What is the AAC format?

To date, the AAC-format has not yet reached mass distribution on audio media, but it surpasses all existing types of audio compression by a number of parameters, which means it is worthy of our attention.

What it is?

Let's start with the definition: AAC audio format is a proprietary (patented) version of audio file compression. At the same time, it has less quality loss during encoding compared to MP3 under the same bitrate conditions. In addition, the AAC format is a broadband algorithm for audio encoding, using two main coding principles to significantly reduce the amount of data that is required to transmit high-quality digital audio. This solution is recognized as one of the most high-quality, compression-based technologies with losses. The format supports most modern equipment, even portable. It should be noted that ringtones in AAC format can be purchased in the iTunes Store, and in this store music is represented, compressed only with the help of this solution. It must also be said that the AAC format was originally created as an MP3 successor, which can provide an improved encoding quality. The solution was published back in 1997 as a new, 7th, part of the MPEG-2 family.

Principle of operation

When encoding into this format, the following processes are performed: unrecoverable components from the signal are deleted, the encoded audio signal is cleared of redundancy. After that, the data is processed in accordance with the MDCT method according to their complexity. In the next step, codes are added to correct various internal errors. Finally, the signal is transmitted or stored.

All the details

It is interesting that the AAC-format has a sampling frequency within 8-96 kHz, as well as the number of channels in the segment 1-48. MP3 uses a hybrid filter set. In turn, AAC refers to the Modified Discrete Cosine Transformation with the increased size of the "window", which reaches 2048 points.

Thus, AAC is much more suitable for encoding audio having a stream of complex pulses, as well as rectangular signals, compared to MP3. The format has the ability to dynamically switch in MDCT block lengths within 2048-256 points. In the case of a short or a single shift, a small "window" of 256 points is used to achieve a better resolution. At the same time, by default a 2048-point large "window" is used to maximize the efficiency of encoding. AAC has a number of advantages over the usual MP3. Among them it should be noted: the realization of a large number of audio channels (up to 48), significant coding efficiency under conditions of constant and variable bit rate, as well as the sampling frequency in the range from 8 Hz to 96 kHz (in MP3 this figure is 8 Hz to 48 kHz) And a more flexible special regime called Joint stereo. As for the solution, "AAS +" is a codec that is oriented to work with low bitrate. It is a combination of SBR and AAC LC, which achieves good sound in the range of 32-48 kbps.