Polycom C16 - Manual (Page 45)

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SoundStructure Design Concepts
Before creating designs for the SoundStructure devices, the concepts of
physical channels, virtual channels, and virtual channel groups will be
introduced. These concepts form the foundation of SoundStructure audio
designs. In addition, the concepts of defining control virtual channels and
control array virtual channels from the logic input and output pins will be
introduced.
Introduction
All audio devices have inputs and outputs that are used to connect to other
devices such as microphones and audio amplifiers. These inputs and outputs
are labeled on the front or rear-panel (depending on the product) with specific
channel numbers, such as inputs 1, 2, 3, etc., and these labels refer to particular
inputs or outputs on the device. For instance, it is common to connect to input
“1” or output “3” of an audio device. This naming convention works well --
meaning that it provides a unique identifier, or name, for each input and
output -- as long as only a single device is used. As soon as a second device is
added, input “1” no longer uniquely identifies an input since there are now
two input ”1” ’s if a system is made from two devices.
Traditionally, to uniquely identify which input “1” is meant, there’s additional
information required such as a device identification name or number,
requiring the user to specify input “1” on device 1 or input “1” on device 2 in
order to uniquely identify that particular input or output. This device
identification is also required when sending commands to a collection of
devices to ensure the command affects the proper input or output signal on the
desired device.
As an example, consider what must happen when a control system is asked to
mute input 1 on device 1. The control system code needs to know how to
access that particular input on that particular device. To accommodate this
approach, most audio systems have an API command structure that requires
specifying the particular device, perhaps even a device type if there are
multiple types of devices being used, and, of course, the particular channel
numbers to be affected by the command. This approach requires that the
designer manually configure the device identification for each device that will