A widely used lossy audio codec (AAC) and its common file container (M4A) that achieves better quality than MP3 at equivalent bitrates.
Advanced Audio Coding (AAC) is a lossy audio compression format standardized by ISO/IEC as the successor to MP3, using more efficient psychoacoustic modeling to retain more perceived audio detail at a given file size. M4A is the MPEG-4 container file that typically holds AAC audio; the same container using the .mp4 extension may also carry video.
Why it matters
AAC at 256 kbps is the standard quality for iTunes Store purchases and Apple Music streaming, and is generally considered transparent (indistinguishable from lossless) for most listening contexts. DJs sourcing tracks from iTunes Store purchases will encounter DRM-free M4A files, so understanding the format helps when assessing whether a file is suitable for club playback or needs to be replaced with a higher-quality source. Note that tracks downloaded for offline listening through an Apple Music subscription are DRM-protected and cannot be played on CDJs or non-Apple hardware, even though they also use the M4A container.
In practice
Most CDJs and standalone players read AAC/M4A natively, but some older hardware requires the file to be on a FAT32 or exFAT drive and may have bitrate limits. If a venue's gear rejects an M4A, transcoding to WAV is an option, though transcoding a lossy file to lossless does not recover the discarded audio data.
