CMX8341 datasheet(45/70 Pages) CMLMICRO

45 / 70 page

Digital PMR (dPMR) Baseband Processor

CMX8341

 2014 CML Microsystems Plc

D/8341/7

identifies that a block has been written out of sequence, then it is likely that a data overrun condition has

occurred and some data has been lost.

8.5.12 Vocoder Section

– Pass-through Mode

To allow the host to communicate directly with the Vocoder section for test and configuration purposes, a

Pass-through mode is available which allows any Vocoder section C-BUS register to be read or written (as

appropriate). This mode uses the Modem Control, TxData0, RxData0, IRQ Status and Program Blocks on

the CMX8341.

To write to the Vocoder section:

o Set the CMX8341 to 'Pass-through' mode (Modem Control register, $C1=$0004)

o Wait for the PRG flag to be set ($C6 b0)

o Write the Vocoder section data value to the Txdata0 register ($B5)

o Write the Vocoder section C-BUS address to the Programming register ($C8) with b15=0 and b14=1

o Wait for the PRG flag to be set ($C6 b0).

To read from the Vocoder section:

o Set the CMX8341 to 'Pass-through' mode (Modem Control register, $C1=$0004)

o Wait for the PRG flag to be set ($C6 b0)

o Write the Vocoder section C-BUS address to the Programming register ($C8) with b15=1 and b14=1

o Wait for the PRG flag to be set ($C6 b0)

o Read the Vocoder section data value from the RxData0 register ($B8).

Vocoder section C-BUS addresses are all 8 bits long and should be written to bits 0-7 of the Programming

8-bit). Unused bits should be cleared to zero. When an 8-bit register is read or written, the data occupies

the lower 8 bits of the appropriate data register (TxData0 or RxData0).

8.5.13 Vocoder Section

– Noise Gate

The Vocoder section has an optional, programmable noise gate. This is controlled by the addition of five

new 16-bit C-BUS write registers, accessed by Pass-through mode (see section 8.5.12). Once the

Function Image™ has been loaded, these new registers will all be set to an initial value of zero. For details

of how to configure these registers, see section 10.3.

The purpose of the encoder's noise gate is to remove background noise in between speech pauses. This

could be used to remove noise generated in any front-end analogue circuitry, or could be used to help

reduce the effects of ambient noise, perhaps with some degree of user control.

Three parameters control this noise gate. An upper threshold level controls the point at which the gate

opens and allows audio to pass. A lower threshold level controls the point at which the gate closes,

thereby preventing audio from passing. These two parameters together control the hysteresis and prevent

'chattering'. A third parameter controls how many consecutive frames of audio must be below the lower

threshold before the gate closes. This 'gate shut delay' prevents the tail end of words (like a trailing 's')

from being clipped.

Once the gate shut delay has expired, the gate does not shut abruptly, but closes over a period of 16

frames. Each 20ms frame has progressively more attenuation applied, until the frames are silent. This

happens in approximately 6dB steps.

CMX8341 Datasheet(PDF) 45 Page - CML Microcircuits

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