Multimedia synchronization can be divided into two

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Unformatted text preview: PEG audio standard (known as perceptual coding) takes advantage of this perceptual weakness of human ears (pshychoacoustic phenomena). In perceptual coding, the audio spectrum is divided into a set of a narrow frequency bands, called critical bands, to reflect the frequency selectivity of human hearing. Then a filtering mechanism is used to filter coding noise sharply to force it to stay close to the frequency of the frequency components of the audio signal b6ing coded thereby masking out the noise. By reducing the coding noise when no audio is present and allowing strong audio to mask the noise at other times, the sound fidelity of the original signals can be perceptually perceived. The basic steps of MPEG audio (MPEG-1 audio, to be more precise) encoding, shown in Figure 19.14(a), are as follows: 1. The audio signal to be compressed is first transformed from the time domain to the frequency domain. 2. The resultant audio spectrum is then passed through a filter bank, which divides it into multiple sub-bands. 3. The input audio signal is simultaneously subjected to a psychoacoustic model, which exploits auditory masking and determines the signal-to-noise ratio of each sub-band. 4. Each sub-band is then quantized using a quantizer. A bit/noise allocation algorithm is used to determine the number of code bits allocated to quantize each sub-band so that the audibility of quantization noise is minimized. 5. The quantized samples are then formatted into a series of bit streams, which is the encoded bit stream for the input audio signal. As shown in Figure 19.14(b), the decoder performs the inverse operation of the encoder for reconstructing the time-domain audio signal from the encoded bit stream. To provide different quality and different bit rates, the standard supports three distinct layers for compression layer I is the most basic and simplest coding algorithm, and layers II and III are enhancements of layer I. Recently, a new version of MPEG audio (MPEG-2 audio) has been developed for low-bit-rate coding of multichannel audio. It has 5.1 channels of audio, including three front channels (i.e., left, center, right) plus two surrounds, and a lowfrequency bass-effect channel called th...
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This document was uploaded on 04/07/2014.

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