I often ask people: What’s the most important thing you need to have a successful fishing trip? I get a lot of different answers about bait, equipment, and boats. Some people tell me beer. But the best answer, in my opinion, is fish. Without fish, you are sure to come home empty-handed.

On a recent visit to Bletchley Park, I thought about this and how it relates to World War II codebreaking. All the computers and smart people in the world won’t help you decode messages if you don’t already have the messages. So while Alan Turing and the codebreakers at Bletchley are well-known, at least in our circles, fewer people know about Arkley View.

The problem was apparent to the British. The Axis powers were sending lots of radio traffic. It would take a literal army of radio operators to record it all. Colonel Adrian Simpson sent a report to the director of MI5 in 1938 explaining that the three listening stations were not enough. The proposal was to build a network of volunteers to handle radio traffic interception.

That was the start of the Radio Security Service (RSS), which started operating out of some unused cells at a prison in London. The volunteers? Experienced ham radio operators who used their own equipment, at first, with the particular goal of intercepting transmissions from enemy agents on home soil.

At the start of the war, ham operators had their transmitters impounded. However, they still had their receivers and, of course, could all read Morse code. Further, they were probably accustomed to pulling out Morse code messages under challenging radio conditions.

Over time, this volunteer army of hams would swell to about 1,500 members. The RSS also supplied some radio gear to help in the task. MI5 checked each potential member, and the local police would visit to ensure the applicant was trustworthy. Keep in mind that radio intercepts were also done by servicemen and women (especially women) although many of them were engaged in reporting on voice communication or military communications.

Early Days

The VIs (voluntary interceptors) were asked to record any station they couldn’t identify and submit a log that included the messages to the RSS.

The hams of the RSS noticed that there were German signals that used standard ham radio codes (like Q signals and the prosign 73). However, these transmissions also used five-letter code groups, a practice forbidden to hams.

Thanks to a double agent, the RSS was able to decode the messages that were between agents in Europe and their Abwehr handlers back in Germany (the Abwehr was the German Secret Service) as well as Abwehr offices in foreign cities. Later messages contained Enigma-coded groups, as well.

Between the RSS team’s growth and the fear of bombing, the prison was traded for Arkley View, a large house near Barnet, north of London. Encoded messages went to Bletchley and, from there, to others up to Churchill. Soon, the RSS had orders to concentrate on the Abwehr and their SS rivals, the Sicherheitsdienst.

Change in Management

In 1941, MI6 decided that since the RSS was dealing with foreign radio traffic, they should be in charge, and thus RSS became SCU3 (Special Communications Unit 3).

There was fear that some operators might be taken away for normal military service, so some operators were inducted into the Army — sort of. They were put in uniform as part of the Royal Corps of Signals, but not required to do very much you’d expect from an Army recruit.

Those who worked at Arkley View would process logs from VIs and other radio operators to classify them and correlate them in cases where there were multiple logs. One operator might miss a few characters that could be found in a different log, for example.

Going 24/7

It soon became clear that the RSS needed full-time monitoring, so they built a number of Y stations with two National HRO receivers from America at each listening position. There were also direction-finding stations built in various locations to attempt to identify where a remote transmitter was.

Many of the direction finding operators came from VIs. The stations typically had four antennas in a directional array. When one of the central stations (the Y stations) picked up a signal, they would call direction finding stations using dedicated phone lines and send them the signal.

The operator would hear the phone signal in one ear and the radio signal in the other. Then, they would change the antenna pattern electrically until the signal went quiet, indicating the antenna was electrically pointing away from the signals.

The DF operator would hear this signal in one earpiece. They would then tune their radio receiver to the right frequency and match the signal from the main station in one ear to the signal from their receiver in the other ear. This made sure they were measuring the correct signal among the various other noise and interference. The DF operator would then take a bearing by rotating the dial on their radiogoniometer until the signal faded out. That indicated the antenna was pointing the wrong way which means you could deduce which way it should be pointing.

The central station could plot lines from three direction finding stations and tell the source of a transmission. Sort of. It wasn’t incredibly accurate, but it did help differentiate signals from different transmitters. Later, other types of direction-finding gear saw service, but the idea was still the same.

