dBpoweramp m4a Codec

 
 This m4a codec handles:

  • Apple Lossless: a lossless audio compression, encoding and decoding supported (16 & 24 bit)
  • Advanced Audio Compression (installed separately), also known as AAC: a lossy compression like mp3, decoding supported (installed on demand). To encode AAC install the m4a Nero AAC Encoder.

m4a is a container, it can contain either Apple Lossless or AAC. 

Portions of the ALAC codec are (c) Apple and are licensed under the Apache License.

Supported by this Codec       

  • Encoding: Yes   [.m4a (Apple Lossless, for AAC see m4a Nero AAC Encoder) ]
  • Multi-processor Encoding: Yes   (with dBpoweramp reference)
  • Decoding: Yes   [.m4a, .mp4, .m4b]
  • ID Tag Reading: Yes   [iTunes Comments]
  • ID Tag Writing: Yes   [iTunes Comments]
  • Unicode Tagging: Yes
  • Supports Album Art: Yes
  • Gapless Encoding & Decoding: Yes
  • Explorer Audio Popup Information: Yes
  • Explorer Columns: Yes
  • Unicode Filenames: Yes

Compatibility

Windows 98 or newer, dBpoweramp R12 or newer required.

 Apple Lossless Compression Options 

Apple Lossless does not have many options, it does not need any (being lossless).

After Encoding Verify.... will read back the compressed file and compare it with source audio ensuring error free compression (both hardware & codec are verified).

 Codec Background 

Successor to mp3, m4a (and mp4) is a container that can store many sub-formats:

  • Apples Lossless format (ALAC) this format is like Zip for audio and looses no audio quality (audio tracks purchased from Apples iMS store are lossy aac).
  • Advanced Audio Compression (AAC), of which are different types: LC (low complexity), HE (high efficiency) to name a few. To encode your audio to be compatible with most audio players, use the LC compression type (used as default by this encoder). High Efficiency (HE AAC) uses SBR technology (not too different as mp3PRO) if the player knows about HE the extra frequency information can be decoded, otherwise half the audio quality will be missing on playback. A newer standard again for ultra low bitrates is AAC+ or AAC HEv2, this uses parametric stereo for even lower bitrates, dBpoweramp is able to decode every AAC type (excluding iTunes m4p protected). 

File Extensions:

  • .m4a are audio content only,
  • .mp4 can contain both audio and video,
  • .m4b these are identical to .m4a except the iPod knows to use them as audio books and allow book marking,
  • .m4v are video files.
  • .m4p protected audio format (tracks purchased from iTunes Music Store are protected), whilst dBpoweramp cannot decode these see Forum for details on fair use.

Adding ID tags to mp4 has standardized on the Apple iTunes format, although a competing (read, non-compatible) Nero tagging format has emerged.

 Advanced Options 

dBpoweramp Configuration offers advanced options for this codec (dBpoweramp Configuration >> Codecs >> Advanced Options):

m4a / mp4 Tagging

m4a Layout can be 'Optimized for Streaming' (where the audio details and tags are placed before audio data), or 'Streaming Incompatible' where audio data appears first.

Tag Padding  by using a padding, minor tag changes save the whole file being shuffled (as tags are generally before the audio)

Compatibility Sony and Nokia players require an id3v1 tag, whilst strictly breaking the m4a standard, this option will write those for compatibility.

m4a / mp4 Decoder

Output (AAC) 16 bit, 24 or 32 bit gives the option to select the output from the decoder, 32 bit is automatically selected when an encoder can use the higher resolution. AAC stores internally encodings as floating point, but generally speaking encoding a 16 bit audio CD to Ogg Vorbis then back to 32 bit point wave will not yield extra bit depth. This option requires dBpoweramp Reference.

Corrupt AAC File gives the option to continue decoding an aac file after a stream error has been detected (ie corrupt audio file), or stop on such an error.

 Version Changes  

Complete Version Changes

Huge thanks go to David Hammerton, for his work on reverse engineering the Apple Lossless format.