Signal Processing - May 2017 - 42

Surround sound is the more commonly known name for
multichannel stereophony. There exist several reproduction setups,
such as 5.1, 7.1, 10.2, and 22.2, which use five, seven, ten, and 22
main channels, respectively, and one or two low-frequency channels, as described in an International Telecommunication Union
(ITU) report (ITU-R BS.2159-4). There are also commercial,
object-based formats such as Dolby ATMOS (http://www.dolby
.com /us/en / brands/dolby-atmos.html), DTS-X (http://
dts.com/dtsx), and Auro-3D (http://www.auro-3d.com), which are
very flexible and are likely to dominate the cinematic sound
industry into the foreseeable future, considering the new International Organization for Standardization/International Electrotechnical Committee standards such as MPEG-D and MPEG-H.
Commercial microphone arrays for multichannel recording
exist, but these arrays are based more on practice in the field than
on a solid theory and understanding of the underlying acoustic
processes. The microphone arrays used for recording 5.1 multichannel audio typically include cardioid, supercardioid, or
hypercardioid microphones positioned on a tree structure [34],
[35]. These arrays can in general be separated into two groups:
1) five-channel main microphone techniques and 2) front-rear
separation techniques. The former uses five closely positioned
microphones mapped directly to the five main channels of a
5.1 reproduction setup. The latter uses two separate arrays to
record direct field and ambience separately. For some arrays
(such as the INA-5 [36]), there is a one-to-one correspondence
between the microphone and loudspeaker channels. For some
other arrays (such as the Soundfield Microphone [37], Fukada

0.175 m

1m 1m 1m

Tree [38], or Hamasaki Tree [39]), the signals obtained from
individual microphone channels need to be mixed.
Some well-known multichannel arrays used for recording multichannel audio are shown in Figure 2. It may be
observed that a variety of microphone arrangements exist
that try to address the common objective of obtaining an
authentic auditory perspective and a high level of envelopment and immersion using existing first-order microphone
directivity patterns. The microphone arrays for recording
10.2 multichannel stereophony are still rather experimental
(see the ITU-R BS.2159-4 report). Similarly, recording for a
22.2 reproduction system will depend strongly on the venue
and context. In fact, multichannel stereophonic systems with
higher channel counts, by virtue of the degrees of design
freedom they provide, allow for more flexibility but also
make it more difficult to design recording setups with strict
perceptual rationale.
Recommended reproduction setups for multichannel systems
are either standardized (e.g., ITU-R BS.775-1) or in the process of
standardization by different standardization bodies [40]. These
setups rely mainly on the frontal channels for the presentation
of audio content that accompanies visual content (usually films
or games). The left and right front channels typically correspond
exactly to the two-channel stereophonic setup for cross- and
backward compatibility. The difference in these setups is mainly
about how ambience is played back. Some of the standards, such
as ITU-R BS.1116.1 and ITU-R BS.1534.1, define formal procedures for the subjective evaluation of these systems.

Omnidirectional

Direction of the
Direct Sound

0.175 m

Figure Eight
Cardioid
Hypercardioid

1m
<1.8 m

0.6 m

0.70 m
Optimal Cardioid
Triangle

2m
Fukada Tree
Ideal Cardioid
Arrangement
(INA-5)
Soundfield
Microphone

<0.15 m

2m
1.5 m

0.3 m

1.5 m

2m
Decca Tree

1.5 m
2-3 m

Hamasaki Tree

DPA 5100

Supercardioid

0.08 m

0.27 m

2m
Hamasaki Square

0.2 m

0.92 m
Ideal Cardioid
Arrangement (INA-3)

0.25 m
IRT Cross

(b)

(c)

(a)

FIGURE 2. Some different microphone array configurations for recording 5.1 multichannel audio showing (a) complete arrays, (b) arrays used to record
the frontal scene, and (c) rear-only arrays. The dimensions of the arrays are not drawn to scale. IRT: Institut für Rundfunktechnik.

IEEE Signal Processing Magazine

May 2017

http://www.dolby http://http:// http://www.dts.com/dtsx http://www.auro-3d.com

Table of Contents for the Digital Edition of Signal Processing - May 2017