The magic of adaptive filters in amateur radio

Examples of a speech signal (top),
a sinusoidal, and noise (bottom)

Radio amateurs have been using adaptive noise reduction and adaptive notch filters for decades. They came with the advent of audio digital signal processing. 

Few however, understand how different they are from ordinary frequency selective filters and for many they appear to do something magic, very different from ordinary filters.

Customize your own list of favorite screens for the Multi-face GPS Clock

Fav 2 subset with 11 entries (Example in text has 9)

The Clock subset menu of the Multi-face GPS Clock gives you a choice already between:

• All, 50 screens
• Fav 1, 34 screens
• Fav 2, 9 screens
• Calendar, 13 screens
• Clocks, 27 screens
• Astro, 17 screens
• Radio, 12 screens 

QMX+ mechanical improvements

My QMX+ has some minor mechanical improvements:

1. Slimmer knobs. I decided to replace the knobs as I often touched the bottom one while adjusting the top one and vice versa. The slimmer knobs solved that. I found knobs on AliExpress with the same looks, only a smaller diameter as the image shows. Unfortunately they don't have screws, so they aren't so well attached as the original ones.
2. Folding laptop feet which I first saw on the AE5X blog.

Why is speech quality so poor in DMR, D-STAR, and System Fusion?

Example of a speech signal which transitions
from an unvoiced to a voiced sound

Digital VHF and UHF communications is increasingly popular among radio amateurs using DMR, D-STAR and System Fusion. 

Very few radio amateurs, however, have much understanding of the signal processing required to compress a speech signal so it can be transmitted with the low bit rates available. Here is an attempt at an explanation.

My favorite faces for the Multi Face GPS Clock

Weekly progress bar with Monday as day 1
in week 9 and yearly progress bar
as of day 56 in the year.

Among the 50 different faces (or screens) now implemented on the Multi face GPS clock, it is hard to select favorites. 

So I start with the latest one that was implemented, i.e. the progress bars for week and year shown to the right. It was new in version 2.3.0 dated 2025-02-04.

A QMX+ coming together

I got an unassembled QMX+ for Christmas this year, and here are pictures of it coming together. First the winding of toroids for the low-pass filters:

The unphysical free-space path loss

Propagation of wave energy to distance d, 2d, 3d
 showing how the size of the surface that the energy
spreads over increases in proportion to 
d², (2d)², and (3d)²

Many places it is stated that the lower the frequency, the smaller the free space loss. 

This is very counterintuitive, because if one solves the wave equation, the amplitude of the solution is independent of frequency. The physics of the problem is that intensity just falls with distance squared according to how the surface of a sphere increases with the radius as shown in the figure.

The Friis formula used in link budgets says something else. It gives received power, Pr, as a function of transmitter power, Pt, receiver and transmitter antenna gains, Gr and Gt; wavelength, lambda, and distance, d:

The Little Known Fourth Method of Generation of Single-Sideband Signals

Envelope Elimination and Restoration, from Kahn 1952 

There is a method for SSB generation which is now becoming popular with digital signal generation and digital output stages. This is the Envelope Elimination and Restoration method or Polar Modulation method. The signal is interpreted as a complex signal in the polar format, i.e., with an envelope and a phase.

This method is receiving renewed interest among radio amateurs as it is used in the SSB implemention of e.g. the interesting QRPLabs QMX transceiver. 

In the QMX a single frequency signal (the phase) is generated in the digital domain and then phase modulated (upper branch of figure). It then has a digital output stage with very high efficiency as it only switches between on and off. The output stage is amplitude modulated with the envelope signal from the lower branch.This gives a much more efficient power amplification than is possible with the three conventional methods.

The three methods

How does it compare with other methods? In 1956, Donald K. Weaver, Jr. wrote a paper with title "A Third Method of Generation and Detection of Single-Sideband Signals".