Skywave Linux: Recent KiwiSDR System Updates

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KiwiSDR: 21st Century Radio for the People
A listener's handbook by and for KiwiSDR enthusiasts.

As much as the KiwiSDRs seem to be nearly perfect HF and longwave receivers, there are sometimes little annoyances which need to be fixed, along with a regular flow of updates and tweaks from developer John Seamons (a.k.a "JKS"). If you check the messages about KiwiSDR software updates, the first thing to notice is that there are a lot of mundane, unexciting changes coming out, but which keep the receivers working for the users. There are a fair number of listeners with specialized interests who often find a need for small tweaks and fixes at the outer limits of the KwiSDR's functionality. Taken as a whole, these have the effect of making the KiwiSDR a performant and robust receiver system.

The Adjacent Channel Anti-Aliasing Fix

Let us start with recent updates which went out in early 2024. In January, KiwiSDR system version 1.649 fixed a problem which had endured for a long time. The problem was peculiar to the way the digital downconverter (DDC) worked, causing the desired signals to receive interference from adjacent frequencies.

In my past writings about direct sampling SDRs, I have often described the DDC as a virtual direct conversion tuner, or as a phasing method single sideband receiver implemented in DSP code. That sort of works, when describing how the radio extracts individual signals from the broad spectrum being sampled. Except, there is more to what is actually happening in the digital realm. Instead of imagining the DDC as simply a virtual direct conversion module, consider that we're still dealing with digital samples. The DDC output will be at a sample rate of at least twice the bandwidth of the received signal, respecting the Nyquist limit.

Consider now, what problems may come from operating in a crowded spectrum, where RF frequencies adjacent adjacent to the desired signal also contain strong signals. If not filtered out, those neighbors will fall into the spectral range above the desired signal, up to the sampling frequency. They wouldn't be a serious problem in the analog world, and could be filtered out with some properly valued resistors and capacitors. In the digital realm, signals above the Nyquist limit must be filtered out, otherwise the desired signal will suffer interference as the adjacent signal is aliased.

On a crowded amateur voice band, this aliasing interference sounds as though a disruptive operator is transmitting on the wrong sideband. More annoying still, if you try to tune across that interloper, it seems to go the wrong way - pitch going up instead of down, and vice versa. On the KiwiSDR, this aliasing problem is not a terrible flaw, but it can be found when the bands are packed with strong signals. Software update v1.649 adds a filter to the DDC function, which suppresses those troublemaking adjacent signals.

TDoA Waterfall Preview and Spiderfied Maps

Software update 1.664 brought improvements to the TDoA extension, enabling users to see a preview of the spectrum waterfall for the tuned frequency, but for other receiver locations. With that enhancement, it is possible to see how well a signal is being picked up by other receivers. It is a great time saver, as users do not need to actually load the other receivers to verify thet they can pick up a station targeted for geolocation.

The other notable improvement for this update is how the user interacts with the TDoA map and spiderweb of receivers available for geolocation attempts. The issue was such that overlapping receiver clusters could be difficult to navigate on the map. If one cluster was expanded, hovering over an element of another cluster would cause the first cluster to zoom out or compact, while opening or zooming in on the new cluster (even if it was not the intended user action). This unintentional behavior could be bothersome, and degrade the user experience.

The new fix temporarily removes other clusters from the map, if one is spiderfied. To de-spiderfy, just move the cursor away from cluster's elements. Then, other clusters may be picked for the spiderfied view. It is a more clean and intuitive way to work in the TDoA extension.

KiwiSDR System v1.666: The Beast of Burdens

This update brought so many changes. Some were sensible, while others caused me to wonder WTF was in the minds of some operators. Let's start with the sensible and mundane. JKS moved some of the controls to more sensible locations, and made the tabs scrollable. For example, antenna switching went to the "RF" tab, while the noise blanker and DSP filters went to the "Audio" tab.

A new passband marker is available, which places a transparent bar onto the waterfall, to indicate which frequencies are within the receiver passband. You may have seen a similar feature on other online SDRs, such as the one run by NA5B.

KiwiSDR operators have been vexed by automatic programs - bots - which show up and stream certain frequencies for long periods of time. To thwart the bots, owners have asked for a gatekeeping feature, intended to restrict access to humans. Therefore, JKS set up a required text entry field, where users must enter some kind of text before getting access to the KiwiSDR. Goddamn, I hate those things. I understand the reasons why operators want it, but whatever humans can be forced to do, robots can do just as well. Bots can handle text entry fields. Bots can create accounts. People? We just get bothered and comply.

Another interesting adjustment is the available GNSS resolution, for indicating where the receiver is located. Operators may select a low resolution option, which gives the geographic coordinates limited to two significant digits. Selecting high resolution gives the location to a six digit precision.

There is a nice efficiency boost introduced in system 1.666. User settings and site data, which had been stored in cookies, have been moved to browser local storage. This reduces bloat in the data transfer between the browser and remote KiwiSDR, as there is no longer a redundant passage of cookie data.

Last, but not least, KiwiSDRs with system version 1.666 or later shall be identified on maps and listings as either "KiwiSDR 1" or KiwiSDR 2" as appropriate.

Better Motions in v1.694

Tablet and smart phone users will like this update, as it enables nice features for touch screens. I have sometimes had a hard time adjusting the passband edges and moving in small tuning increments. Now it is possible to use the audio tab to tweak the passband. Not only that, but mouse wheel tuning is also available.

I got a big kick out of finding that the keyboard is also available for tuning, using the -, +, i, and j or arrow keys. LOL we're not quite up to true Vim keys, but it is a recognition that keybinds can bring efficiency.

Closing Words

You have now seen some of the small and not so small fixes and improvements which have been rolled out fro the KiwiSDR. As good as the radio is, there are nearly always small tweaks which can make it better. Everyone may not always agree that they want each feature change, such as the name / callsign gatekeeper, but the KiwiSDR trends toward improvement with each system upgrade.



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