Creating A Numbers Station Of Your Very Own

March 17, 2024 by Lewin Day 13 Comments

Numbers stations are a weird phenomenon where odd voices read out long strings of numbers or random codewords to the confusion of the vast majority of the listening audience. If you’ve ever wanted to build one of your own, you could follow the example of [AudioWanderer].

NumberMumble, as it’s called, is a numbers station emulator. It doesn’t signal spy networks or reveal national secrets. Instead, it randomly plays audio samples using an Arduino, including characteristic bursts of white noise that make it sound more authentic. It relies on the Mozzi library to help with audio tasks, including generating white noise and playing back samples. It’s also kitted out with a filter knob for varying the tone. Audio output is via PWM.

If you want to confuse your neighbours with oddball audio, put this thing on a radio transmitter and get broadcasting. But don’t, because that’s illegal without the proper licenses or — you know — if you happen to be a real spy. Video after the break.

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Playing Audio On The Pi Pico With No DAC To Speak Of

March 12, 2024 by Lewin Day 24 Comments

Normally, if you want to play music or other audio on a microcontroller, you need to get yourself a DAC. Or at least, that’s the easiest way to go about it and the one most likely to get you good, intelligible audio. You don’t have to go that way, though, as [antirez] demonstrates.

[antirez] decided to do this with a Pi Pico, but it’s applicable to other microcontrollers too. It’s all done with a single pin and a PWM output. The PWM output is set to a very high frequency beyond human hearing. In this case, it was 100 KHz. Then, the duty cycle of the PWM is changed to essentially output various average voltage levels at the pin. Vary the output voltage as per your desired sound file by using each sample to vary the duty cycle of the PWM. Voila! You can output whatever sound you want on that pin! [antirez] steps through the basics of doing this, including processing simple WAV files into a raw format that can be dumped into MicroPython code.

There’s no sound sample on the project page, and we’d have to assume it sounds pretty crunchy when hooked up to a speaker. And yet, it could prove a useful technique if you’re designing your own audio greeting cards or something, so keep that in mind!

Reverse Engineering The Behringer Ultranet Protocol

March 5, 2024 by Richard Baguley 26 Comments

Ultranet is a protocol created by audio manufacturer Behringer to transmit up to 16 channels of 24-bit sound over a Cat-5 cable. It’s not an open standard, though: Behringer doesn’t offer an API or protocol description to build your own Ultranet devices. But that didn’t stop [Christian Nödig], thanks to a defective mixer, he poked into the signals and built his own Ultranet receiver.

Ultranet runs over Cat-5 ethernet cables but isn’t an ethernet-based protocol. The electrical protocols of Ultranet are identical to Ethernet, but the signaling is different, making it a Level 1 protocol. So, you can use any Cat-5 cable for Ultranet, but you can’t just plug an Ultranet device into an Ethernet one. Or rather, you can (and neither device should explode), but you won’t get anything out of it.

Instead, [Christian]’s exploration revealed that Ultranet is based on another standard: AES/EBU, the bigger professional brother of the SPD/IF socket on HiFi systems. This was designed to carry digital audio over an XLR cable, and Behringer has taken AES/EBU and tweaked it to run over a single twisted pair. With two twisted pairs in the cable carrying a 192 kbps signal, you get sixteen channels of 24-bit audio in total over two twisted pairs inside the Cat-5 cable.

That’s a bit fast for a microcontroller to decode reliably, so [Christian] uses the FPGA in an Arduino Vidor 4000 MKR in his receiver with an open-source AES decoder core to receive and decode the Ultranet signal into individual channels, which are passed to an ADC and analog output.

In effect, [Christian] has built a 16-channel mixer, although the mixing aspect is too primitive for actual use. It would be great for monitoring, though, and it’s a beautiful description of how to dig into protocols like Ultranet that look locked up but are based on other, more open standards.

