CMX649 CML Microcircuits, CMX649 Datasheet - Page 31

• Manufacturer Part Number: CMX649
• Description: ADM Codec
• Manufacturer: CML Microcircuits

ADM Codec
Local Decoder
Output Select
(Bits 10 – 9)
Idle Channel
Enhance
(Bit 8)
Encoder VAD
Decay Time
Constant
(Bits 7 – 5)
Encoder VAD
Attack Time
Constant
(Bits 4 – 3)
Encoder VAD
Output Source
(Bits 2 – 1)
2003 CML Microsystems Plc
This bit improves the perceived low-level sound quality by enabling the automatic
tracking of offsets, which reduces internal offset levels. Some increase in harmonic
distortion may result from the use of this bit. If automatic compensation is not
required, this bit should be set to logic 0 and the ENCODE OFFSET LEVEL Register
($E3) should also be set to logic 0. This bit should normally be set to a logic 1 to
allow analog offsets to be automatically compensated.
When running the encoder as a digital ADM to PCM transcoder this bit should be set
to 0 since the local ADM decoder runs outside a feedback loop.
When enabling offset compensation, ensure the encoder ADM input selection is set
either for feedback from the comparator or for PCM to ADM transcoding. Also load
the ENCODE OFFSET LEVEL Register ($E3) with a small positive constant. If
transcode feedback is selected then the PCM filter must also be set to interpolate.
These bits allow selection of the Voice Activity Detector decay time constant.
Allows selection of the Voice Activity Detector attack time constant.
Allows selection of the Voice Activity Detector output source.
Bit 10
Bit 7
Bit 4
Bit 2
0
0
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
Bit 9
Bit 6
Bit 3
Bit 1
0
1
0
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
ADM estimator output drives local decoder
output.
ADM estimator output summed with PCM
interpolation filter output drives local
decoder output.
For encoder, ADM output tracks sum of MIC
input analog signal plus a PCM signal input
over the C-BUS interface.
Direct PCM test mode.
Interpolated PCM output.
(VAD Decay Time Constant)/4
(VAD Decay Time Constant)/8
(VAD Decay Time Constant)/16
(VAD Decay Time Constant)/32
Nominal VAD operation.
ADM bits are driven over VAD pin at the ADM bit
rate (may be useful when transcoding or verifying
proper application of the burst interface).
VAD output driven to 0.
VAD output driven to 1.
Bit 5
0
1
0
1
0
1
0
1
Selected Local Decoder Output
Attack Time Constant (ms)
128/(Bit Rate)
256/(Bit Rate)
512/(Bit Rate)
1024/(Bit Rate)
2048/(Bit Rate)
4096/(Bit Rate)
8192/(Bit Rate)
16384/(Bit Rate)
Decay Time Constant (ms), Bit Rate in kbps
31
VAD Output
CMX649
D/649/2