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Voice Multi-transcoder IC

CMX7261 - Linear PCM, G.711, G.729A, G.723.1, CVSD


  • No Audio Codec Required
  • No DSP  nor DSP Skills Required
  • Full and Half-duplex Operation
  • Encoding, Decoding and Transcoding
  • Mutiple Voice Coding Algorithms Supported
    • Linear PCM
    • G.711 ( µ-law, A-law)
    • G.729A
    • G.723.1
    • CVSD
  • Transcoder Routing:
    • Choice of Input Sources:
      C-BUS/Host, PCM Codec, Analogue Audio Inputs
    • Choice of Output Sources:
      C-BUS/Host, PCM Codec, Analogue Audio Outputs
    • Voice Activity Detection (VAD)
  • C-BUS Host Serial Data/Control Interface
    • SPI compatible with Register Mapping
    • Read/Write 128byte FIFOs
    • Streaming data transfer
  • Auxiliary GPIO Functions
  • Master/Slave PCM Serial Interface for External Audio Codec


  • Duplex and Half Duplex Digital Radio Systems
  • Personal Area Network Voice Links
  • Privacy-type Digital Voice Communications
  • Wireless PBX
  • Voice over IP (VoIP) Applications
  • Digital Software Defined Radio (SDR)

Supply Requirement:

  • 3.0 to 3.6 V

The CMX7261 Multi-transcoder IC is a member of the CML DuraTALK family of digital voice products providing multiple speech coders and an analogue audio codec in a single chip.

The CMX7261 features voice encoding and decoding through an Analogue Front End or transcoding operations over its digital interfaces. Both half-duplex and full-duplex modes are available.

Input data and control are via an SPI compatible C-BUS which has a write-through ability to manage an external SPI controlled device using only one host SPI interface. C-BUS supports single SPI transactions or streaming modes via embedded hardware FIFOs.

The CMX7261 utilises CML’s proprietary FirmASIC® component technology. On-chip sub-systems are configured by a Function Image data file that is uploaded during device initialisation and defining the device's function and feature set. The Function Image™ Host can be loaded by the host or auto-loaded from an external serial memory.

Two Function Images are currently available:

7261FI-1.x    linear PCM, CVSD and G.729A speech coders
                     8, 16 and 32 kHz sampling
                     Packed and unpacked frame formats
                     µ-law, A-law PCM compression
                     Voice Activity Detection


7261FI-2.x    linear PCM and G.723.1 speech coders
                     G.723.1 at 5.3kbps and 6.3kbps
                     8, 16 and 32 kHz sampling
                     µ-law, A-law compression
                     DTX function with Voice Activity Detection
                     Lost Packet Recovery

The device's functions and features can be enhanced by subsequent Function Image™ releases, simplifying in-the-field upgrades.

Block Diagram


Design Support Information



Q.  Should pull-up or pull-down resistors be connected to CMX7261 device GPIO pins (GPIOA, GPIOB, GPIOC, and GPIOD)?

A.  The CMX7261 features flexible GPIO pins that can be independently configured via C-BUS serial interface commands to operate in one of the following modes: CMOS digital output, CMOS digital input or CMOS digital input with bus hold. Bus hold automatically engages a 75kohm internal pull-up or pull-down resistor that reinforces the current logic state of the input signal. As is normally the case, the state of a CMOS digital input will be indeterminate and may increase device supply current when the input’s voltage is not at valid ‘0’ or ‘1’, i.e. when the node voltage is “floating.”

When the CMX7261 is RESET (by asserting its RESETN pin, issuing a C-BUS General RESET or by triggering an internal power on reset) all its GPIO pins are immediately placed in CMOS digital input mode. Any GPIO pins not being pulled either up or down by an external load will be floating. Once a Function Image™ is loaded into the device then all its GPIO pins are placed in CMOS digital input with bus hold.

If user circuits require one or more GPIO pins to be held at a deterministic ’0’ or ‘1’, i.e. not floating after RESET and before the Function Image™ is loaded then that circuit should connect external pull-up or pull-down circuits to such pins.

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