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FIGURE 27.32 AC-3 encoder.<br />

FIGURE 27.33 ATRAC encoder.<br />

Finally, the dynamic bit allocation algorithm based on maximization of the mask-to-noise ratio MNR i(m)<br />

defined with Eq. (27.60) assign bits to each block and each subband.<br />

Dolby AC-3 Standard<br />

Dolby AC-3 standard gives the possibility for multichannel audio compression (from 1 to 5.1 channels)<br />

[3]. The bit stream ranges from 32 to 640 kb/s. Coding operation in AC-3 format is realized using time<br />

division aliasing cancelation (TDAC) filter bank. The sample block of length 512 transformed into<br />

frequency domain and each sample block is overlapped by 256 samples. Spectral components are represented<br />

in floating-point format and the exponents and mantissas are coded separately. A set of exponents<br />

is coded into the spectral envelope. One AC-3 synchronization frame is composed for six audio blocks<br />

(1536 audio samples). Simplified AC-3 encoder is presented in Fig. 27.32. The psychoacoustic model<br />

used in the AC-3 standard divides the audio band (0–24 kHz) into 50 subbands.<br />

Adaptive Transform Acoustic Coding ATRAC Standard<br />

An adaptive transform acoustic coder (ATRAC) is developed by Sony and is designed for the MiniDisk<br />

system [3]. The bit stream of 16-bit audio signal with sampling rate 44.1 ksamples/s (705.6 kbps) is<br />

reduced into 146 kbps. The input audio signal (512 samples per channel) is decomposed into spectral<br />

coefficients, which are grouped into 52 block floating units (BFUs). The spectral coefficients are normalized<br />

in each BFU, which then are quantized to the specified word length. Using QMF filters, the timefrequency<br />

analysis unit divides input signal into three subbands: 0–5.5 kHz, 5.5–11 kHz, and 11–22 kHz.<br />

Each subband is transformed into the frequency domain using the MDCT. Fig. 27.33 presents the general<br />

scheme of the ATRAC encoder.<br />

© 2002 by CRC Press LLC<br />

PCM<br />

audio<br />

Analysis<br />

Filter<br />

Bank<br />

Mantissas<br />

Mantissa<br />

Quantization<br />

Quantized<br />

Mantissas<br />

Exponents<br />

PCM<br />

audio Time-Frequency<br />

Analysis<br />

Spectral<br />

Envelope<br />

Encoding<br />

Bit Allocation Information<br />

AC-3 Frame Formatting<br />

Bit<br />

Allocation<br />

Spectrum<br />

Quantization<br />

Bit<br />

Allocation<br />

Spectral<br />

Envelope<br />

Parameters<br />

AC-3<br />

Bit Stream<br />

Parameters<br />

Quantized<br />

Spectra

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