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FAA20 Embedded NEXCOM Vocoder

Board Manual

This manual has been prepared for the Federal Aviation Administration.

FA100-00107 (March 2004) – Rev 1.0.0 Page 25

5.3.3 Packet Voice Router

The packet voice router controls the flow of compressed voice packets

through the FAA20. The router has three ports: the vocoder port (VOC),

the nibble port (NIB), and the external communication port (COM).

Figure 11: Packet Voice Router

Unlike the mixer, each output port can be connected to one and only one

input port. Although the output port is limited to one connection, the input

port can have multiple connections. For example, the VOC input port can

be routed to both the NIB port and the COM port.

Although the router contains three ports, only the NIB and VOC ports are

functional when not in the TEST mode. Only in TEST mode does the

FAA20 software forward packets between the COM queue and the serial

port. In this mode and when TEST BITEXACT is enabled, the queue data

is forwarded to the serial port for vector testing.

5.3.4 PCM Rate Adapter

The PCM rate adapter converts 8 ksps linear voice data to 6.67 ksps, the

sample rate required for truncated timing mode. The rate adapter is useful

for implementations that require constant rate PCM interface while

supporting both normal and truncated timing modes. For example, external

digitally interfaced telecommunication equipment typically supports voice

sampled at a fixed rate of 8 kbps. The rate adapter is supported for the

PCM linear interface only. The analog linear interface does not require a

rate adapter since its sample rate can be adjusted as needed.

The PCM rate adapter is implemented with a multi-rate filter which includes

interpolation, decimation, and filter components. The rate adapter adjusts

the sample rate by the factor L/M, i.e., a ratio of two integers. Since the

truncated timing mode requires a rate reduction from 4.8 kbps to 4.0 kbps

NIB

VOC

COM

PACKET VOICE ROUTER

VOC

Port

NIB

Port

COM

Port

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Digital Voice Systems VC-20-ATC-10B Especificaciones Pagina 31