Digital Voice Systems VC-20-ATC-10B Especificaciones descargar pdf (Pagina 27)

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 21

The FAA20 nibble signaling requirements are identical the VC20 nibble

signaling requirements. Figure 8 provides signal timing information.

Figure 8: NIB Interface Timing Diagram

The NIB clock rate can be changed “on-the-fly” to support switching

between normal and truncated timing modes; however, the transition

should be glitch-free.

The FAA20 supports a special nibble framing mode for a frame size of 230

clocks. This special mode may enable some host systems that have a

fixed nibble clock rate to support truncated timing mode. As already

presented, the PCM interface provides a rate adapter to perform the

required sample rate adjustment. For the nibble interface, the clock rate

can remain at 9600 Hz and the frame size can be changed to reflect the

truncated timing frame rate. Although the actual frame size required for

truncated timing is not an integral frame size, i.e. 230.4 clocks at 9600 Hz,

the FAA20 supports alternating frame sizes of 230 and 231 clocks to

support an average frame size of 230.4 clocks. Thus, the truncated timing

mode can be supported and compressed packets are not accumulated or

depleted.

When the FAA20 nibble timing source is set to internal, the frame size is set

to 230 for truncated timing mode, and the truncated timing rate is 9600 Hz,

then the FAA20 will generate a special set of framing pulses that average

the 230.4 frame size. Specifically, the following five frame sizes are

repeated sequenced: 230, 231, 230, 230, and 231. Externally timed

interfaces can use the same framing sequence. Audio quality is not

affected since the vocoder is not affected by data transmission methods.

ENCCK

FMEN

ENCx

CVD

FMDE

DECx

Data clocked out with rising edge of

ENCCK

FMEN sampled on falling edge of ENCCK

Data

sampled on falling

edge of SCLK

FSX sampled on falling edge of SCLK

CVD

DECCK

DCS

CVD

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 108 109

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Digital Voice Systems VC-20-ATC-10B Especificaciones Pagina 27

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