What is CW Skimmer?

CW Skimmer is a software package, available from Alex Shovkoplyas, VE3NEA, at www.DXAtlas.com. A 30-day full-featured trial version of release 1.1 is available free of charge. Combined with your current transceiver, or with Software Defined Radio (SDR) hardware that is already available, CW Skimmer will enable operators to be aware of everything that is going on across large swaths of any band. You can pick out individual signals and enhance their readability by putting them through a tight audio DSP filter, or click on a station and move your transceiver to its frequency, but what is truly different about Skimmer is its mega-multi-tasking decoders. It looks at the entire swath of spectrum it can “hear”, identifies CW signals, and decodes them all. Meanwhile, it looks at the decoded text and works to identify stations newly arrived on the band, stations calling CQ, etc. It generates a time-stamped list of these stations and their frequencies, and makes them available via Telnet to your logging program, or via the Internet to the hub server of the reverse beacon network (more on this below).

Here’s a sample of CW Skimmer at work on 40 meters with an inexpensive  SoftRock SDR  (figure 1). It was “listening” to a brief period of a recent CQWW CW and decoding what it saw. The first figure shows the main screen.

The waterfall display only shows about 5 KHz from the center frequency of the principal decoder (the green zone at the bottom), but in fact, the SoftRock is receiving the entire slice of spectrum that the soundcard can handle. In this case, 326 different signals (or possible signals) were being copied across the band, and 154 calls were identified in less than 2 minutes (figure 2).

The callsign list is dynamic – stations are constantly being added or dropped when they are no longer there.

The Skimmer software is very simple to operate and seems quite able to copy CW at or near the noise floor, though not quite as well as a good CW operator. The software automatically adjusts the number of decoders available, to keep your CPU utilization below 100 percent, so it resumes skimming as soon as it can.

There’s a lot of work yet to be done. Some contest experience shows that CW Skimmer is somewhat prone to multiple miscopies of a single callsign, dependent on QRM and signal strength. Because it currently tries to do all its analysis in a 5-10 millisecond interval while listening to a given decoder, Skimmer can’t figure out that a station working one caller after another on a given frequency is, in fact, running, unless it sends tip-offs like “QRZ?” It requires a lot of CPU horsepower – my 2.2 GHz Celeron, for example, is stretched to the limit (sometimes beyond) by the decoding requirements of a contest-busy band. Alex is aware of all this, and working hard on it. Don’t forget, this is the first public release; there will doubtless be many improvements in coming months.