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DACS App Note

DACS TDM Ring Network (#9)


ASTi's Remote Interface Unit (RIU) is a low-cost, self-contained DSP sub-system that provides digital-to-analog and analog-to-digital conversion of audio and control signals at remote operator locations in radio and intercom simulator systems. RIUs work in connection with ASTi's Digital Audio Communications System (DACS) and are controlled by ASTi's Model Builder software. Model Builder provides a high-level, object-oriented environment for setting up and running RIUs. Through an intuitive software user interface you can control such parameters as: DSP sampling rates, audio channel assignments, digital IO assignments, audio channel mixing. The internal operation of the RIUs can also be monitored through Model Builder.

Communication between a centrally located DACS platform and RIUs is via high-speed time division multiplex ring networks (TDM Rings). A TDM Base module, resident in the DACS, provides TDM network control and provides real-time mixing of any combination of RIU audio channels as well as synthesized sounds and archive sound files from the system hard disk. Radio effects, such as noise injection, fade, frequency matching and squelch, are also handled. Multiple TDM Rings can be connected to a single DACS and each TDM Ring can be populated with multiple RIUs, providing interface for a large number of remote operator stations.

RIUs mount adjacent to radio control heads and other operator panel equipment so that digital audio conversion and digital IO is accomplished locally. Important benefits are realized from this approach.

Audio and control signals are digitally distributed to remote locations - these digital signals have far greater immunity to electro-magnetic interference (EMI) and line-loss degradation than line level audio signals. This benefit makes ASTi's TDM / RIU technology particularly attractive for applications where many operator stations are dispersed across an installation site with connection to a central voice communication hub.

TDM Ring/ RIU interconnection is made with Category 5 off-the -shelf computer network cable - this simplifies cable routing and results in potentially great cost savings for large installations.

RIUs contain interfaces for multiple audio inputs and outputs, control signal interfaces for input switch sensing and control signal interfaces that provide logic outputs for control of operator panels. This solves most problems associated with radio and intercom simulation systems.

Operator switch state sense and control tasks are often routed through a main host computer and various digital signal conversion peripherals. Switch state information is then communicated from the host across an ethernet link to a DACS. This is an acceptable solution so long as panel-to-host interface and propagation delays requirements are met.

However, in many upgrade or retrofit installations, the new panel interfaces to the host have not yet been developed and special cabling is needed. Usually an individual conductor is required for each switch, knob, button, aural annunciator and visual indicator on the panel. The non-recurring development time and cost issues associated with such a task can be prohibitive.

ASTi's TDM / RIU technology provides a solution for all of these problems by creating a stand-alone radio / intercom simulation environment. Local signal conversion means better quality audio. And the standard set of digital inputs built into the RIU mean that interface to new or existing panels is simplified.

TDM Ring Details

The TDM Base unit is based on a DSP platform that is installed inside the DACS. The TDM Base carries audio signals and digital control signals between the DACS and multiple RIUs over link lengths of up to 500 feet. (150 meters)

As many as 3 TDM Base units may be installed in a single DACS. One of the TDM Bases is designated as a master with the remainder designated as slaves. Multiple TDM Bases are linked together inside the DACS via a high-speed parallel bus. Each TDM ring can simultaneously process up to (24) sixteen-bit, 8 Hz sampled audio channels.

As many as 14 RIUs may be connected to a single TDM ring; the maximum number of RIU nodes on a ring is dependant on specific application requirements, such as: the number of communication channels and the complexity of the simulation models running on the DACS.

RIU Details

Audio Interfaces:
The RIU utilizes digital signal processing with 16-bit delta / sigma converters for superior sound quality. The Audio Interface provides connection to a wide range of audio equipment such as: microphones, headsets, low power speakers and other line level audio devices.

Each Audio Interface channel provides a differential audio input and a differential audio output. The differential line configuration minimizes common mode noise, such as inductively coupled interference and cross talk.

Each channel also includes a Digital Input for switch state sensing and a Digital Output line, for controlling low power circuits, like LEDs.

The RIU provides four Audio Interface channels.

Digital Inputs (DIs):
One DI per audio channel is provided to sense remote switch closure. Each DI includes a ground reference and a buffered sense pin for direct connection to single pole single throw switches.

Digital Outputs (DOs):
DOs provide general purpose logic control signals. DOs can be used as switches in low-current LED circuits or to drive the inputs of digital logic circuits.

DOs are opto-isolated logic gates, each providing the equivalent of a single pole - single throw switch circuit. To use DOs, simply connect an external voltage source and low-current load to the RIU interface.

RIU Specifications

Audio Outputs
  • Quantity: (4) standard
  • <100 Ohm Output Impedance
  • Balanced, differential
Audio Inputs
  • Quantity: (4) standard
  • Input Impedance: <10 kOhm , Balanced
  • Max Input Level: ±2.5Vpeak
  • Input Gain: Variable 0 to +60 dB (jumper configurable for 20, 30, 40, 60 dB)
  • Mic-Power Selection: 5 VDC, Configurable
Digital Inputs
  • Quantity (4) PTT Ground Sense Inputs and ground sources (one on each Audio Connector)
  • Each Digital Input sense line is paired with a Ground Reference pin on the connector.
Digital Outputs
  • Quantity (4), one on each Audio Connector
  • Outputs are opto-isolated switches
  • External current-limited voltage source and return is required
  • 500 foot (150 meter) Max Ring Length
  • The number of RIUs per TDM Ring is application dependent.
  • Interconnection Cabling: Standard 4 Pair, Category 5 Twisted Pair Network Cable
  • Power Consumption: less than 4 watts (Wall Mount AC/DC Converter included with unit)