Interesting VIs

Most of the VIs, like most hams at the time, were men. But there were a few women, including Helena Crawley. She was encouraged to marry her husband Leslie, another VI, so they could be relocated to Orkney to copy radio traffic from Norway.

In 1941, a single VI was able to record an important message of 4,429 characters. He was bedridden from a landmine injury during the Great War. He operated from bed using mirrors and special control extensions. For his work, he receive the British Empire Medal and a personal letter of gratitude from Churchill.

Results

Because of the intercepts of the German spy agency’s communications, many potential German agents were known before they arrived in the UK. Of about 120 agents arriving, almost 30 were turned into double agents. Others were arrested and, possibly, executed.

By the end of the war, the RSS had decoded around a quarter of a million intercepts. It was very smart of MI5 to realize that it could leverage a large number of trained radio operators both to cover the country with receivers and to free up military stations for other uses.

Meanwhile, on the other side of the Atlantic, the FCC had a similar plan.

The BBC did a documentary about the work the hams did during the war. You can watch it below.

The Functionalist design philosophy that Dieter Rams brought to Braun from the 50s to the 90s still inspires the look of a few devices, including Apple’s iPod, Teenage Engineer’s synthesizers and recorders – and [2dom]’s IR7 streaming radio.

The streaming radio was inspired by Braun’s portable radios, particularly the SK2, TP1, and the T3 pocket radio. [2dom] started with the T3’s circular pattern of holes and experimented with several variations, finally settling on a cylindrical shape with a central display; a prototype with a low-power monochrome rectangular display was eventually rejected in favor of a circular LCD. The housing consists of four 3D-printed components: an upper and lower shell, a resonator for the speaker, and a knob for a rotary encoder.

Electronics-wise, an ESP32 handles the computing requirements, while the LCD and rotary encoder provide a user interface. For audio, it uses a VS1053 MP3 decoder, PAM8403 amplifier, and a wideband speaker, with an audio isolation transformer to clean up the audio. To reduce power consumption, a MOSFET cuts power to the peripheral components whenever the device is in sleep mode. The full design is available on GitHub.

The end result of this effort is a quite authentic-looking 21st-century adaptation of Rams’s original designs. If you’re interested in more Braun designs, check out this replica of one of their desk fans. We’ve also seen a restoration of one of Braun’s larger radios, the TS2.

Software Defined Radio (SDR) is the big thing these days, and why not? A single computer can get rid of a room full of boat anchors, and give you better signal discrimination than all but the best kit. Any SDR project needs an RF receiver, and in this project [mircemk] used a single 6J1 vaccum tube to produce an SSB SDR that combines the best of old and new. 

Single-tube radios are a classic hack, and where a lot of hams got started back in the day, but there is a reason more complicated circuits tend to be used. On the other hand, if you can throw a PC worth of signal processing at the output, it looks like you can get a very sensitive and selective single-sideband (SSB) receiver. 

The 6J1 tube is convenient, since it can run on only 6 V (or down to 3.7 as [mircemk] demonstrates). Here it is used as a mixer, with the oscillator signal injected via the screen grid. Aside from that, the simple circuit consists of a receiving coil, a few resistors and a variable capacitor. How well does it work? Quite well, when paired with a PC; you can judge for yourself in the video embedded below.

We’ve featured a lot of [mircemk]’s projects over the years, like this handsome OLED VU meter, or this frequency analyzer with a VFD  and even a virtual pinball cabinet made from scraps, among many others.

With few exceptions, amateur radio is a notably sedentary pursuit. Yes, some hams will set up in a national or state park for a “Parks on the Air” activation, and particularly energetic operators may climb a mountain for “Summits on the Air,” but most hams spend a lot of time firmly planted in a comfortable chair, spinning the dials in search of distant signals or familiar callsigns to add to their logbook.

There’s another exception to the band-surfing tendencies of hams: fox hunting. Generally undertaken at a field day event, fox hunts pit hams against each other in a search for a small hidden transmitter, using directional antennas and portable receivers to zero in on often faint signals. It’s all in good fun, but fox hunts serve a more serious purpose: they train hams in the finer points of radio direction finding, a skill that can be used to track down everything from manmade noise sources to unlicensed operators. Or, as was done in the 1940s, to ferret out foreign agents using shortwave radio to transmit intelligence overseas.

