On Wednesday 20th March, the club hosted a very interesting talk and demonstration by Charles, G4DXP. Charles has been experimenting with line-of-sight transmission of audio, using lasers and pulse width modulation. He brought along his two home-built laser transceivers for members to play with, and gave us an explanation of how they work and how they are constructed.
Charles’ laser transceivers are based on a design by Ron Jones, K7RJ, and are elegant in their simplicity. In fact, most of the complexity in making the system work is in finding a reliable means for aligning the lasers over long distances, rather than in the electronics.
If you’re interested in trying to build something like this, always remember that even low power lasers can be hazardous. Never use a laser in a way that might endanger another person, or where the beam might come close to anyone’s face. Many countries have legal restrictions on the use of lasers, so do your research before buying or using any laser light sources.
Charles clearly loves his construction projects, as he also brought along a differential windlass that he’d built for members to have a play with.
On behalf of Colburn and Richmondshire District Amateur Radio Society, we’d like to extend our thanks to Charles for taking his time to come and visit us, and show off his various projects.
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:
Conventional FM for communication with non-digital users.
V/D (Simultaneous Voice and Data)
Error corrected digital voice. This is the “normal” C4FM mode.
Voice FR (Full Rate)
Higher bitrate digital voice, allowing for better audio quality when error correction is not required.
Data FR (Full Rate)
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.