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With the popularization of digital audio there is a growing demand for audio compression and audio transmission techniques. The use of perceptual coding (based on the psychoacoustic model) has been a major breakthrough in the compressión of digital audio. But it doesn't solve the problem of multi-channel audio encoding. The evolution of multichannel audio technology has been growing slowly beginning with stereo evolving into 5.1 surround sound systems and then systems of 10 or even more channels. These systems are no longer just for cinema or recording studios, but they are now used in what we call (home cinema). These "home cinema" systems employ Dolby 5.1 which has 5 channels and a sixth channel for the low frequencies. However, there are new applications that use many more channels. However, it seems important to emphasize the fact that all multichannel systems don't have the same behavior. We could talk about two different categories: Category 1 includes: sound films intended to be reproduced in "home cinema 5.1" systems or in the theatrical business. The cross-correlation between channels tends to be high for symmetrical channels (L-Ls, R-Rs, C-R, C-L) but not among the rest of the channels. Category 2 would include those live signals obtained using multiple microphones to capture the acoustic properties of a room. The signals oriented for generate acoustic fields, obtained by a linear grouping of microphones, belong to this group. Such signals have a very high cross correlation between all channels. ==Class 1 signals== To class 1 signals, the system most commonly used today is Dolby Digital. This is used both in cinemas and in home cinema. The system compressión is Dolby AC-3. The distribution of the speakers in Dolby 5.1 is composed of three front channels (left, right and center), two surround channels (left and right) and a channel devoted to strengthening the bass effects. This channel is severely limited to a band of 20 to 120 Hz, while the other five have a frequency response of 20 Hz to 20 kHz, so we are talking about a system of 5.1 channels. The digital audio encoding that is used in the Compact Disc (16-bit PCM) achieves a dynamic range of 96 dB at the expense of working at a frequency 44.1 kHz with samples of 16-bit, which is a lot of data over to be stored or transmitted in a cost-effective manner, particularly in multichannel systems. That is why we need compression algorithm. The Dolby AC-3 achieves compression rates 10:1 also allowed for different bit rates depending on the number of channels encoded or quality required. Dolby AC-3 has been designed to maximize the time and frequency masking characteristic of human hearing. This happens every signal to encode for a bank of filters, distributes bits that will be quantified with the espectral components of different bands in the light of the spectral characteristics of the encoded signal. An internal model that simulates the frequency masking and temporary hearing allows the encoder vary its resolution espectral-temporal depending on the nature of sound, in a way that ensures a minimum number of bits to describe each band signal in ensuring that the noise becomes totally masked. This model makes those masquerading frequency spectral components of the sound that will be masked by other are not encrypted. AC-3 also distributes the bits between the various channels so as to get a bit rate stable, allocating more bits to channels with a higher frequency content. The algorithm AC-3 considers the six channels as a single entity by adding a single bit frame, which gets a bit rate less than separating each channel in a different frame. The most important blocks of this algorithm are the following ones: * Buffer-entry. * Filtering * Detector transient * Precombinación carrier 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Multichannel code」の詳細全文を読む スポンサード リンク
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