Digital Voice Systems VC-20-ATC-10B Especificaciones descargar pdf (Pagina 34)

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 28

5.4 System Timing and Control

This section addresses the following timing and control topics:

• Truncated Timing Mode

• Voice Delay

• Programmable LEDs

• Command Processor

5.4.1 Truncated Timing Mode

The truncated timing mode enables the FAA20 to produce a 4.0 kbps

compressed data stream instead of the nominal 4.8 kbps. This lower data

rate is required to support NEXCOM communication when accurate timing

is not available from a ground site.

Truncated timing is implemented using the clock slow-down method. With

this method, the entire FAA20 voice and vocoder operating rate is reduced

by a factor of 5/6. The linear sample rate is reduced from 8.0 ksps to 6.67

ksps. The compressed packet rate is reduced from 50 pps to 41.67 pps.

There are four input controls for truncated timing control:

• DIN Signal Conductors (TMRX and TMTX signals)

• Expanded Control Nibble Bits (TRUN and TSTAT bits)

• Terminal Command (TRUN command)

• Configuration Switch (TRUN switch – SW4)

The FAA20 uses a first-come, first-serve approach for controlling timing

based on these inputs. For example, the TRUN command can enable

truncated timing, and then the TMRX signal can disable it. Control events

are processed in the order received.

When the DIN signal conductors or the expanded control nibble is used, the

FAA20 switches timing modes during the active state of the FMDE signal.

These two input controls do not function in the TEST operational mode.

When the terminal command is used, the FAA20 switches timing modes as

soon as the command is processed. When the configuration switch is

used, the FAA20 switches timing modes during the power up.

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 108 109

Sin comentarios

Digital Voice Systems VC-20-ATC-10B Especificaciones Pagina 34