That was the primary mission of the Radio Intelligence Division, a rapidly assembled organization tasked with protecting the United States by monitoring the airwaves and searching for spies. The RID proved to be remarkably effective during the war years, in part because it drew heavily from the amateur radio community to populate its many field stations, but also because it brought an engineering mindset to the problem of finding needles in a radio haystack.

Winds of War

America’s involvement in World War II was similar to Hemingway’s description of the process of going bankrupt: Gradually, then suddenly. Reeling from the effects of the Great Depression, the United States had little interest in European affairs and no appetite for intervention in what increasingly appeared to be a brewing military conflict. This isolationist attitude persisted through the 1930s, surviving even the recognized start of hostilities with Hitler’s sweep into Poland in 1939, at least for the general public.

But behind the scenes, long before the Japanese attack on Pearl Harbor, precipitous changes were afoot. War in Europe was clearly destined from the outset to engulf the world, and in the 1940s there was only one technology with a truly global reach: radio. The ether would soon be abuzz with signals directing troop movements, coordinating maritime activities, or, most concerningly, agents using spy radios to transmit vital intelligence to foreign governments. To be deaf to such signals would be an unacceptable risk to any nation that fancied itself a world power, even if it hadn’t yet taken a side in the conflict.

It was in that context that US President Franklin Roosevelt approved an emergency request from the Federal Communications Commission in 1940 for $1.6 million to fund a National Defense Operations section. The group would be part of the engineering department within the FCC and was tasked with detecting and eliminating any illegal transmissions originating from within the country. This was aided by an order in June of that year which prohibited the 51,000 US amateur radio operators from making any international contacts, and an order four months later for hams to submit to fingerprinting and proof of citizenship.

A Ham’s Ham

The man behind the formation of the NDO was George Sterling. To call Sterling an early adopter of amateur radio would be an understatement. He plunged into radio as a hobby in 1908 at the tender age of 14, just a few years after Marconi and others demonstrated the potential of radio. He was licensed immediately after the passage of the Radio Act of 1927, callsign 1AE (later W1AE), and continued to experiment with spark gap stations. When the United States entered World War I, Sterling served for 19 months in France as an instructor in the Signal Corps, later organizing and operating the Corps’ first radio intelligence unit to locate enemy positions based on their radio transmissions.

After a brief post-war stint as a wireless operator in the Merchant Marine, Sterling returned to the US to begin a career in the federal government with a series of radio engineering and regulatory jobs. He rose through the ranks over the 1920s and 1930s, eventually becoming Assistant Chief of the FCC Field Division in 1937, in charge of radio engineering for the entire nation. It was on the strength of his performance in that role that he was tapped to be the first — and as it would turn out, only — chief of the NDO, which was quickly raised to the level of a new division within the FCC and renamed the Radio Intelligence Division.

To adequately protect the homeland, the RID needed a truly national footprint. Detecting shortwave transmissions is simple enough; any single location with enough radio equipment and a suitable antenna could catch most transmissions originating from within the US or its territories. But Sterling’s experience in France taught him that a network of listening stations would be needed to accurately triangulate on a source and provide a physical location for follow-up investigation.

The network that Sterling built would eventually comprise twelve primary stations scattered around the US and its territories, including Alaska, Hawaii, and Puerto Rico. Each primary station reported directly to RID headquarters in Washington, DC, by telephone, telegraph, or teletype. Each primary station supported up to a few dozen secondary stations, with further coastal monitoring stations set up as the war ground on and German U-boats became an increasingly common threat. The network would eventually comprise over 100 stations stretched from coast to coast and beyond, staffed by almost 900 agents.

Searching the Ether

The job of staffing these stations with skilled radio operators wasn’t easy, but Sterling knew he had a ready and willing pool to pull from: his fellow hams. Recently silenced and eager to put their skills to the test, hams signed up in droves for the RID. About 80% of the RID staff were composed of current or former amateur radio operators, including the enforcement branch of sworn officers who carried badges and guns. They were the sharp end of the spear, tasked with the “last mile” search for illicit transmitters and possible confrontation with foreign agents.

