C4FM for Beginners

On 18th July, members were given a beginner’s introduction to simplex operation using Yaesu’s C4FM (Fusion) digital mode.

Yaesu have an introductory video for Fusion, which I found quite helpful when preparing for the talk:

In the UK, 144.6125 is designated as the 2m digital calling channel. Use this for CQ calls using C4FM or other digital modes. Once you’ve established contact, move to a standard FM simplex channel, or use the all modes section of the band. The 144.6375 – 144.7875 range is recommended.

Most Yaesu C4FM radios are capable of five different modes of operation:

Mode Appears as Used for
FM FM Conventional FM for communication with non-digital users.
V/D (Simultaneous Voice and Data) DN Error corrected digital voice. This is the “normal” C4FM mode.
Voice FR (Full Rate) VW Higher bitrate digital voice, allowing for better audio quality when error correction is not required.
Data FR (Full Rate) DW Sending text or images between C4FM radios. You cannot select this mode manually. Your transmitter will automatically switch to this mode when you use a feature of your radio that requires it.
AMS (Automatic Mode Select) Bar above active mode The radio will transmit using the displayed mode, but will automatically switch to other modes upon receiving a transmission using one of the other modes.

The following two videos provide an introduction to analog to digital conversion, and to digital error correction techniques.

A few things to take away from these videos:

  • We need to sample our audio at twice the highest audio frequency we wish to transmit.
  • We have a fixed amount of bits that we can transmit over a 12.5KHz FM channel.
  • Higher audio quality requires more bits to be dedicated to audio data.
  • Error correction requires more bits to be dedicated to checks on our audio data, at the expense of audio quality.
  • We have to make trade-offs between audio quality and reliability, hence the two different voice modes with different priorities.