FX709 Codec

CVSD Digital Voice Codec

FX707 CML Code Selector & Automatic Repeat Encoder

  • CVSD Encode + Decode
  • Programmable Clock Rates
  • Programmable Voice Filters
  • Voice Power Output
  • Voice Spectrum Monitor
  • 8-bit Memory/Instruction I/O
  • Processor Interface

  • Voice Message Mailbox
  • Status Annunciators
  • Re-try Message Forward
  • Voice Security Scrambling
  • Voice Data Communications
  • Time/Frequency Companding
  • Audio Delay Functions


The FX709 is an audio-digital interface codec for microprocessor-controlled Voice Store and Retrieve applications.
In encode, audio input signals are band-limited by a low-pass filter and digitised by a CVSD 1-bit serial encoder. After conversion to 8—bit parallel format, encoded data is read to the I/O bus for storage in memory.
In decode, memory contents written into the I/O port are converted back to 1-bit serial form and decoded by a CVSD decoder. The decoder output is low pass filtered and output as retrieved audio.
The audio encode/decode functions are independently controlled, permitting concurrent or asynchronous VSR operations to be performed. Time and frequency companding is available via independently programmable encode/decode data rates and filter cutoffs.
The power assessment register provides support for VOX functions and ‘Pause’ memory management.
This register contains two 4-bit numbers representing the average signal levels into the data encoder and a replica encoder over a programmable averaging period.
The device instruction set includes input/output signal switching and a standby power-down function. The FX709 is a low power CMOS circuit and uses a single 5 volt supply.