Audio system design
has been an
As new technologies
emerge they add to
and complement what’s
come before, or replace
Analog transitioned to
digital. Single signal per
cable wiring shifted to multiple multiplexed
signals such as AES3 and MADI. Mixers with
dedicated inputs and outputs have morphed
into mixing control surfaces that act on
sources wired to a central electronics hub.
Distribution amplifiers are being replaced
by Ethernet switches to allow multiple control
surfaces and destinations access to any
signals within a closed network.
The next step in this evolution looks
to be audio-over-IP, or rather more
of it. With the publication of AES67-2013,
“AES standard for audio applications of networks—
High-performance streaming audio-
over-IP interoperability,” in September
2013, greater opportunities for integrating
IP technology are forecast.
Such was the sentiment expressed at
the DTV Audio Group Forum held in New
York in December. The forum presented an
overview of AoIP, how it compares to other
interconnection technologies, and application
“Looking forward, AES67 is enabling
where the future is going,” said Greg Shay,
chief science officer, Telos Alliance, and one
of the DTV Audio Forum’s presenters. “It’s
not just an interface technology, but how
you interconnect and define a facility. We
want the studio to fit into the whole world.”
While high-performance AoIP technology
and equipment have been around for at
least a decade, my impression is that these
systems have been more prevalent in radio
than TV installations, and as a way of distributing
audio within closed systems.
A stumbling block for more extensive
applications was that systems made by
different manufacturers weren’t compatible
with each other. Similar, yes; IP-based,
yes; but not close enough. AES67 should
change that, and if the standard is followed,
then different manufacturers’ AoIP gear
should be able to communicate with each
other and pass and interpret AoIP packets
from one system to another, both inside a
facility, and outside to wide area networks.
This interoperability with its potential
for outside-world connectivity is the key
to expanding AoIP opportunities for broadcast,
TV, audio and video post and music recording
facilities, TV trucks, stadiums and
other venues. Facilities can be geographically
apart, yet bound together virtually.
Or be closer, such as a mobile production
truck parked outside of a stadium.
AES67 was developed for what’s considered
high-performance audio, that is, at
least 16 bit/44.1 kHz full-bandwidth digital
audio with a low latency, less than 10 msec.
The standard covers synchronization, media
clocks, transport, quality of service, encoding
and streaming and session description.
“The interoperability of AES67 is based
on 10 years of industry experience,” Shay
said. For AES67, no new protocols were devised.
It is based on existing protocols and
standards from organizations such as AES
and IEEE, and on the Open Systems Interconnect
(OSI) model of network architecture
using abstract or virtual layers.
The layer structure needs some explanation to understand how AES67 AoIP compares
with Ethernet-based technologies.
Andreas Hildebrand, senior product manager
at ALC NetworX GmbH, provided a brief
overview at the DTV Audio Forum.
Layer 1, the lowest layer, is the physical layer,
which can be a copper or fiber connection
over which data is transferred between a
transmitting device and receiving device.
(Radio-link connections are covered by this
Layer 2 is the data link layer, the layer used
by Ethernet-based technologies like AVB
(Audio Video Bridging) and TDM (time division
AES67 uses Layer 3 for AoIP. This is the network
layer, with an IP-based suite of protocols
for packet forwarding between networks
(including the internet). According
to the standard, Layer 3 “is responsible for
packet forwarding and routing of variable-length
data sequences from a source to a
AES67 also uses Layer 4, the transport layer,
which, according to the standard, “provides
end-to-end communications between devices
on a network. The layer handles issues of
packet loss and reordering and implements
multiplexing so that a single network connection
can serve multiple applications on
the end station.” The standard calls for realtime
transport protocol (RTP) for AoIP.
There are three other OSI Layers—5, 6
and 7—but they aren’t relevant here.
Each layer is “stacked” on top of the
next lower layer, with data passing down
in a specific way from one layer to the next
lower one. But devices on a given layer can
virtually communicate with each other.
As Hildebrand explained, Layer 1 systems
tend to be proprietary, mostly based on fiber
or copper connections, using point-to-point
or chain topologies. Switches tend to be custom-
built, devices tend to be fixed, with limited
channel capacity and support for selected
media formats. Because of their proprietary
nature, these systems are typically based on
a single manufacturer’s products, but for the
same reason are generally quite rugged and
have very low latency.
Layer 2 systems also tend to be proprietary.
These are Ethernet-based, with channel
capacity determined by the bandwidth of
the Ethernet channel. Devices are connected
in a local area network, but “you can’t cross
out of the LAN,” Hildebrand said. AVB systems
fall into this category. “You need to have an
AVB switch,” Hildebrand said. “If you stay inside
the AVB cloud, all data is preserved by
built-in quality of service mechanisms. [But]
none of the AVB streams can pass across a
non-AVB bridge to another cloud.”
For Layer 3 systems, Hildebrand said that
although there are proprietary systems, most
are based on standardized Internet protocols
supported by standard switches. This allows
networks to connect to other networks.
In this layer, “the size of the network is not
limited,” Hildebrand said. It is “flexible, scalable,
with a flexible choice of media format.
Latency varies depending on the size of the
network and the utlized payload format.”
As mentioned, AES67 was developed to
have low latency, less than 10 msec. Clocking
and synchronization, packet size and other
encoding and streaming criteria all play a
role. These will be discussed next time.
Mary C. Gruszka is a systems design engineer
and consultant based in New York.
She can be reached via TV Technology.