MPEG Audio Compression: Psychoacoustics, Critical Bands, and Masking in ECE160 Lecture 14, Study notes of Electrical and Electronics Engineering

Download MPEG Audio Compression: Psychoacoustics, Critical Bands, and Masking in ECE160 Lecture 14 and more Study notes Electrical and Electronics Engineering in PDF only on Docsity!

ECE Lecture 14 1

ECE160 / CMPS

Multimedia

Lecture 14: Spring 2007

MPEG Audio Compression

ECE Lecture 14 2

Psychoacoustics

• The range of human hearing is about

20 Hz to about 20 kHz

• The frequency range of the voice is typically only

from about 500 Hz to 4 kHz

• The dynamic range, the ratio of the maximum

sound amplitude to the quietest sound that

humans can hear, is on the order of about 120 dB

ECE Lecture 14 4

Threshold of Hearing

• Threshold of human hearing, for pure tones: if a sound is above the dB level shown then the sound is audible
• Turning up a tone so that it equals or surpasses the curve means that we can then distinguish the sound
• An approximate formula exists for this curve:

ECE Lecture 14 5

Frequency Masking

• Lossy audio data compression methods, such as MPEG/Audio encoding, do not encode some sounds which are masked anyway
• The general situation in regard to masking is as follows:
  1. A lower tone can effectively mask (make us unable to hear) a higher tone
  2. The reverse is not true - a higher tone does not mask a lower tone well
  3. The greater the power in the masking tone, the wider is its influence - the broader the range of frequencies it can mask.
  4. As a consequence, if two tones are widely separated in frequency then little masking occurs

ECE Lecture 14 7

Frequency Masking Curve

ECE Lecture 14 8

Frequency Masking Curve

ECE Lecture 14 10

Critical Bands and Bandwidth

ECE Lecture 14 11

Bark Unit

• Bark unit is defined as the width of one critical

band, for any masking frequency

• The idea of the Bark unit: every critical band

width is roughly equal in terms of Barks

ECE Lecture 14 13 Temporal and Frequency Masking

ECE Lecture 14 14 Temporal and Frequency Masking

• For a masking tone that is played for a longer time,

it takes longer before a test tone can be heard.

Solid curve: masking tone played for 200 msec;

Dashed curve: masking tone played for 100 msec.

ECE Lecture 14 16

MPEG Layers

• MPEG audio offers three compatible layers :
  • Each succeeding layer able to understand the lower layers
  • Each succeeding layer offering more complexity in the psychoacoustic model and better compression for a given level of audio quality
  • Each succeeding layer, with increased compression effectiveness, accompanied by extra delay
• The objective of MPEG layers: a good tradeoff

between quality and bit-rate

ECE Lecture 14 17

MPEG Layers

• Layer 1 quality can be quite good - provided a

comparatively high bit-rate is available

• Digital Audio Tape typically uses Layer 1 at around 192 kbps
• Layer 2 has more complexity; was proposed for

use in Digital Audio Broadcasting

• Layer 3 (MP3) is most complex,

and was originally aimed at audio transmission

over ISDN lines

• Most of the complexity increase is at the

encoder, not the decoder - accounting for the

popularity of MP3 players

ECE Lecture 14 19

MPEG Audio Strategy

• Frequency masking : by using a psychoacoustic

model to estimate the just noticeable noise level:

• Encoder balances the masking behavior and the available number of bits by discarding inaudible frequencies
• Scaling quantization according to the sound level that is left over, above masking levels
• May take into account the actual width of the

critical bands:

• For practical purposes, audible frequencies are divided into 25 main critical bands
• For simplicity, adopts a uniform width for all frequency analysis filters, using 32 overlapping subbands

ECE Lecture 14 20 MPEG Audio Compression Algorithm