CMX138 CML Microcircuits, CMX138 Datasheet - Page 57

• Manufacturer Part Number: CMX138
• Description: Audio Scrambler and Sub-Audio Signalling Processor
• Manufacturer: CML Microcircuits

Audio Scrambler and Sub-Audio Signalling Processor
$C8 (P4.4-5)
The Programmed value is subtracted from the output signal. Can be used to compensate for inherent
offsets in the output path via AUDIO (Output 1 Offset) and MOD (Output 2 Offset). It is recommended
that the offset correction is kept within the range +/-50mV. This adjustment occurs before the coarse
output gain adjustment (register $B1), therefore an alteration to the latter register will require a
compensation to be made to the output offsets.
$C8 (P4.6)
The MOD ramp-up and ramp-down rates can be independently programmed and enabled (via bits 0,1 of
register $A7). The ramp rates should be programmed before ramping any outputs.
Ramp up starts from when the transmit mode starts (Mode Control Register bit 1 set = ‘1’). Ramp down
starts from when transmit mode is turned off (Mode Control Register bit 1 cleared = ‘0’).
$C8 (P4.7)
This unsigned number sets the clipping point (maximum deviation from the centre value) for the MOD
output. The maximum setting ($3FFF) is V
The limiter is set to maximum following a C-BUS Reset or a Power-Up Reset. The levels of internally
generated signals may need to be adjusted by setting appropriate transmit levels to avoid un-intentional
limiting. The limiter is active whenever either of the 12.5 or 25kHz Channel filters are selected (both in Rx
or Tx).
$C8 (P4.8)
Reserved – set to $119A
$C8 (P4.9)
Bit 13 selects the Hard Limiter in the Audio Processing path when set to 1, instead of the default Soft
Limiter.
The lower 8 bits set the order of the Audio Filter processing. This feature can be used to optimise the
signal to noise performance of particular radio hardware designs. Each filter/process block can be
specified in any order. Each two-bit field specifies the order in which the process will be executed in Tx
mode, therefore it is imperative that each set of bit fields be different. The reverse sequence is used in Rx
mode. The Voice Filter and Soft Limiter will always be implemented as the final block in the Tx sequence.
© 2008 CML Microsystems Plc
P4.4
P4.5
P4.6
P4.7
P4.9
P4.
Bit:
Bit:
Bit:
Bit:
Bit:
15
15
15
15
15
0
0
0
0
0
0
14
14
14
14
14
Output 1 Offset and Output 2 Offset
0
0
Ramp Rate Control
0
Transmit Limiter Control
0
Reserved
0
Audio Filter sequence
0
Time to ramp down to zero gain = (1 + RRD) × 1.333ms
lim
13
13
13
13
13
Time to ramp-up to full gain = (1 + RRU) × 1.333ms
2’s complement offset for Output 1 (AUDIO), resolution = AV DD / 65536 per LSB
2’s complement offset for Output 2 (MOD), resolution = AV DD / 65536 per LSB
Ramp Rate Up Control (RRU)
12
12
12
12
12
0
11
11
11
11
11
0
10
10
10
Limiter Setting, resolution = AV DD /16384 per LSB
10
10
0
BIAS
9
9
9
9
9
0
± (AV DD /2) i.e. output limited from 0 to AV DD .
Page 57
Reserved – set to $119A
8
8
8
8
8
0
Pre-emp
7
7
7
7
7
6
6
6
6
6
Ramp Rate Down control (RRD)
5
5
5
5
5
Comp
4
4
4
4
4
Scramble
3
3
3
3
3
2
2
2
2
2
D/138_FI1.0/5
1
1
1
1
1
300Hz
CMX138
0
0
0
0
0