CMX649 CML Microcircuits, CMX649 Datasheet - Page 6

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

ADM Codec
5.
The CMX649 encodes and decodes analog audio signals to/from ADM, Linear PCM, µ-law PCM or A-law
PCM. It has programmable clock dividers that enable it to use a range of 4-16 MHz crystal clocks and to
sample the data over a large range of data rates. Programmable current sources for on-chip op-amps
enable the overall power consumption to be optimised for any given supply voltage and clocking scheme,
thus achieving extremely low working power levels.
Anti-Alias Image filters and gain controls are fully programmable. All the time constants and other
parameters of the ADM can be programmed for optimum performance.
The CMX649 also includes a Microphone Amplifier, Data Clock Recovery, Data Scrambler/De-Scrambler
and Voice Activity Detector (VAD) circuits.
All of these parameters are controlled via C-BUS.
5.1
The CMX649 contains a full duplex speech codec supporting common Adaptive Delta Modulation (ADM)
and non-linear PCM coding algorithms. In addition, it supports linear PCM coding for DSP interface
applications. This codec offers simple interface and application, yet is configurable to support a wide
variety of speech quantisation systems.
5.1.1 ADM Coding Engine
ADM is a differential waveform coding technique predominantly applied to speech. Figure 3 illustrates the
ADM encoder employed. The device is for speech quantising applications and is based on popular
Continuously Variable Slope Delta (CVSD) encoder approaches, with optional modifications and
improvements configurable through the ENCODE and DECODE ADM CONTROL Registers ($E1 and
$D1). Optional second order integration in the feedback loop provides improved speech quality at a
given bit rate or similar quality at a lower bit rate. Toll quality is achieved at bit rates much lower than for
PCM. The decoder is embedded in the encoder, as is the case with most differential encoders. Note the
symmetry between the encoder and decoder of Figures 3 and 4 respectively. The signal flows for ADM
are shown in bold.
INPUT
2003 CML Microsystems Plc
General Description
Block Descriptions
+
-
REG $E6
OUTPUT
PCM
+
REG $E7
INPUT
PCM
PCM TO ADM TRANSCODE FEEDBACK
Figure 3 ADM Encoding
ENCODER FEEDBACK
1
st
ESTIMATOR INTEGRATORS
ENCODE
REG $E0
SETUP
OFFSET NULLING
REGS $E3 & $E5
6
ADM CTL
REG $E1
2
nd
PCM OUT
LINEAR
PCM IN
LINEAR
x
REGISTER
DELAY
PROGRAMMABLE
STEP SIZE
CONTROL
REG $E8
INPUT
ADM
REG $EA
OUTPUT
ADM
CMX649
D/649/2
ADM OUT