Chapter 12 MPEG-2 Audio

Index

Goal MPEG-2 LSF , MPEG-2.5 Mutichannel MPEG-2 BC

Goals

Address low sampling frequencies Low data rate over internet MPEG-2 LSF , MPEG-2.5

Muti-channel extensions MPEG-2 BC

MPEG-2 LSF , MPEG-2.5

Sampling rate MPEG-2 LSF: 24 , 22.5 , 16 kHz MPEG-2.5 : 12 , 11.025 , 8 kHz

MP3 MPEG-2.5 Layer3 extensions

其餘與 MPEG-1 大都相同 Bitstream & coder

MPEG-2 LSF , MPEG-2.5

Fig.1 p.316 ID

00: MPEG-2.5 01: reserved 11: MPEG-1 10: MPEG-2 LFS

Muti-channel

5.1 channel configuration (3/2/.1) 5 full channel – 20kHz

3 – front 2 – side & rear

1 frequency enhancement channel , LFE – 200Hz Enable high-level sounds at frequencies below 200Hz

See Fig.2 p.319

Muti-channel

Challenge Minimize data rate without sacrificing audio quality

In order to increases PCM sample precision High sampling rate : 44.1 48 96 192 kHz Long sample word length: 16 - 24 bits See table 2 p321

MPEG-2 BC

Backwards compatible with MPEG-1 Provide mono- and two-channel info concurrently

with muti-channel X: need large data capacity

Embrace backward matrixing technique MPEG-2 BC O: low data capacity ( 與上面比較 ) X: constrains the design of coder X: unmasking of quantization noise on dematrixing stage

?? 我看不懂 在 p323 倒數最後一段

MPEG-2 BC

Matrixing Lo = c ( L + a C + b Ls ) Ro = c ( R + a C + b Rs ) a = b = 1/sqrt(2) , c = 1 / ( 1 + sqrt(2) )

Fig.3 p323

MPEG-2 BC

Bitstream Format Fig 4 5 6 7 p325 p326 課本也沒多說

The mutichannel field is stored in the ancillary data portion of the mpeg-1 bitstream

是否用 extension part : If mutichannel data rate exceeds the MPEG-1 dat

a rate Ext Bitstream Present bit: fig 6

MPEG-2 BC

Mutilingual channels Fig 9 p327:

就只是把 mutilingual 也放進 encoder 裡 最多支援 7 個

MPEG-2 BC

Encoding Process Fig 10 p.329 幾乎跟 MPEG-1 相同 Matrixing Transmission channel allocation Dynamic crosstalk

如何把聲音分佈到不同的聲道去