What methods are used to compress digital audio effectively?
Currently, the most famous are Audio MPEG, PASC and ATRAC. All of them
use the so-called “perceptual
encoding) in which information is removed from the sound signal,
perceptible to the ear. As a result, despite the change in shape and spectrum
signal, your hearing perception is practically unchanged, and the degree
Compression accounts for the slight reduction in quality. Such encoding
refers to lossy compression methods, when
it is no longer possible to accurately reconstruct the original waveform from the compressed signal
shape.
The techniques to eliminate part of the information are based on the characteristics of the human being.
who to listen to, called masking: if there is a high
strong peaks (dominant harmonics) weaker frequency content
hear in the immediate vicinity of them practically no
accepted (masked). When encoding, the entire audio stream is divided
is divided into small squares, each of which becomes a spectral
presentation and is divided into several frequency bands. Within the stripes there are
performs the definition and removal of masked sounds, after which each frame
it undergoes adaptive coding directly in spectral form. All
these operations can significantly reduce (several times) the volume
data while maintaining acceptable quality for most listeners
I read.
Each of the encoding methods described is characterized by a bit rate
the bitrate with which the compressed information should come
on the cable box when the audio signal is restored. Decoder converts
a series of instantaneous signal spectra compressed into a conventional digital waveform
shape.
MPEG Audio – A group of MPEG standardized audio compression methods
(Moving Pictures Experts Group – a group of experts to process motion
images). MPEG audio methods exist in various
types – MPEG-1, MPEG-2, etc .; currently the most common
not MPEG-1 type.
There are three layers of MPEG-1 audio for stereo compression.
your signals:
1 – 1: 4 compression ratio with a data stream of 384 kbps;
2-1: 6..1: 8 at 256..192 kbps;
3 – 1: 10..1: 12 at 128..112 kbps.
The minimum data rate in each layer is defined as 32
kbps; specified bit rates maintain signal quality
roughly at the level of a CD.
All three levels use the input split spectral transformation
changing the frame in 32 frequency bands. The most optimal in relation
data volume and sound quality recognized as level 3 with bit rate
128 kbps and a data density of approximately 1 Mb / min. When compressed from a bottom
at what speeds the forced limiting of the frequency band starts to
15-16 kHz, and channel phase distortions also occur (effects such as
phaser or flanger).
MPEG audio is used in computer sound systems, CD-i / DVD,
CD-ROM “audio”, digital radio / television and other systems
massive sound transmission.
PASC (Precision Adaptive Subband Coding – Precise Adaptive Intraband
coding) – a special case of Audio MPEG-1 Layer 1 with a speed
Stream 384 kbps (1: 4 compression). Used in the DCC system.
ATRAC (Adaptive TRansform Acoustic Coding – acoustic coding
adaptive transformation) is based on stereophonic sound
16-bit quantized format with a 44.1 kHz sample rate.
When compressed, each frame is divided into 52 frequency bands, resulting in
transmission speed: 292 kbps (1: 5 compression). Applied in the system
