In-Band Signaling: Coded Squelch Systems

In the first part of our series on in-band signaling, we discussed one of the most common and easily recognizable forms of audio control, familiar to anyone who has dialed a phone in the last fifty years – dual-tone multifrequency (DTMF) dialing. Our second installment will look at an in-band signaling method that far fewer people have heard, precisely because it was designed to be sub-audible — coded squelch systems for public service and other radio services. Continue reading “In-Band Signaling: Coded Squelch Systems”

My Beef with Ham Radio

My amateur radio journey began back in the mid-1970s. I was about 12 at the time, with an interest in electronics that baffled my parents. With little to guide me and fear for my life as I routinely explored the innards of the TVs and radios in the house, they turned to the kindly older gentleman across the street from us, Mr. Brown. He had the traditional calling card of the suburban ham — a gigantic beam antenna on a 60′ mast in the backyard – so they figured he could act as a mentor to me.

Mr. Brown taught me a lot about electronics, and very nearly got me far enough along to take the test for my Novice class license. But I lost interest, probably because I was an adolescent male and didn’t figure a ham ticket would improve my chances with the young ladies. My ham ambitions remained well below the surface as life happened over the next 40 or so years. But as my circumstances changed, the idea of working the airwaves resurfaced, and in 2015 I finally took the plunge and earned my General class license.

The next part of my ham story is all-too-familiar these days: I haven’t done a damn thing with my license. Oh, sure, I bought a couple of Baofeng and Wouxun handy-talkies and lurked on the local repeaters. I even bought a good, solid HF rig and built some antennas, but I’ve made a grand total of one QSO — a brief chat with a ham in Texas from my old home in Connecticut on the 10-meter band. That’s it.

Continue reading “My Beef with Ham Radio”

An Amateur Radio Repeater Using An RTL-SDR And A Raspberry Pi

An amateur radio repeater used to be a complex assemblage of equipment that would easily fill a 19″ rack. There would be a receiver and a separate transmitter, usually repurposed from commercial units, a home-made logic unit with a microprocessor to keep an eye on everything, and a hefty set of filters to stop the transmitter output swamping the receiver. Then there would have been an array of power supply units to provide continued working during power outages, probably with an associated bank of lead-acid cells.

More recent repeaters have been commercial repeater units. The big radio manufacturers have spotted a market in amateur radio, and particularly as they have each pursued their own digital standards there has been something of an effort to provide repeater equipment to drive sales of digital transceivers.

But what if you fancy setting up a simple repeater and you have neither a shed full of old radios or a hotline to the sales department of a large Japanese manufacturer? If you are [Anton Janovsky, ZR6AIC], you make your own low-powered repeater using an RTL-SDR, a low-pass filter, and a Raspberry Pi.

[Anton]’s repeater is a clever assemblage through pipes of rtl_sdr doing the receiving, csdr demodulating, and [F5OEO]’s rpitx doing the transmitting. As far as we can see it doesn’t have a toneburst detector or CTCSS to control its transmission so it is on air full-time, however we suspect that may be a feature that will be implemented in due course.

With only a 10 mW output this repeater is more of a toy than a useful device, and we’d suggest any licensed amateur wanting to have a go should read the small print in their licence schedule before doing so. But it’s a neat usage of a Pi and an RTL stick, and with luck it’ll inspire others in the same vein.

We’ve touched on the Pi as a transmitter before, from a straightforward broadcast FM unit to crossing continents with WSPR, and even transmitting digital TV in another [F5OEO] hack.

Commandeering public video screens: real or fake?

It’s time for everyone’s favorite comment thread game: Real or Fake? This week’s edition comes in from a tip that [Phil] sent about a way to take over video screens in Times Square. Watch the video after the break to see the hackers using a two-part solution to rebroadcast video from an iPhone onto a screen in the busy urban setting. The first part is a transmitter that plugs into the iPhone, the second is a signal repeater that, when held close to a video screen, overrides the clip currently being displayed with the video from the handheld. The image above shows the repeater being floated up to the big screen using a giant red balloon which you can make out in the black bar to the left of the replayed video.

Our first thought is that someone just watched Tron: Legacy and wanted to have a little Sci-Fi fun with the Internets. We can’t imagine a hardware solution that would actually make this work, but please do share your thoughts about that in the comments. We’d suspect this is more of a video hack that uses After Effects, similar to how the stopped motion candle video of the eyelid shutter glasses videos were faked. But apparently there is a follow-up video on the way that will show how the prototype was made so we could be wrong.

update: [Phil Burgess] points out that the “repeater” looks awfully familiar.

Fake for a variety of already-stated reasons (e.g. video out the headphone jack?). But the smoking gun, watching the 720P video on YouTube, is that I plainly recognize the hardware they’re using as the “repeater”: it’s simply the internals from a Digipower JS1-V3 cell phone USB boost charger (having torn apart a few myself):

Continue reading “Commandeering public video screens: real or fake?”