Digital Voice Systems VC-20-ATC-10B Especificaciones descargar pdf (Pagina 32)

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 26

for compressed voice, a rate factor of 5/6 is required. Two filters are

required. The receive filter down-converts the 8 ksps to 6.67 ksps before

sending the data to the encoder. The transmit filter up-converts the 6.67

ksps decoded voice to 8 ksps before sending the data to the PCM output

port. Figure 12 provides a block diagram for the PCM rate adapter.

Figure 12: Rate Adapter Block Diagram

The interpolator uses the zero-insertion method to maintain spectral purity.

For example, for an interpolation factor of X, every real data sample is

followed by X-1 inserted zero-valued samples. While this approach

maintains spectral purity, it reduces the overall signal level (after filtering)

by a factor equal to the interpolation rate. Thus, a gain adjustment (G) is

required to compensate for signal loss.

The low pass filter characteristics are depicted in Figure 13. The 120-tap

finite impulse response (FIR) filter is designed with a reasonably flat pass

band, a narrow transition band and a small voice delay, i.e. 1.5 ms.

5.3.5 Tone Generator

The tone generator outputs tones in the frequency range of 0 to 0.5 Fs

(where Fs is the sample rate). The resolution is 1 Hz. The output level is

set at 3 dBm0. To route a 0 dBm0 tone out another port, use a mixer value

of 0x5A9D.

L = 5

Low Pass Filter

N = 120

G = 5

M = 6

L = 6

Low Pass Filter

N = 120

G = 6

M = 5

8 ksps

6.67 ksps 40 ksps

PCM

MIXER/

VOCODER

L = interpolating factor

M = decimating factor

G = gain factor

N = number of FIR filter taps

1 2 ... 27 28 29 30 31 32 33 34 35 36 37 ... 108 109

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Digital Voice Systems VC-20-ATC-10B Especificaciones Pagina 32