precisely than treating every AAC-labelled source as identical.