Unprotected AAC audio file
M4A is a file extension for an audio file encoded with advanced audio coding (AAC) which is a lossy compression. M4A was generally intended as the successor to MP3, which had not been originally designed for audio only but was layer III in an MPEG 1 or 2 video files. M4A stands for MPEG 4 Audio.
Both M4A and MP3 file extensions are used for audio-only files. Audio-only MPEG 4 container files usually have an M4A file extension. M4A files are unprotected. Protected files usually have an M4P file extension.
AAC or M4A file quality is better and file sizes smaller compared to MP3 files. M4A files sound better than MP3 files when encoded at the same bit rate due to some of the enhancements made to the format such as:
- Compression based on perception -- if it is not within human perception, that sound data may be lost without notable effect to quality.
- Smaller sample block size of 120 or 128 samples rather than 192 for changing (or transient) signals, allowing for more precise details where they are needed.
- Larger block sizes in stationary signals, 1024 or 960 vs MP3's 576 sample blocks, allowing for less data to represent a portion of the music that does not have the same complexity.
Learn more about M4A-related files:
Andoni Oconchubhair defines MPEG standards at Search SOA
Look for answers to M4A-related questions on IT Knowledge Exchange.
Visit the MPEG webpage.