Sign in Subscribe

AAC (Advance Audio Coding)

Last updated on 

Advanced Audio Coding (AAC) is a lossy compression audio format that is designed to offer high-quality sound at lower bitrates. It is an audio coding standard that was developed by the MPEG (Moving Picture Experts Group) as part of MPEG-2 and later as part of MPEG-4. AAC is widely used in applications such as digital television, radio, and streaming services like Spotify and Apple Music.

History of AAC

AAC was first developed in 1997 by a group of researchers from the Fraunhofer Institute for Integrated Circuits in Erlangen, Germany. It was developed as an improvement over the existing audio coding formats like MP3, which had been developed by the same institute.

MP3 had become the de facto standard for music compression by the mid-1990s, but it had some limitations in terms of sound quality at low bitrates. AAC was developed to overcome these limitations and provide a better audio compression format that would be suitable for digital broadcasting and other applications.

In 1999, AAC was adopted as part of the MPEG-2 standard, which was used for digital television and DVD. Later, AAC was included in the MPEG-4 standard, which also included video codecs and other multimedia features. The inclusion of AAC in the MPEG-4 standard helped to popularize it as a format for online streaming and other digital applications.

AAC Encoding Process

The AAC encoding process involves converting analog audio signals into digital data and compressing them using an algorithm. The encoding process involves several stages, including:

  1. Sampling: The analog audio signal is sampled at a fixed rate to convert it into digital data.
  2. Quantization: The digital data is converted into a series of numbers, each of which represents the amplitude of the audio signal at a particular time.
  3. Frequency analysis: The digital data is analyzed to identify the frequency components of the audio signal.
  4. Compression: The frequency components are quantized and then compressed using various techniques such as Huffman coding, psychoacoustic modeling, and entropy coding.

AAC uses a combination of different compression techniques to achieve high-quality sound at lower bitrates. The main techniques used in AAC are:

  1. Huffman coding: This is a technique used to compress the quantized frequency components. It works by assigning shorter codes to the most frequently occurring values and longer codes to less frequently occurring values.
  2. Psychoacoustic modeling: This is a technique that takes into account the characteristics of the human ear and the way it perceives sound. It is used to identify the parts of the audio signal that are most important to human hearing and to allocate more bits to those parts.
  3. Entropy coding: This is a technique that is used to further compress the frequency components by removing redundant data. It works by encoding the frequency components using fewer bits than their actual value.

AAC Bitrates and Quality

AAC can provide high-quality sound at lower bitrates than other audio formats like MP3. The quality of the sound depends on the bitrate used for encoding. Higher bitrates result in better sound quality, but also result in larger file sizes. Lower bitrates result in smaller file sizes but lower sound quality.

The AAC format supports bitrates ranging from 8 kbps to 320 kbps. The recommended bitrate for AAC is 128 kbps, which provides a good balance between sound quality and file size. However, for applications like digital broadcasting and streaming services, lower bitrates are often used to reduce the bandwidth required for transmission.

AAC also supports different sampling rates, including 44.1 kHz, which is the same as CD quality, and 48 kHz, which is used for digital broadcasting and other applications.

AAC vs. MP3

AAC and MP3 are both lossy compression audio formats, but AAC is considered to be a more advanced and efficient format than MP3. AAC uses more advanced compression techniques, such as psychoacoustic modeling, to achieve better sound quality at lower bitrates. This means that AAC can achieve the same sound quality as MP3 at lower bitrates, resulting in smaller file sizes.

AAC also has better support for multichannel audio, which is important for applications like surround sound in movies and games. AAC can support up to 48 channels, while MP3 is limited to only 2 channels.

Another advantage of AAC over MP3 is its ability to support higher sampling rates. AAC supports sampling rates up to 96 kHz, while MP3 is limited to 48 kHz. This makes AAC a better format for applications that require high-quality audio, such as music production and mastering.

One disadvantage of AAC compared to MP3 is its compatibility with older devices and software. MP3 is a more widely supported format, and can be played on a wider range of devices and software. However, as newer devices and software are developed, AAC is becoming more widely supported, and is now the standard format for many digital applications.

AAC in Streaming Services

AAC is the standard format used by many streaming services, including Apple Music, Spotify, and YouTube. Streaming services use AAC to deliver high-quality audio over the internet, while keeping file sizes and bandwidth requirements low.

Streaming services typically use lower bitrates for AAC than recommended for music production or mastering, to reduce bandwidth requirements. For example, Spotify uses a bitrate of 96 kbps for its standard quality stream, and 320 kbps for its highest quality stream. Apple Music uses a bitrate of 256 kbps for its standard and high-quality streams.

AAC is also used for streaming video, as part of the MPEG-4 standard. The AAC audio codec is often paired with the H.264 video codec, which provides high-quality video compression.

Conclusion

AAC is a lossy compression audio format that is widely used in digital broadcasting, streaming services, and other applications. It was developed as an improvement over MP3, with more advanced compression techniques and better support for multichannel audio and higher sampling rates.

AAC provides high-quality sound at lower bitrates than MP3, resulting in smaller file sizes and reduced bandwidth requirements. It is the standard format used by many streaming services, and is becoming more widely supported by devices and software.

Overall, AAC is an important audio format that has revolutionized digital audio, providing high-quality sound in a more efficient and compact format.