Today, AAC-format still has not reached the mass media to spread the sound, but the number of parameters it surpasses all current types of audio compression, and thus worthy of our attention.
What is it?
Let's start with definitions: AAC audio format is proprietary (patented) version of the audio file compression.He also has a smaller loss of quality during coding than MP3 at the same bit rate conditions.Moreover, the format-AAC is a wideband audio coding algorithm that uses two main principles encoding to substantially reduce the number of data required for transmission of high-quality digital audio.This decision is recognized as one of the highest quality, technology implemented lossy compression.The format supports most modern equipment, even portable.Note that the ringtones AAC format is available in iTunes Store, and in this product represents music, compressed only by means of said solutions.It is also necessary to say that the AAC-format was originally created as a successor to MP3, which can provide improved quality coding.The decision was published in 1997 as a new, 7th, part of a family of MPEG-2.
The principle of
When encoding to this format, the following processes: imperceptible components of the signal are removed, cleaned from the encoded audio signal redundancy.The data are processed in accordance with the method according to the complexity of the MDCT.In a next step adds correction codes to the various internal errors.Finally, the signal is transmitted or persists.
Interestingly, the AAC-format has a sampling frequency in the range 8-96 kHz, as well as the number of channels in the interval 1-48.MP3 Hybrid uses a set of filters.In turn, AAC refers to a modified discrete cosine transform at an increased rate of "windows", which reaches 2048 points.
Thus, AAC much more suited to encode the audio stream having the complex pulse and square-wave signals, compared with MP3.The format was the ability to dynamically switch to the block length of MDCT in the range 2048-256 points.In case if there is a short-term or a single shift, applied a small "window" of 256 points in order to achieve better resolution.This is the default 2048-point large "window" to maximize the coding efficiency.AAC has a number of advantages over the usual MP3.Among them we should mention: the realization of a large number of audio channels (up to 48), significant coding efficiency under conditions of constant and variable bit rate and sampling frequency in the range from 8 Hz to 96 kHz (for MP3, this figure is from 8 Hz to 48 kHz)and a more flexible special mode called Joint stereo.As for solutions, "AAC +" - a codec, which is focused on working with low bit rate.It is a combination of SBR and AAC LC, thus achieving good sound in the range of 32-48 kbit / s.