Overview
ISO/IEC 23003-5:2020 specifies how uncompressed audio is represented and carried inside files based on the ISO base media file format (commonly known as MPEG‑4 file format). It covers mono, stereo and multi‑channel Linear Pulse Code Modulation (LPCM) audio and floating‑point PCM, including common word lengths and sampling rates. The standard defines the on‑disk/container syntax and semantics required for interoperable handling of uncompressed audio in MPEG‑4 containers (and notes that identical representations also appear in other container formats such as RIFF WAV and AIFF).
Key topics and technical requirements
- PCM configuration box (
pcmC): A mandatory box inside an audio sample entry when the SampleEntry codingname is ipcm (integer PCM) or fpcm (floating PCM). It documents the PCM format for the audio stream.
- Endianness: Default is big‑endian; a
format_flags bit (0x01) indicates little‑endian when set.
- Sample sizes:
- Integer PCM: allowed bit depths are 16, 24, 32 bits.
- Floating‑point PCM: allowed sizes are 32, 64 bits (IEEE 754 representation; full‑scale magnitude = 1.0).
- Integer representation: two’s complement for integer PCM.
- PCM sample/frame definition: A PCM audio frame (access unit) is a concatenated set of per‑channel PCM samples for the same sample time, with each sample occupying the specified number of bits without intervening bits.
- Multi‑channel handling: If channel count > 1, a ChannelLayout (as defined in ISO/IEC 14496‑12) is mandatory to specify channel count and ordering.
- Syntax: The document provides the
PCMConfig FullBox structure (pcmC) with fields format_flags and PCM_sample_size.
Applications and who uses it
- Implementers of MPEG‑4 file writers and readers, media frameworks and multimedia players that must store or decode uncompressed PCM streams.
- Audio tool and DAW developers exporting lossless PCM into MPEG‑4 containers.
- Content distributors, broadcasters, preservationists and archiving systems needing precise, interoperable uncompressed audio metadata.
- Developers of converters between container formats (MPEG‑4 ↔ WAV/AIFF) ensuring correct sample size, endianness and channel mapping.
Related standards
- ISO/IEC 14496‑12 - ISO base media file format (framework and ChannelLayout definitions).
- IEEE 754 - floating‑point representation referenced for floating PCM semantics.
This standard ensures consistent encoding of uncompressed audio in MPEG‑4 containers, improving interoperability across players, production tools and distribution systems handling LPCM and floating‑point audio.
Open preview (PDF)Information technology — MPEG audio technologies — Part 5: Uncompressed audio in MPEG-4 file format
