F EATURE
ARTICLE
by Clayton Gumbrell (Australia)
QRSS-Rx A Network for Radio Communication The QRSS-Rx features an mbed at the core of a QRSS receiver, which is an RF receiver that can digitize a small bandwidth of RF signals and send them to a server for processing into a spectrum image for real-time display on a website. This project is intended to improve the QRSS grabber system.
February 2012 – Issue 259
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hat is QRSS? QRSS is one of many modes of communication and experimentation used by radio amateurs. It is the transmission and reception of very slow-speed Morse transmissions at very low-power levels—typically less than 1 W, and often a lot less. The term QRSS comes from the QRS Q-code (a message encoding system, or a brevity code, used by radio amateurs among others) meaning, “please send slower.” Due to the extreme slowness of this mode, QRS is adapted to QRSS. Morse speeds are usually specified in words per minute (WPM), but for QRSS speed is instead expressed in the period of a Morse dot. The commonly used speed of QRSS3 means a dot lasting for three seconds. This equates to about 0.4 WPM. Due to the very slow speed—and using the magic of digital signal processing (DSP)—these transmissions can be extracted from below the noise and picked up from thousands of kilometers away. Reception is performed using “grabbers.” Audio from a receiver is fed into a computer sound card where software processes it to produce a spectrum image of a small portion of the radio spectrum, typically 100-Hz wide. This image is made available on a website, providing almost real-time access to how a signal is being received across the globe (see Photo 1). The hardware involved with a grabber receive system is not insignificant. It requires a frequency-stable radio receiver, a PC (with soundcard), and an Internet connection. The receiver audio is fed into the PC soundcard. Software then converts the audio to an image. Additional software makes these images available on a webpage. There is a variety of different software configurations used, with
each grabber having its own look and feel. Each grabber is hosted on its own webpage—although there are some websites that will aggregate a number of grabbers onto the same page. Finding your signal can therefore require the inspection of a number of websites. Dealing with such a small RF spectrum requires the grabber receiver to be precisely tuned to the frequency. Typically, a receiver will drift, so keeping a grabber on frequency can be challenging. Because of the challenges and required resources, there are few grabber stations operating, with only about a dozen grabbers commonly available.
ENTER THE QRSS-Rx This project—the QRSS-Rx—is designed to improve the QRSS grabber system by putting together a “QRSS Network.”
Photo 1—An example of a QRSS grabber image. This image represents 10 minutes of spectrum activity. Note that some signals are sent as slow Morse, but others are graphical where the transmitted signal varies in frequency to create symbols or text in the spectrum image. (Photo courtesy of Peter Mulhare, ZL2IK) CIRCUIT CELLAR®
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