Glossary
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Alphabetical list of terms
This is not intended to be a full glossary of
ambisonic terms —that would run to hundreds of entries.
Rather it aims to explain the terms used in the pages of this
section of this website, and where necessary, compare and contrast
them with other usages.
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Further terms, and further meanings for some of the terms
listed here, are in the deprecated terms
section, below.
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- ambisonic channel number
- An ambisonic file or stream consists of a collection of
ambisonic channels. These have
traditionally been described by letters: all ambisonic files/streams
contain what were described as W, X and Y. Most files/streams
contain other channels as well.
This letter code system cannot be extended to higher ambisonics,
instead a unique integer (from zero upwards) is assigned to
designated each channel.
That integer (the ambisonic channel number) is related to the
degree (l) and
order (m) as follows:
ACN = l * ( l + 1) + m
(‘Ambisonic component number’ has
been discussed as an alternative term for this.)
- ACN
- See ambisonic channel number.
- channel
- Just as a stereo file or a stereo audio stream has a left channel
and a right channel, so ambisonics uses multi-channel files/streams.
The term ‘signal’ is sometimes used with the
same meaning.
- degree
- Each spherical harmonic is defined by an degree
and an order. There are (potentially)
an infinite number of degrees, numbered by integers, from
zero upwards.
The symbol l is used for the degree
number.
Ambisonic channels are each associated
with a specific spherical harmonic and can be designated by
the same numbers as the associated SH.
- deprecated term
- See below.
- l
- Symbol for degree.
- m
- (1) Symbol for order (meaning (1)).
(2) see also the deprecated usage of
m.
- order
- (1) Each spherical harmonic is defined by a
degree
and an order. For three-dimensional SHs (periphonic
ambisonics) the number of orders in each degree is one more
than twice the degree (2l+1) and these are designated
by numbers from m = -l to m = l
The symbol m is used for the order
number.
Ambisonic channels are each associated
with a specific spherical harmonic and can be designated by
the same numbers as the associated SH.
(2) An ambisonic file (or stream, etc.) is described
as being first order if it contains (only) zero and first degree
channels; second order if it contains (only) zero, first and second
order channels; etc. (A zero order ambisonic file is -it could be
argued- possible: it would be an omnidirectional recording (or
synthesised piece).)
(3) see also the deprecated usage of
order.)
Deprecated terms and meanings
- m
- Some texts use m as a symbol for degree.
- n
- Some texts use n as a symbol for order
(meaning (1)).
- order
- This term has been used for some time, with two distinct meanings
in ambisonic literature. Firstly to refer to the total collection
of channels, a meaning retained in this glossary.
Secondly to refer to the components in that total collection of
channels that are of that degree.
This glossary deprecates the latter, and uses the term degree
(which is common across the rest of the literature on SHs). It
continues the usage for the totality of channels: for example,
“a third order file”, “a second order microphone”,
“a first order recording”.
See also the discussion below.
- range
- The term order (meaning (1)),
is now preferred.
Discussion
In ambisonics a first-order file has channels W, X, Y and
(possibly) Z (channel numbers 0, 1, 3 and, possibly, 2). That is it
has components which satisfy both l = 0 and l = 1.
Chris Travis has suggested that the word ‘order’ be
retained for usages such as ‘a first-order file’,
‘a second-order microphone’, etc.
Personally, I will try not to say “It is a second-order component” when
I mean “It is a degree-2 component”. But I will continue to talk about
e.g. “second-order” microphones and recordings. And I think “Higher
Order Ambisonics” is OK too.
but that for the other usage (e.g. ‘X is a first-order
component’) the more generally accepted term of degree is
used (e.g. ‘X is a first-degree component’).
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Copyright:
This page copyright © 2008 The Ambisonics Association.
Acknowledgements:
Grateful thanks to Fons Adriaensen and Philip Cotterell for their
encouragement … and corrections.
Published: October 2008. Revised: October 2008.