But before the fedora-sporting, Tommy-gun toting G-men could swoop in to make their arrest came the tedious process of detecting and classifying potentially illicit signals. This task was made easier by an emergency order issued on December 8, 1941, the day after the Pearl Harbor attack, forbidding all amateur radio transmissions below 56 MHz. This reduced the number of targets the RID listening stations had to sort through, but the high-frequency bands cover a lot of turf, and listening to all that spectrum at the same time required a little in-house innovation.

Today, monitoring wide swaths of the spectrum is relatively easy, but in the 1940s, it was another story. Providing this capability fell to RID engineers James Veatch and William Hoffert, who invented an aperiodic receiver that covered everything from 50 kHz to 60 MHz. Called the SSR-201, this radio used a grid-leak detector to rectify and amplify all signals picked up by the antenna. A bridge circuit connected the output of the detector to an audio amplifier, with the option to switch an audio oscillator into the circuit so that continuous wave transmissions — the spy’s operating mode of choice — could be monitored. There was also an audio-triggered relay that could start and stop an external recorder, allowing for unattended operation.

The SSR-201 and a later variant, the K-series, were built by Kann Manufacturing, a somewhat grand name for a modest enterprise operating out of the Baltimore, Maryland, basement of Manuel Kann (W3ZK), a ham enlisted by the RID to mass produce the receiver. Working with a small team of radio hobbyists and broadcast engineers mainly working after hours, Kann Manufacturing managed to make about 200 of the all-band receivers by the end of the war, mainly for the RID but also for the Office of Strategic Services (OSS), the forerunner of the CIA, as well as the intelligence services of other allied nations.

These aperiodic receivers were fairly limited in terms of sensitivity and lacked directional capability, and so were good only for a first pass scan of a specific area for the presence of a signal. Consequently, they were often used in places where enemy transmitters were likely to operate, such as major cities near foreign embassies. This application relied on the built-in relay in the receiver to trigger a remote alarm or turn on a recorder, giving the radio its nickname: “The Watchdog.” The receivers were also often mounted in mobile patrol vehicles that would prowl likely locations for espionage, such as Army bases and seaports. Much later in the war, RID mobile units would drive through remote locations such as the woods around Oak Ridge, Tennessee, and an arid plateau in the high desert near Los Alamos, New Mexico, for reasons that would soon become all too obvious.

Radio G-Men

Once a candidate signal was detected and headquarters alerted to its frequency, characteristics, and perhaps even its contents, orders went out to the primary stations to begin triangulation. Primary stations were equipped with radio direction finding (RDF) equipment, including the Adcock-type goniometer. These were generally wooden structures elevated above the ground with a distinctive Adcock antenna on the roof of the shack. The antenna was a variation on the Adcock array using two vertical dipoles on a steerable mount. The dipoles were connected to the receiving gear in the shack 180 degrees out of phase. This produced a radiation pattern with very strong nulls broadside to the antenna, making it possible for operators to determine the precise angle to the source by rotating the antenna array until the signal is minimized. Multiple stations would report the angle to the target to headquarters, where it would be mapped out and a rough location determined by where the lines intersected.

With a rough location determined, RID mobile teams would hit the streets. RID had a fleet of mobile units based on commercial Ford and Hudson models, custom-built for undercover work. Radio gear partially filled the back seat area, power supplies filled the trunk, and a small steerable loop antenna could be deployed through the roof for radio direction finding on the go. Mobile units were also equipped with special radio sets for communicating back to their primary station, using the VHF band to avoid creating unwanted targets for the other stations to monitor.

Mobile units were generally capable of narrowing the source of a transmission down to a city block or so, but locating the people behind the transmission required legwork. Armed RID enforcement agents would set out in search of the transmitter, often aided by a device dubbed “The Snifter.” This was a field-strength meter specially built for covert operations; small enough to be pocketed and monitored through headphones styled to look like a hearing aid, the agents could use the Snifter to ferret out the spy, hopefully catching them in the act and sealing their fate.

