Introduction to MPEG Surround

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Introduction to MPEG Surround

• ???
• 2/9/2005

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Outline

• Background
• Motivation
• Perception of sound in space
• Pricicple of MPEG Surround
• Downmixing to one channel
• Estimation of spatial cues
• Synthesis of spatial cues
• Conclusions Reference

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Motivation

• The vast majority of audio playback equipment use
  traditional two-channel presentations (stereo)
• More reproduction channels (multi-channel audio
  or surround sound) is quite visible in the
  market place
• A non-disruptive transition from stereo to
  multi-channel audio requires media formats that
  can serve both those using conventional stereo
  equipment and those using next-generation
  multi-channel equipment.

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Perception of sound in space

• HRTF(Head Related Transfer Function) modeling the
  path of sound from a source to the left and right
  ear entrances.

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Perception of sound in space(cont.)

• Three parameters(cues) describing how human
  localize sound in the horizontal plane
• Interaural level difference (ILD)
• Interaural time difference (ITD)
• Interaural coherence (IC)

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ITD (Interaural time difference) ILD
(Interaural level difference)
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ITD (Interaural time difference) ILD
(Interaural level difference) (cont.)

• ITD and ILD between a pair of headphone signals
  determine the location of the auditory event
  which appears in the frontal section of the upper
  head.

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IC (Interaural coherence)

• The spatial impression of the auditory enent is
  related to IC

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Two sound source Summing localization

• Inter-channel time difference (ICTD)
• Inter-channel level difference (ICLD)
• Inter-channel coherence (ICC)

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Two sound source Summing localization (cont.)
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MPEG Surround

• MPEG Surround exploits inter-channel differences
  in level, phase and coherence equivalent to the
  ILD, ITD and IC cues to capture the spatial image
  of a multi-channel audio signal
• Downmix signal and encodes these cues in a very
  compact form such that the cues and the
  transmitted signal can be decoded to synthesize a
  high quality multi-channel representation.
• Provide backward compatibility with stereo/mono
  audio systems.

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Coding Scheme
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Downmixing to one channel (1/2)

• The sum signal is generated by adding the input
  channels in a subband domain
• Multiplying the sum with a factor in order to
  preserve signal power

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Downmixing to one channel (2/2)
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Estimation of spatial cues (1/4)

• The spatial cues, ICTD, ICLD, and ICC are
  estimated in a subband domain. The spatial cue
  estimation is applied independently to each
  subband

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Estimation of spatial cues(2/4)

• ICTD (samples)with a short-time estimate of
  normalized cross-correlation functionwhere
  and is a short-time estimate of
  the mean of

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Estimation of spatial cues(3/4)

• ICLD (dB)
• ICC

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Estimation of spatial cues(4/4)

• For multi-channel audio signals, ICTD and ICLD
  are defined between the reference channel and
  each other C-1 channels

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Synthesis of spatial cues(1/3)

• ICTD are synthesized by imposing delays, ICLD by
  scaling, and ICC by applying de-correlation
  filters.

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Synthesis of spatial cues(2/3)

• The delays are determined by the ICTDs

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Synthesis of spatial cues(3/3)

• The scale factors are determined by the ICLDs
  satisfying
• After delays and scaling, we need to reduce
  correlation between the subbands.This is
  achieved by designing the filters hc controlled
  as a function of ICC.

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Conclusions (1/2)

• Well-known perceptual audio coders, such as MP3,
  primarily exploit a single channels ability to
  mask its own quantization noise.
• In contrast, spatial perception is primarily
  attributed to three parameters ILD, ITD, and
  IC.

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Conclusions (2/2)

• MPEG Surround provides an extremely efficient
  method for coding of multi-channel sound via the
  transmission of a compressed stereo (or even
  mono) audio program plus a low-rate
  side-information channel.
• MPEG Surround is the latest technology for
  bitrate efficient and backward compatible
  presentation of multi-channel audio.

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Reference

• ISO/IEC JTC1/SC29/WG11 (MPEG), Document N7390,
  Tutorial on MPEG Surround Audio Coding , July
  2005, Poznan, Poland
• C. Faller, Parametric coding of spatial audio,
  in Proc. DAFx (Digital Audio Effects), October
  2004.