A Homebrew Radio, As All The Best Homebrew Radios Should Be

It started with an old TV sound chip, and some curiosity. The TDA1701 that [Philip Bragg] found in a box of junk is a complete FM IF strip and audio power amplifier from the golden age of analogue PAL televisions, and while it was designed for the 5.5 MHz or 6 MHz FM subcarrier of European broadcast TV, he found it worked rather well at the more usual 10.7 MHz of a radio receiver. There followed a long thread detailing the genesis bit-by-bit of a decent quality VHF radio receiver, built dead-bug-style on a piece of PCB material.

The TDA1701 was soon joined by a couple of stages of IF amplification with a ceramic filter, and then by several iterations of a JFET mixer. A varicap tuned MOSFET RF amplifier followed, and then a local oscillator. Finally it became a fully-functional FM radio, with probably far better performance than most commercial radios. He admits tuning is a little impractical though, with what appears to be a cermet preset potentiometer covering the entire band.

We suspect this project isn’t finished, and we hope he posts the schematic. But it doesn’t really matter if he doesn’t, because the value here isn’t in the design. Instead it lies in the joy of creating an ad-hoc radio just for the fun of it, and that’s something we completely understand.

We’ve covered a lot of radios in our time, and while it might be the first to feature a TV sound chip, it’s not the first built on bare PCB.

The Price Of Domestic Just In Time Manufacturing

Hardware is hard, manufacturing only happens in China, accurate pricing is a dark art. Facts which are Known To Be True. And all things which can be hard to conquer as an independent hardware company, especially if you want to subvert the tropes. You may have heard of [Spencer Wright] via his superb mailing list The Prepared, but he has also been selling an unusual FM radio as Centerline Labs for a few years. Two years ago they relaunched their product, and last year the price was bumped up by a third. Why? Well, the answer involves more than just a hand wave about tariffs.

The Public Radio is a single-station FM radio in a mason jar. It’s a seemingly simple single purpose hardware product. No big mechanical assemblies, no complex packaging, not even any tangential accessories to include. In some sense it’s an archetypically atomic hardware product. So what changed? A normal product is manufactured in bulk, tested and packaged, then stored in a warehouse ready to ship. But TPR is factory programmed to a specific radio station, so unless Centerline wanted one SKU for each possible radio station (there are 300) this doesn’t work. The solution was domestic (US) just in time manufacturing. When a customer hits the buy button, a unit is programmed, tested, packed, and shipped.

As with any business, there is a lot more to things than that! The post gives the reader a fascinating look at all the math related to Centerline Labs’ pricing and expenses; in other words, what makes the business tick (or not) including discussion of the pricing tradeoffs between manufacturing different components in Asia. I won’t spoil the logical path that led to the pricing change, go check out the post for more detail on every part. 

We love hearing about the cottage hardware world. Got any stories? Drop them in the comments!

Cross-Correlation Makes Quick Work Of Ads

Once relegated to the proverbial Linux loving Firefox user, ad blocking has moved into public view among increased awareness of privacy and the mechanisms of advertising on the internet. At the annual family gathering, when That Relative asks how to setup their new laptop, we struggle through a dissertation on the value of ad blockers and convince them to install one. But what about mediums besides the internet? Decades ago Tivo gave us one button to jump through recorded TV. How about the radio? If available, satellite radio may be free of The Hated Advertisement. But terrestrial radio and online streams? [tomek] wasn’t satisfied with an otherwise sublime experience listening streaming Polish Radio Three and decided to build a desktop tool to detect and elide ads from the live audio stream.

[tomek] was aware of this hip knowledge domain called Digital Signal Processing but hadn’t done any of it themselves. Like many algorithmic problems the first step was to figure out the fastest way to bolt together a prototype to prove a given technique worked. We were as surprised as [tomek] by how simple this turned out to be. Fundamentally it required a single function – cross-correlation – to measure the similarity of two data samples (audio files in this case). And it turns out that Octave provides it in the box. After snipping the start-of-ad jingle out of a sample file and comparing it to a radio program [tomek] got the graph at the left. The conspicuous spike is the location of the jingle in the audio file.

At this point all that was left was packaging it all into a one click tool to listen to the radio without loading an entire analysis package. Conveniently Octave is open source software, so [tomek] was able to dig through its sources until they found the bones of the critical xcorr() function. [tomek] adapted their code to pour the audio into a circular buffer in order to use an existing Java FFT library, and the magic was done. Piping the stream out of ffmpeg and into the ad detector yielded events when the given ad jingle samples were detected.

[tomek] packaged that tool into a standalone executable, but the gem here is the followup post. After removing ads in the online stream they adapted a RaspberryPi to listen to an FM receiver and remote control their Yamaha tuner over the network. So when the tuner is playing Radio Three the Pi notices and ducks the audio appropriately to avoid those pesky ads. Video of this after the break.

