Ambisonics

Ambisonics Logo. See http://en.wikipedia.org/wiki/Ambisonics#/media/File:AmbisonicLogo.svg

Ambisonics is a method of codifying a sound field taking into account its directional properties. Instead of every channel having the signal what every loudspeaker should be emitting, as in stereo or 5.1 surround, every Ambisonics channel has information about certain physical properties of the acoustic field, such as the pressure or the acoustic velocity.

In the two related lectures we will review the Ambisonics theory, going from the recording and encoding to the exhibition, passing by the transmission and format codification. We will mostly concentrate on first order Ambisonics, although we will also make a brief introduction to higher order Ambisonics (HOA).

Since it is relatively difficult to find good introductory material about Ambisonics, you can find some preliminary lecture notes at:

Lecture notes on Ambisonics (version 0.3)

This material has been elaborated by Daniel Arteaga and published under Creative Commons Attributtion-ShareAlike 4.0 International License.

Many things are missing or can bee improved, like a better list of references, historical remarks, a better exposition of concepts, et a revision of the English, typographical revision, etc. Additionally, it will probably contain errors. Use it under your own risk!

Update 9/06/2015: Lecture notes have been updated with some minor corrections.

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Stereo and multi-loudspeaker reproduction

Some notions before starting:

from http://stereos.about.com/od/introductiontostereos/a/soundformats.htm

Monophonic Sound

Monophonic sound is sound created by one channel or speaker and is also known as Monaural or High-Fidelity sound. Monophonic sound was replaced by Stereo or Stereophonic sound in the 1960s.

Stereophonic Sound

Stereo or Stereophonic sound is created by two independent audio channels or speakers and provides a sense of directionality because sounds can be heard from different directions.

The term stereophonic is derived from the Greek words stereos, which means solid and phone, which means sound. Stereo sound can reproduce sounds and music from various directions or positions the way we hear things naturally, hence the term solid sound. Stereo sound is a common form of sound reproduction.

Multichannel Surround Sound

Multichannel sound, also known as surround sound, is created by at least four and up to seven independent audio channels or speakers placed in front of and behind the listener that surrounds the listener in sound. Multichannel sound can be enjoyed on DVD music discs, DVD movies and some CDs.

This Lecture we describe the principles of two channel stereo, analise the most common configurations for Multichannel reproduction and briefly describe the most used Stereo Recording techniques.

An detailed overview is depicted below:

  1. Introduction
  2. Two loudspeaker Stereo – More info in [1,2,3]
    1. Two channel (2-0) stereo
      1. Basic principles of loudspeaker stereo: ‘Blumlein Stereo’
      2. Cross-Talk
      3. Basic principles of loudspeaker stereo
      4. Intensity Stereo
      5. Time Difference Stereo
    2. Basic two-channel signal formats
    3. Limitations of two-channel loudspeaker stereo
  3. Multichannel stereo and surround systems – More info in [1]
    1. Three channel stereo (3-0)
    2. Four-channel surround (3-1 stereo)
    3. Channel Surround (3-2 stereo)
    4. Other multichannel configurations
      1. (7.1 channel surround)
      2. (10.2 channel surround)
  4. Surround Sound Systems – More info in [1]
  5. Matrixed surround sound systems – More info in [1]
    1. Dolby Stereo, Surround and Prologic
    2. Circle Surround
    3. Lexicon Logic 7
    4. Dolby EX
  6. Digital surround sound formats – More info in [1]
    1. Dolby Digital
    2. MPEG
  7. Stereo Recording Techniques – More info in [3, 4]
    1. X-Y technique
    2. A-B technique
    3. ORTF technique (Mix technique)

References:

[1] F. Rumsey and T. McCormick – Sound and recording (Chapter 3 and 4)

[2] V. Pulkki “Compensating displacement of amplitude-panned virtual sources.” Audio Engineering Society 22th Int. Conf. on Virtual, Synthetic and Entertainment Audio pp. 186-195. 2002 Espoo, Finland

[3] Bennett et al. – A new approach to the assessment of stereophonic

[4] Bruce Barlett, Jenny Barlett – On Location Recording Techniques

[5] http://en.wikipedia.org/wiki/Microphone_practice

Spatial Audio Psychoacoustics

From [2]

Most research into the mechanisms underlying directional sound perception conclude that there are two primary mechanisms at work, the importance of each depending on the nature of the sound signal and the conflicting environmental cues that may accompany discrete sources. These broad mechanisms involve the detection of timing or phase differences between the ears, and of amplitude or spectral differences between the ears. The majority of spatial perception is dependent on the listener having two ears, although certain monaural cues have been shown to exist – in other words it is mainly the differences in signals received by the two ears that matter.

In this lecture we cover issues related to the perception and cognition of spatial sound as it relates to sound recordings and reproduction. The overview of the class is as follows:

  1. 3D Sound and Spatial Audio
  2. Important terms
  3. Geometric convention
  4. Introduction to sound localization
  5. The minimum audible angle (MAA)
  6. Acoustic cues used in localization
  7. Measurements
  8. Subjective Attributes of Spatial Sound (please read **, pages from 35-39 )
  9. Conclusions

More info at:

[1] A.Gelfand – Hearing: an introduction to psychological and physiological acoustics (Chapter 13)

[2] F. Rumsey and T. McCormick – Sound and recording (Chapter 2) **

Even more ….

[3] G. Kendall – A 3-D Sound Primer: Directional Hearing and Stereo Reproduction

[4] W. Yost – Fundamentals of hearing : an introduction

[5] J. Blauert – Spatial hearing : the psychophysics of human sound localization 

[6] B. Moore – An introduction to the psychology of hearing

Binaural Reproduction using Loudspeakers

Can we listen to binaural recordings through normal speakers?

Of course. Even when binaural recordings for headphones still sound fantastic on speakers – you just won’t be able to get the 3D effect. That’s because as the sound travels through room, the left and right channels mix and your brain can’t make sense of the directional cues (cross-talk). Various works “has been/are being” developed to cancel this crosstalk and thus, allow proper 3D listening on speakers.

In this lecture, we revise the principles of Transaural recordings and learn the basis of CrossTalk Cancellation proposed by Schroeder. The contents of the lecture and the related references are depicted below:

1.- Introduction

2.- Crosstalk Introduction

3.- CrossTalk Cancellation

4.- Conclusions

More info at:

Binaural Reproduction using Headphones

Binaural recordings are reproductions of sound “the way human ears hear it”. Actually, the word “binaural” literally just means “using both ears.” When you listen to a binaural recording through headphones, you perceive distinct and genuine 360° sound.

It’s the purest, most natural way to record and listen to music.

In this lecture, we cover the following topics about Binaural recording using Headphones. Further information can be found in the related references.

1.- Introduction to Binaural Audio

2.- The Stereo Reproduction of 3D Sound

3.- Headphone Reproduction: Quick Review

4.- Binaural Principles

    4.1.- Problems of Binaural Systems

5.- Binaural Recording

     5.1.- Measurement of Binaural IR

     5.2.- Excitation Signal

     5.3.- Using DSP for HRTFs

     5.4.- Collecting HRTF Measurements

     5.5.- Equalization

     5.6.- Data Reduction of HRTFs

     5.7.- Head Tracking