Advanced audio coding

1. The components of the signal that are not perceived by the person are removed.
2. The redundancy in the encoded audio signal is removed.
3. Then the signal is processed by the MDCT method according to its complexity.
4. Correction codes for internal errors are added.
5. The signal is stored or transmitted.

The MPEG-4 container does not require a single or small set of highly efficient compression schemes.

• The MPEG-4 family of audio coding algorithms covers a range from low-quality speech coding (up to 2 kbps ) to high-quality audio (64 kbps per channel and higher).
• AAC has a sampling frequency of 8 to 96 kHz and the number of channels from 1 to 48.
• Unlike the hybrid MP3 filter set, AAC uses a modified discrete cosine transform ( MDCT ) along with an increased window size of 2048 points. AAC is more suitable for encoding audio with a stream of complex pulses and square signals than MP3 .

AAC can dynamically switch between MDCT block lengths from 2048 points to 256.

• If a single or short-term change occurs, a small window of 256 points is used for better resolution.
• By default, a large 2048-point window is used to improve coding efficiency.

• Up to 48 audio channels;
• Greater coding efficiency with both constant and variable bitrate ;
• Sampling frequencies from 8 to 96 kHz;
• Flexible Joint stereo mode;

• Low Complexity (LC-AAC);
• High-Efficiency Advanced Audio Coding (HE-AAC);
• Main Profile;
• Scalable Sample Rate (SSR);
• Long Term Prediction (LTP) is more complex and resource-intensive (but also better) than all the others.

High Efficiency Advanced Audio Coding (AAC +)

AAC + - low bit rate profile. It is a combination of AAC LC, but with a sampling frequency half that of the original, which significantly reduces the bitrate overhead, then the Spectral Band Replication technology is used by predicting it and using some additional information for recovery. Naturally, this approach is not very accurate and is suitable only in cases where it is very necessary to reduce the bitrate.

• .aac - AAC-MPEG2;
• .mp4 is one of the profiles in the MP4 container (the MPEG-4 Part 14 specification provides for packing in a container not only several audio streams, but also several video streams, as well as several subtitle streams).

Despite the fact that the MPEG-4 Part 14 specification ( MP4 file format ) defines only one extension for the container - .mp4, Apple uses the container to pack AAC streams, but uses extensions that are not provided for in the MPEG-4 Part 14 standard:

• .m4a - Standard extension;
• .m4b - AAC file that supports bookmarks; used for audio books and podcasts ;
• .m4p - protected AAC file; It is used to protect the file from copying while downloading copyrighted music legally in online stores, such as the iTunes Store ;
• .m4r - ringtone file used by Apple iPhone and Windows 10 Mobile

• Apple AAC ( MacOS , Windows )
• Fraunhofer FDK AAC ( Linux , MacOS, Windows)
• Nero AAC (Linux, Windows)
• FFmpeg native AAC encoder (Linux, MacOS, Windows)
• FAAC (Linux, MacOS, Windows) ^[3]

On December 1, 2012, AAC was introduced as the state standard for coding audio broadcasting signals with reduced redundancy for transmission over digital communication channels in the Russian Federation . ^[four]

• HE-AAC
• MP3
• Vorbis
• Opus
• Digital Sound Formats