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Edwin Armstrong’s Battle For FM Radio

Chances are you have at least one radio that can receive FM stations. Even though FM is becoming less used now with Internet and satellite options, it still is more popular than the older AM radio bands. FM was the brainchild of an inventor you may have heard of — Edwin Armstrong — but you probably don’t know the whole story. It could make a sort of radio-themed soap opera. It is a story of innovation, but also a story of personal vanity, corporate greed, stubbornness, marital problems, and even suicide. The only thing missing is a long-lost identical twin sibling to turn it into a full telenovela.

Early Days

Armstrong grew up in New York and because of an illness that gave him a tic and caused him to be homeschooled, he was somewhat of a loner. He threw himself into his interest in electric and mechanical devices. By 1909 he was enrolled in Columbia University where professors noted he was very focused on what interested him but indifferent to other studies. He was also known as someone more interested in practical results than theory. He received an electrical engineering degree in 1913.

Unlike a lot of college graduates, Armstrong didn’t go work for a big firm. Instead, he set up a self-financed independent lab at Columbia. This sounded good because it meant that he would own the patents on anything invented there. But it would turn out to be a two-edged sword.

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Learn Something About Phase Locked Loops

The phase locked loop, or PLL, is a real workhorse of circuit design. It is a classic feedback loop where the phase of an oscillator is locked to the phase of a reference signal using an error signal in the same basic way that perhaps a controller would hold a temperature or flow rate in a physical system. That is, a big error will induce a big change and little errors induce little changes until the output is just right. [The Offset Volt] has a few videos on PLLs that will help you understand their basic operation, how they can multiply frequencies (paradoxically, by dividing), and even demodulate FM radio signals. You can see the videos below.

The clever part of a PLL can be found in how it looks at the phase of two signals. For signals to be totally in phase, they must be at the same frequency and also must ebb and peak at the same point. It should be clear that if the frequency isn’t the same the ebbs and peaks can’t line up for any length of time. By detecting how much the signals don’t line up, an error voltage can be generated. That error voltage is used to adjust the output oscillator so that it matches the reference oscillator.

Of course, it wouldn’t be very interesting if the output frequency had to be the same as the reference frequency. The clever trick comes by dividing the output frequency. For example, a 100 MHz crystal oscillator is difficult to design. But taking a voltage-controlled oscillator at 100 MHz (nominal) and dividing its output by 100 will give you a signal you can lock to a 1 MHz crystal oscillator which is, of course, trivial to build.

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Art Deco Radio Gets FM Reception

Taking a vintage radio and cramming it full of modern, Internet-connected, guts has long been a staple of the hacking and making scene. While some might see it as a crime to take what’s arguably a legitimate piece of history and turn it into nothing more than a slipshod case for the Raspberry Pi, we have to admit there’s a certain appeal to the idea. Taking the beauty of classic design and pairing it with more modern capabilities is getting the best of both worlds.

But this project by [Nick Koumaris] is a somewhat unique take on the concept. Rather than sacrificing a real vintage piece of hardware to house the electronics, he’s designed a 3D printable case that looks like a classic 1936 AWA Radiolette. But what’s really interesting to us is that he then puts a basic FM radio inside of it.

That’s right, no Internet radio streaming or smartphone Bluetooth compatibility here. It’s just a regular FM radio, not entirely unlike the kind of hardware you’d expect to be inside of a classic radio. Of course, it’s much more modern, and [Nick] actually built it himself from a TEA5767 FM radio module and an Arduino Pro Mini.

While functionally it might not be terribly exciting, we do appreciate that he went through the trouble to make a vintage-looking user interface for the radio. While physical buttons would arguably have been more appropriate given the era, the art deco inspired font and graphics that show on the device’s Nokia 5110 LCD do look really slick.

Purists will surely be happy to see another project where a piece of vintage piece of audio equipment wasn’t sacrificed at the Altar of Hack, but we’ve also played host to many projects which weren’t nearly as concerned with historical preservation.

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34C3: Microphone Bugs

Inspiration can come from many places. When [Veronica Valeros] and [Sebastian Garcia] from the MatesLab Hackerspace in Argentina learned that it took [Ai Weiwei] four years to discover his home had been bugged, they decided to have a closer look into some standard audio surveillance devices. Feeling there’s a shortage of research on the subject inside the community, they took matters in their own hands, and presented the outcome in their Spy vs. Spy: A modern study of microphone bugs operation and detection talk at 34C3. You can find the slides here, and their white paper here.

Focusing their research primarily on FM radio transmitter devices, [Veronica] and [Sebastian] start off with some historical examples, and the development of such devices — nowadays available off-the-shelf for little money. While these devices may be shrugged off as a relic of Soviet era spy fiction and tools of analog times, the easy availability and usage still keeps them relevant today. They conclude their research with a game of Hide and Seek as real life experiment, using regular store-bought transmitters.

An undertaking like this would not be complete without the RTL-SDR dongle, so [Sebastian] developed the Salamandra Spy Microphone Detection Tool as alternative for ready-made detection devices. Using the dongle’s power levels, Salamandra detects and locates the presence of potential transmitters, keeping track of all findings. If you’re interested in some of the earliest and most technologically fascinating covert listening devices, there is no better example than Theremin’s bug.

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