Digital Voice Systems VC-20-ATC-10B Especificaciones descargar pdf (Pagina 35)

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 29

5.4.2 Voice Delay

The software is designed to reduce the overall voice delay through the

FAA20. There are many factors to consider in reducing this delay.

• vocoder execution time

• buffering/filtering delays

• processor overhead

• data transmission speed

• vocoder process alignment

Of these factors, vocoder process alignment is the most significant factor in

controlling/minimizing delay. It is controlling the execution start time for the

encoding/decoding tasks based on the timing of the frame sync signals on

the nibble interface. If proper alignment is not maintained, delays can be

easily be increased by up to 20 ms or more, i.e. the width of a compressed

nibble frame. For example, the decoder should start processing a frame

immediately after the frame has been received. The encoder should start

processing voice as late as possible so that the compressed data frame is

ready just before it is needed for transmission.

The FAA20 triggers the encoder/decoder tasks based on known execution

time requirements and the positioning of the compressed nibble frame sync

signals. Execution time requirements for the encoder and decoder vary

based on voice content. In addition, execution times must account for

overhead and higher priority interrupt processing. The higher priority

interrupt processing is required to support hardware data transfers and

useful FAA20 features such as the mixer, voice activity/peak detection, and

GAM volume control. Finally, if both the encoder and decoder tasks

overlap, the FAA20 delays or extends the execution of the decoder task in

favor of minimizing the encoder task execution time.

The vocoder RUN signal (DIN signal pin A6) indicates encoder/decoder

task execution timing (see Figure 14).

The DELAY command prints throughput delays between the linear (AUD or

PCM) and compressed (NIB) interfaces. The reported delay does not

include collection delay (nominally 20 ms) and/or AMBE+ algorithm delay

(nominally 60 ms). Table 6 provides typical delay values for the normal

timing mode.

1 2 ... 30 31 32 33 34 35 36 37 38 39 40 ... 108 109

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Digital Voice Systems VC-20-ATC-10B Especificaciones Pagina 35