Convert AAC to MP3

Yes, and it is important to understand why. Both AAC and MP3 are lossy compression formats: when the original AAC file was created, certain auditory information was already permanently discarded. When converting from AAC to MP3, the decoding process restores the audio to PCM (with the AAC compression artifacts already present) and then re-encodes it with the MP3 algorithm, discarding additional information. This phenomenon is known as generational loss. The magnitude of the additional loss depends on the bitrate used: if the original AAC is 256 kbps and the output MP3 is 192–256 kbps, the difference is imperceptible in casual listening and very small even in controlled ABX tests. If the original AAC is 128 kbps and the output MP3 is 128 kbps, the accumulated artifacts may be audible in complex musical content with quality headphones. The practical recommendation is: always use the highest available bitrate for output (minimum 192 kbps, preferably 256 kbps) to minimize additional degradation.

To minimize quality degradation in AAC to MP3 conversion, 192 kbps is recommended as a minimum and 256 kbps as the optimal value. The reason is that the MP3 encoder at 192 kbps has enough 'bit space' to faithfully represent the PCM audio output from the AAC decoder (which already contains the original compression artifacts) without adding visible new artifacts. At 128 kbps, the MP3 encoder needs to compress more aggressively and may introduce artifacts that add to those of the original AAC. The LAME encoder (LAME Ain't an MP3 Encoder, the open source reference encoder for MP3 developed since 1998) at -q 2 or higher with VBR, or at 192 kbps CBR, produces optimal results for this type of transcoding. At 256 kbps, the resulting MP3 is practically transparent for any listener under any reasonable listening condition, regardless of the AAC source.

AAC has very broad support on modern devices, but there are specific scenarios where MP3 is necessary or clearly preferable. Older MP3 players: most portable players manufactured before 2010 (first-generation iPods with unupdated firmware, players from brands like iRiver, Creative Zen, first-generation SanDisk Sansa) only accept MP3 and WMA, not AAC. Car audio systems: infotainment systems in vehicles manufactured between 2000 and 2012 frequently support MP3 but not AAC, especially in brands that do not use iOS CarPlay or Android Auto systems. CD/DVD/Blu-ray players: physical disc players that also read data CDs with MP3 (a de facto standard present in almost all DVD players since 2002) rarely support AAC. Some professional PA and playback systems: DJ equipment, PA (Public Address) systems, and non-Apple platform presentation software may require MP3. Older video editing software: some versions of Windows Movie Maker and free video editors do not accept AAC as an audio track.

Audio metadata in AAC/M4A is stored in iTunes atoms (specifically in the 'moov/udta/meta/ilst' atom of the MPEG-4 container), while MP3 uses the ID3 standard (currently ID3v2.4, specified by id3.org in 2000 and updated in 2011). The most common tags have direct equivalences: the song title in iTunes atoms (©nam) corresponds to the ID3 tag TIT2; the artist (©ART) corresponds to TPE1; the album (©alb) corresponds to TALB; the track number (trkn) corresponds to TRCK; the genre (©gen or gnre) corresponds to TCON; and the year (©day) corresponds to TDRC. Album artwork is stored as a binary data blob (usually JPEG or PNG) in both iTunes atoms (covr) and ID3v2 (APIC tag). Correct conversion preserves all these tags by mapping equivalent fields. Cover art is only preserved if the conversion software supports extracting the image binary from the M4A container and inserting it into the ID3v2 APIC tag.

M4A and AAC are conceptually distinct but frequently confused. AAC (Advanced Audio Coding) is the audio compression codec: the algorithm that converts PCM into compressed data. M4A is a file extension indicating an MPEG-4 container (ISO/IEC 14496-12) with AAC audio and no video. The relationship is analogous to MP3 (codec) and .mp3 (extension), or H.264 (codec) and .mp4 (extension). Apple introduced the .m4a extension to distinguish MPEG-4 audio-only files from .mp4 video files and .m4r ringtones. Technically, a .m4a file and a .aac file with the same audio are nearly interchangeable: modern players accept both extensions. The difference is that .aac can be a 'raw' AAC stream (without MPEG-4 container, the ADTS — Audio Data Transport Stream — format) while .m4a is always an MPEG-4 container. iTunes and Apple Music distribute their files in .m4a format (MPEG-4 container with AAC-LC or ALAC). iPhone recordings are saved as .m4a. iTunes Plus purchase files (DRM-free, launched in May 2007) are .m4a at 256 kbps AAC-LC.

The most common sources of AAC/M4A files are: iPhone voice recordings (the Voice Memos app on iOS saves in .m4a at 32 kbps AAC by default, configurable in Settings > Voice Memos to 'Lossless' for WAV LPCM); iTunes/Apple Music purchases (files downloaded from Apple Music for offline playback are .m4a with FairPlay DRM, not convertible without authorization; iTunes Store purchases are .m4a without DRM at 256 kbps if they are iTunes Plus); YouTube videos (YouTube audio streams are AAC in MP4 containers; tools like yt-dlp can extract the audio stream directly as .m4a without re-encoding, preserving the original quality); macOS audio captures (QuickTime Player saves audio recordings in .m4a); and FaceTime and WhatsApp video files on iOS (which use AAC for audio). Converting .m4a with Apple Music DRM to MP3 is not possible without violating Apple's terms of service and the DMCA; only DRM-free files (iTunes Plus or own CD rips) can be legally converted.