A Job (Too) Well Done

For a hastily assembled organization, the RID was remarkably effective. Originally tasked with monitoring the entire United States and its territories, that scope very quickly expanded to include almost every country in South America, where the Nazi regime found support and encouragement. Between 1940 and 1944, the RID investigated tens of thousands, resulting in 400 unlicensed stations being silenced. Not all of these were nefarious; one unlucky teenager in Portland, Oregon, ran afoul of the RID by hooking an antenna up to a record player so he could play DJ to his girlfriend down the street. But other operations led to the capture of 200 spies, including a shipping executive who used his ships to refuel Nazi U-boats operating in the Gulf of Mexico, and the famous Dusquense Spy Ring operating on Long Island.

Thanks in large part to the technical prowess of the hams populating its ranks, the RID’s success contained the seeds of its downfall. Normally, such an important self-defense task as preventing radio espionage would fall to the Army or Navy, but neither organization had the technical expertise in 1940, nor did they have the time to learn given how woefully unprepared they were for the coming war. Both branches eventually caught up, though, and neither appreciated a bunch of civilians mucking around on their turf. Turf battles ensued, politics came into it, and by 1944, budget cuts effectively ended the RID as a standalone agency.

The world’s militaries have always been at the forefront of communications technology. From trumpets and drums to signal flags and semaphores, anything that allows a military commander to relay orders to troops in the field quickly or call for reinforcements was quickly seized upon and optimized. So once radio was invented, it’s little wonder how quickly military commanders capitalized on it for field communications.

Radiotelegraph systems began showing up as early as the First World War, but World War II was the first real radio war, with every belligerent taking full advantage of the latest radio technology. Chief among these developments was the ability of signals in the high-frequency (HF) bands to reflect off the ionosphere and propagate around the world, an important capability when prosecuting a global war.

But not long after, in the less kinetic but equally dangerous Cold War period, military planners began to see the need to move more information around than HF radio could support while still being able to do it over the horizon. What they needed was the higher bandwidth of the higher frequencies, but to somehow bend the signals around the curvature of the Earth. What they came up with was a fascinating application of practical physics: meteor burst communications.

Blame It on Shannon

In practical terms, a radio signal that can carry enough information to be useful for digital communications while still being able to propagate long distances is a bit of a paradox. You can thank Claude Shannon for that, after he developed the idea of channel capacity from the earlier work of Harry Nyquist and Ralph Hartley. The resulting Hartley-Shannon Theorem states that the bit rate of a channel in a noisy environment is directly related to the bandwidth of the channel. In other words, the more data you want to stuff down a channel, the higher the frequency needs to be.

Unfortunately, that runs afoul of the physics of ionospheric propagation. Thanks to the physics of the interaction between radio waves and the charged particles between about 50 km and 600 km above the ground, the maximum frequency that can be reflected back toward the ground is about 30 MHz, which is the upper end of the HF band. Beyond that is the very-high frequency (VHF) band from 30 MHz to 300 MHz, which has enough bandwidth for an effective data channel but to which the ionosphere is essentially transparent.

Luckily, the ionosphere isn’t the only thing capable of redirecting radio waves. Back in the 1920s, Japanese physicist Hantaro Nagaoka observed that the ionospheric propagation of shortwave radio signals would change a bit during periods of high meteoric activity. That discovery largely remained dormant until after World War II, when researchers picked up on Nagoka’s work and looked into the mechanism behind his observations.

Every day, the Earth sweeps up a huge number of meteoroids; estimates range from a million to ten billion. Most of those are very small, on the order of a few nanograms, with a few good-sized chunks in the tens of kilograms range mixed in. But the ones that end up being most interesting for communications purposes are the particles in the milligram range, in part because there are about 100 million such collisions on average every day, but also because they tend to vaporize in the E-level of the ionosphere, between 80 and 120 km above the surface. The air at that altitude is dense enough to turn the incoming cosmic debris into a long, skinny trail of ions, but thin enough that the free electrons take a while to recombine into neutral atoms. It’s a short time — anywhere between 500 milliseconds to a few seconds — but it’s long enough to be useful.

The other aspect of meteor trails formed at these altitudes that makes them useful for communications is their relative reflectivity. The E-layer of the ionosphere normally has on the order of 10⁷ electrons per cubic meter, a density that tends to refract radio waves below about 20 MHz. But meteor trails at this altitude can have densities as high as 10¹¹ to 10¹² electrons/m³. This makes the trails highly reflective to radio waves, especially at the higher frequencies of the VHF band.

