MP4/M4A Format Preferences allow setting up how output MP4/M4A files are encoded. This tab allows specifying MP4/M4A Codec, Bit Rate Mode, Bit Rate, Bit Depth, Compression Level, Sample Rate and Channels.
To open MP4/M4A Format Preferences tab, choose ‘Preferences’ in Application Menu and select Format tab.
- MP4/M4A Codec
- Bit Rate Mode
- VBR Quality
- Bit Rate
- Sample Rate
- Bit Depth
- Compression Level
The list of supported codecs that can be used when converting to MP4/M4A format:
- AAC-LC (Low Complexity AAC) - is the most efficient and mostly used AAC profile, which is targeted to the high-quality encoding of music and voice. AAC-LC provides transparent, near-CD quality at 80 kbps for mono and 128 kbps for stereo input. This codec is selected by default.
- AAC-HE v1 (High-Efficiency AAC v1) - is the extension of AAC which improves audio quality at lower bit rates, where AAC-LC cannot achieve acceptable quality. This codec is targeted to medium-quality encoding at bit rates 24 kbps per channel and higher. AAC-HE v1 is 30% more efficient than AAC-LC. However, it is not a replacement for AAC-LC but rather its extension and should be audio codec of mobile and broadcasting.
- AAC-HE v2 (High-Efficiency AAC v2) - is the extension of AAC which improves audio quality at lower bit rates, where AAC-LC cannot achieve acceptable quality. This codec improves audio quality for stereo signals at low bit rates, such as 32 kbps for stereo input. HE-AAC v2 uses Parametric Stereo technology and 50% more efficient than AAC-HE v1. AAC-HE v2 should be used for mobile, broadcasting and other domains where CD-quality is not essential.
- ALAC (Apple Lossless Audio Codec) - is the only lossless codec natively supported by iTunes and iOS devices. Users who want to use lossless audio with the support of metadata in iTunes, have to use this codec. Audio files compressed with ALAC are about 40% to 60% the size of the originals depending on the kind of audio.
The list of supported Bit Rate Modes:
- Constant - recommended for constant-bit-rate network transmission when decoding in real-time a fixed end-to-end audio delay. However, due to strict constant bit rate constraint, this mode results in lower audio quality and higher complexity than other encoding modes.
- Average - recommended for controlling file size. Unlike Constant mode, this mode does not provide constant delay when using constant bit rate transmission, but this mode provides almost best global quality while still being able to strictly control the resulting file size. This mode is selected by default.
- Variable - recommended for controlling the audio quality. The audio signal is encoded with constant quality and virtually no bit rate constraints. This is the best mode to achieve consistent audio quality with the lowest overall bit rate.
Bit Rate Mode is inapplicable and has no effect when Apple Lossless Audio Codec is selected.
VBR Quality is applicable only when one of AAC codecs is selected and Bit Rate Mode is Variable. Allows to select algorithm allowing to encode audio with the best quality or with the best speed.
The higher the bit rate, the better the quality, but higher bit rates create larger files. 'Auto' lets the application to calculate the best bit rate of encoded audio depending on the bit rate of source files. ‘Auto’ is selected by default.
Select ‘As Source’ when encoded audio should have the same sample rate as source audio. This is the default preferred setting.
A fixed Sample Rate also can be set from 8000 to 192000 Hz. If the Sample Rate of source audio differs from fixed Sample Rate, the output audio will be resampled.
Select ‘As Source’ when encoded audio should have the same number of channels as source audio. This is the default preferred setting.
A fixed number of channels also can be set to:
- Mono (1 Channel). If the source audio has more than one channel, it will be downmixed to a mono.
- Stereo (2 Channels). If the source audio has more than two channels, it will be downmixed to a stereo.
Select ‘As Source’ when the Bit Depth should be assigned to 16, 24 or 32 bit, depending on the bit depth of source audio. ‘As Source’ is the default preferred setting.
Bit Depth is applicable only when ALAC (Apple Lossless Audio Codec) is selected.
Specifies the value of compression level and selects the algorithm allowing to encode audio with the best compression or with the best speed. The default setting is ‘High’ which means highest compression level is in use.
Compression Level is applicable only when ALAC (Apple Lossless Audio Codec) is selected.
Specifies the extension of output files - mp4 or m4a.
MP4 stands for MPEG-4. MP4 is a format commonly used to store video, audio, subtitles and still images. MP4 is also a filename extension used to represent both audio and video files.
M4A stands for MPEG-4 Audio and is a filename extension to represent MP4 files with audio only. MPEG-4 files can contain any number of audio, video, and subtitle streams, so it is impossible to determine the type of streams in a MPEG-4 file by its extension. As a response to this issue, Apple started using and popularizing the .M4A filename extension. M4A is used for MP4 containers with audio encoded with Advanced Audio Coding (AAC) or Apple Lossless (ALAC) codecs.