MPEG Audio Tutorial
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MPEG Audio Tutorial
1
Outlines
•Basics of Audio Perception
•Basics of Audio Compression
•MPEG-1 Audio
•MPEG-2 Audio
•Dolby AC-3
•Audio Frame
•Other Audio Compression
•AAC
2
Basics of Audio Perception •Humans hear in the range of 20 Hz –20 kHz.•Signals are inaudible if their sound pressure level is below the threshold in quiet, which varies by 60 dB over this frequency range.
•Spectral masking causes louder tones to mask presence of nearby, softer tones.
•The bandwidth, around a masking tone, over which spectral masking occurs is called the critical bandwidth.•Temporal masking causes louder sounds to mask presence of softer sounds immediately before or after the occurrence of the louder sound.
3
Critical Bands
Response
…
20 Hz20 kHz •Critical bandwidths are not fixed; they form in response to stimulus.•Critical bandwidth increases with frequency.
•About 24 critical bandwidths fit in the 20 Hz –20 kHz range.
4
5
Spectral Masking and Threshold in Quiet
Sound Pressure Level
Frequency
(dB)
0102030405060700.02
0.05
0.1
0.2
0.5
1
2
5
10
20
kHz
Threshold in Quiet
Masking Threshold
1 kHz Masking Tone
6
Temporal Masking
Sound Pressure Level
Time
(dB)
30
405060708090-40004080
msec
Sound Event
Post-masking
Pre-masking
7
Compression => Quantization
Sound Pressure Level
Frequency
(dB)
0102030405060700.02
0.05
0.1
0.2
0.512
5
10
20
kHz
Threshold in Quiet
Masking Threshold
1 kHz Masking Tone
•Quantization noise is flat across band
•Quantization noise is not heard if it stays in shaded area
Noise level of (m-2)-bit quantizer Noise level of m-bit quantizer
8
Compression Objective
Sound Pressure Level
Frequency
(dB)
0102030405060700.02
0.05
0.10.2
0.51251020
kHz
Threshold in Quiet
Masking Threshold
1 kHz Masking Tone
•Keep quantization noise level below heavy curve.•Hard to do with single band (shown above).
•Better idea –shape noise level using multiple bands.
Quantization noise level
9
Spectrally Shaped Quantization
Sound Pressure Level
Frequency
(dB)
0102030405060700.02
0.05
0.10.2
0.51251020
kHz
Threshold in Quiet
Masking Threshold
1 kHz Masking Tone
•Split full band into subbands (shown 500 Hz wide above).
•Adjust quantization noise level in each subband to stay below heavy curve.
Spectrally shaped quantization noise