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MPEG H transport formats
MPEG-H audio bitstreams can be formatted as either raw Access Units or as a self-contained MPEG-H Audio Stream (MHAS). These transport formats are usually referred to by their ISOBMFF sample type 4CCs (FourCC) of mha1 and mhm1 respectively 1.
The use of Access Units (AU) without additional framing information is the traditional way of storing encoded audio data in ISOBMFF (mp4) files. They are not self-contained and require meta-information from the mp4-file header to be accessible. They come with almost no overhead and can't be used outside mp4 files, e.g. MPEG-2 TS or HDMI. This format is still supported by the decoder, but due to its limitations, the de-facto standard for MPEG-H Audio is MHAS.
MHAS is a packetized transport format for MPEG-H Audio, i.e. individual packets can be addressed inside a bitstream without the use of additional meta-data. This provides the framing mechanism, which is required for MPEG-2 TS transport, where LATM/LOAS was used in the past. MHAS can also be used to carry meta-information in addition to the encoded audio data, which is necessary to use MPEG-H Audio over HDMI and other low-level links - the stream is self-contained. MHAS is the preferred transport format and should be used under all circumstances, if possible.
1: ISO defines the additional sample type mha2 and mhm2 for multi-stream use-cases, but that's beyond the scope of this document.↩