In addition to the short-lived nature of meteor trails, daily and seasonal variations in the number of meteors complicate their utility for communications. The rotation of the Earth on its axis accounts for the diurnal variation, which tends to peak around dawn local time every day as the planet’s rotation and orbit are going in the same direction and the number of collisions increases. Seasonal variations occur because of the tilt of Earth’s axis relative to the plane of the ecliptic, where most meteoroids are concentrated. More collisions occur when the Earth’s axis is pointed in the direction of travel around the Sun, which is the second half of the year for the northern hemisphere.

Learning to Burst

Building a practical system that leverages these highly reflective but short-lived and variable mirrors in the sky isn’t easy, as shown by several post-war experimental systems. The first of these was attempted by the National Bureau of Standards in 1951. They set up a system between Cedar Rapids, Iowa, and Sterling, Virginia, a path length of about 1250 km. Originally built to study propagation phenomena such as forward scatter and sporadic E, the researchers noticed significant effects on their tests by meteor trails. This made them switch their focus to meteor trails, which caught the attention of the US Air Force. They were in the market for a four-channel continuous teletype link to their base in Thule, Greenland. They got it, but only just barely, thanks to the limited technology of the time. The NBS system also used the Iowa to Virginia system to study higher data rates by pointing highly directional rhombic antennas at each end of the connection at the same small patch of sky. They managed a whopping data rate of 3,200 bits per second with this system, but only for the second or so that a meteor trail happened to appear.

The successes and failures of the NBS system made it clear that a useful system based on meteor trails would need to operate in burst mode, to jam data through the link for as long as it existed and wait for the next one. The NBS tested a burst-mode system in 1958 that used the 50-MHz band and offered a full-duplex link at 2,400 bits per second. The system used magnetic tape loops to buffer data and transmitters at both ends of the link that operated continually to probe for a path. Whenever the receiver at one end detected a sufficiently strong probe signal from the other end, the transmitter would start sending data. The Canadians got in on the MBC action with their JANET system, which had a similar dedicated probing channel and tape buffer. In 1954 they established a full-duplex teletype link between Ottawa and Nova Scotia at 1,300 bits per second with an error rate of only 1.5%

In the late 1950s, Hughes developed a single-channel air-to-ground MBC system. This was a significant development since not only had the equipment gotten small enough to install on an airplane but also because it really refined the burst-mode technology. The ground stations in the Hughes system periodically transmitted a 100-bit interrogation signal to probe for a path to the aircraft. The receiver on the ground listened for an acknowledgement from the plane, which turned the channel around and allowed the airborne transmitter to send a 100-bit data burst. The system managed a respectable 2,400 bps data rate, but suffered greatly from ground-based interference for TV stations and automotive ignition noise.

The SHAPE of Things to Come

The first major MBC system fielded during the Cold War was the Communications by Meteor Trails system, or COMET. It was used by the North Atlantic Treaty Organization (NATO) to link its far-flung outposts in member nations with Supreme Headquarters Allied Powers Europe, or SHAPE, located in Belgium. COMET took cues from the Hughes system, especially its error detection and correction scheme. COMET was a robust and effective MBC system that provided between four and eight teletype circuits depending on daily and seasonal conditions, each handling 60 words per minute.

COMET was in continuous use from the mid-1960s until well after the official end of the Cold War. By that point, secure satellite communications were nowhere near as prohibitively expensive as they had been at the beginning of the Space Age, and MBC systems became less critical to NATO. They weren’t retired, though, and COMET actually still exists, although rebranded as “Compact Over-the-Horizon Mobile Expeditionary Terminal.” These man-portable systems don’t use MBC; rather, they use high-power UHF and microwave transmitters to scatter signals off the troposphere. A small amount of the signal is reflected back to the ground, where high-gain antennas pick up the vanishingly weak signals.

