Digital Voice Systems VC-20-ATC-10B Especificaciones descargar pdf (Pagina 21)

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 15

voice should be 20 ms and 24 ms, respectively. The nominal nibble

clock rates are 9600 and 8000 Hz, respectively, for normal and

truncated timing modes. For a special framing nibble framing mode

that supports truncated timing at the 9600 Hz rate, see Section 5.2.3

• Clock Reference (CLKREF). The CLKREF parameter is used to

control the internal linear voice processing task. The parameter

should be set to match the primary linear port which will be used with

the vocoder, i.e. AUD or PCM.

• Rate Adapter (ADAPT). The PCM port rate adapter is designed to

support truncated timing connections for host systems that are

limited to 8 ksps linear voice rates. If enabled, the rate adapter

internally re-samples the PCM audio to create the truncated 6.67

ksps data stream. A rate adapter is not required with the analog

interface. The audio codec sample rate is internally adjusted to the

truncated timing sample rate; thus, the external interface remains

unchanged (except that the signal bandwidth is reduced

accordingly).

For applications which require external NIB timing and use the analog linear

interface, it should be noted that voice samples will be occasionally

added/dropped. This is a result of the small timing differences in the

internal timing base (used to drive audio codec) and the external nibble

timing. Note: This operation is identical with original VC20 implementation.

Other than the CFG command, there are two status tools that can also be

used to verify the timing configuration.

• ERR LED Indicator. The ERR LED lights for each sample/add drop.

If gross timing errors are present, this light will appear solid. For

minor timing variations, this light may blink occasionally.

• COUNT Terminal Command. The sample add/delete counters

provide the most detailed information about timing differences.

4.3.2 Setting up the Audio Flow

Just as the proper timing is required for good audio quality; the proper

signal levels are required for good audio quality. And, signal levels are

equally important on the analog and digital audio interfaces. Since audio

level matching can be more problematic, the FAA20 provides an additional

level setting tool on the analog audio interface.

• Voice Signal Levels (GAM, MIX). The recommended average

speech level for the vocoder is -23 dBm0. The gain, attenuation, and

mute (GAM) feature can be used to digitally adjust the AUD (analog)

port signal level. The GAM provides an adjustment of -36 dB to +21

dB. The mixer feature can provide signal attenuation for both analog

and PCM audio signals.

• Voice Signal Flow (MIX, ROUTE). Insure the voice signal flows are

configured, as required. For NORM mode, the flash configurations

are used to setup the FAA20 at power up. For the VC20 and DEMO

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