Although not directly related to Cold War communications, it’s worth noting that there was a very successful MBC system fielded in the civilian space in the United States: SNOTEL. We’ve covered this system in some depth already, but briefly, it’s a network of stations in the western part of the USA with the critical job of monitoring the snowpack. A commercial MBC system connected the solar-powered monitoring stations, often in remote and rugged locations, to two different central bases. Taking advantage of diurnal meteor variations, each morning the master station would send a polling signal out to every remote, which would then send back the previous day’s data once a return path was opened. The system could collect data from 180 remote sites in just 20 minutes. It operated successfully from the mid-1970s until just recently, when pervasive cell technology and cheap satellite modems made the system obsolete.

In these days of everything-streaming, it’s great to see an old school radio build. It’s even better when it’s not old-school at all, but packed full of modern ICs and driven by a micro-controller like the dsPIC in [Minh Danh]’s dsMP3 build. Best of all is when we get enough details that the author needs two blog posts — one for hardware, and one for firmware — like [Minh Danh] has done.

This build does it all: radio, MP3 playback, and records incoming signals. The radio portion of the build is driven by an Si4735, which allows for receiving both in FM and AM — with all the AM bands, SW, MW and LW available. The FM section does support RDS, though because [Minh Danh] ran out of pins on the dsPIC, isn’t the perfect implementation.

The audio section is a good intro to audio engineering if you’ve never done a project like this: he’s using a TDA1308 for headphones, which feeds into a NS8002 to drive some hefty stereo speakers– and he tells you why he selected those chips, as well as providing broken-out schematics for each. Really, we can’t say enough good things about this project’s documentation.

That’s before we get to the firmware, where he tells us how he manages to get the dsPIC to read out MP3s from a USB drive, and write WAVs to it. One very interesting detail is how he used the dsPIC’s ample analog inputs to handle the front panel buttons on this radio: a resistor ladder. It’s a great solution in a project that’s full of them.

Of course we’ve seen radio receivers before, and plenty of MP3 players, too — but this might be the first time we’ve seen an electronic Swiss army knife with all these features, and we’re very glad [Minh Danh] shared it with us.

[Bill Dudley] had a problem. He had an Onkyo AV receiver that did a great job… until it didn’t. A DSP inside failed. When that happened, the main microprocessor running the show decided it wouldn’t play ball without the DSP operational. [Bill] knew the bulk of the audio hardware was still good, it was just the brains that were faulty. Thus started a 4-month operation to resurrect the Onkyo receiver with new intelligence instead.

[Bill’s] concept was simple. Yank the dead DSP, and the useless microprocessor as well. In their place, an ESP32 would be tasked with running things. [Bill] no longer cared if the receiver had DSP abilities or even the ability to pass video—he just wanted to use it as the quality audio receiver that it was.

His project report steps through all the hard work he went through to get things operational again. He had to teach the ESP32 to talk to the front panel display, the keys, and the radio tuner. More challenging was the core audio processor—the obscure Renaisys R2A15218FP. However, by persevering, [Bill] was able to get everything up and running, and even added some new functionality—including Internet radio and Bluetooth streaming.

It’s a heck of a build, and [Bill] ended up with an even more functional audio receiver at the end of it all. Bravo, we say. We love to see older audio gear brought back to life, particularly in creative ways. Meanwhile, if you’ve found your own way to save a piece of vintage audio hardware, don’t hesitate to let us know!

While ham radio operators have been embracing digital mobile radio (DMR), the equipment is most often bought since — at least in early incarnations — it needs a proprietary CODEC to convert speech to digital and vice versa. But [QRadioLink] decided to tackle a homebrew and open source DMR modem.

The setup uses a LimeSDR, GNU Radio, and Codec2. There are some other open DMR projects, such as OpenRTX. So we are hopeful there are going to be more choices. The DMR modem, however, is only a proof-of-concept and reuses the MMDVMHost code to do the data link layer.

[QRadioLink] found several receiver implementations available, but only one other DMR transmitter — actually, a transceiver. Rather than use an AMBE hardware device or the potentially encumbered mbelib codec, the project uses Codec2 which is entirely open source.

There’s a lot of explanation about the data collection to prepare for the project, and then a deep dive into the nuts and bolts of the implementation. You might enjoy the video below to see things in action.

If you just want to listen to DMR, it’s easy. If Codec2 sounds familiar, it is part of M17.