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Tube Design Tips To Save A Writer’s Project

December 30, 2023 by Jenny List 24 Comments

Most of the stories we cover here are fresh from the firehose, the newest and coolest stuff to interest you during your idle moments. Sometimes though, we come across a page that’s not new, but is interesting in its own right enough to bring to your attention. So it is with our subject here, because when faced with a tube circuit design problem, we found salvation in a page from [The Valve Wizard].

Do you need to apply negative feedback to a triode amplifier? The circuit is simplicity itself, but sadly when we were at university they had long ago stopped teaching the mathematics behind the component values. Step forward everything you need to know about triode amplifier negative feedback.

Negative feedback is a pretty simple idea: subtract a little of the amplifier’s output from the input. It reduces the amplifier’s gain with a flat response, so it’s useful for removing humps in the frequency response and reducing the tendency for distortion. In a single-ended triode amp it’s done with a resistor and capacitor from anode to grid, but the question is, just what resistor or capacitor?. Here the page has all the answers, taking the reader through calculating the desired gain, and picking the value of the capacitor to avoid affecting the frequency response. We wish that someone had taught us this three decades ago!

The website is full of really useful info about valve or tube amps, and it’s worth mentioning that he’s made it available in book format too. There’s no reason not to have a go at vacuum electronics. Meanwhile in case you are wondering what project prompted this, it was a quest to improve upon this cheap Chinese kit amplifier.

A High-End Studio Multiplexer Surrenders To An Arduino

December 27, 2023 by Jenny List 17 Comments

The equipment used in professional radio and TV studios is both extremely high quality and very expensive indeed, and thus out of the reach of an experimenter. Happily as studios are refurbished there’s a steady supply of second-hand equipment which can be surprisingly cheap, but as [Nathan] found out with a Quartz audio router, comes with no control software. What’s to be done with what’s essentially a piece of junk? Remove its brain and replace it with one that can be controlled, of course!

On the PCB alongside a bank of switch matrices is an FPGA which does the heavy lifting. That’s “heavy” in a limited sense, because all it does is handle the chip select lines for the matrices and write data to their registers. This is a task that can be handled by a microcontroller, so in goes an Arduino Nano, which along with a few other board modifications delivers a serial-controlled studio router.

The interesting part for us in this project comes from a look at the date codes on the board, they’re from the early 2000s. This is (roughly) contemporary with the ATmega chip on the Arduino, so we’re curious as to why the designers saw fit to use an FPGA when the microcontrollers of the day were clearly up to the task for much less outlay. We suspect a touch of millennium-era price inflation, but we can’t be sure.

Meanwhile, old broadcast kit has featured here before.

Raspberry Pi Pico Becomes MIDI-Compatible Synth

December 22, 2023 by Lewin Day 13 Comments

ECE 4760 is a microcontroller course that runs at Cornell every year, and it gives students a wide remit to pursue various kinds of microcontroller projects. [Pelham Bergesen] took the class and built himself a MIDI-controllable synthesizer out of a Raspberry Pi Pico.

[Pelham] coded a library to parse MIDI messages on the Pico, with the microcontroller’s UART charged with receiving the input data. MIDI is basically just serial at a baud rate of 31.25k, with a set message structure, after all. From there, the Pico takes the note data and plays the relevant frequencies by synthesizing square waves using a PWM output. A second PWM channel can also be blended with the first to generate more complex tones. The synthesizer is designed to be used with a source of MIDI note data such as a keyboard controller; [Pelham] demonstrates the project in use with a Roland JD-XI. It’s a fairly basic synthesizer, but [Pelham] does a good job of explaining all the steps required to get this far. If you’ve never done an audio or MIDI project before, you might find his guide very helpful for the way it steps through the basics.

[Pelham] didn’t get to implement fancier features like direct digital synthesis (DDS) or analog audio effects before the class closed out. However, that would be an excellent project for anyone else developing their own Pico synthesizer. If you whip up something that sounds good, or even just interesting, be sure to notify us on the tipsline. Video after the break.

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It’s An Audio Distortion Analyzer, Just Not The One You Were Hoping For

December 19, 2023 by Jenny List 7 Comments

An audio distortion analyzer is a specialist piece of analogue test equipment that usually costs a lot of money and can be hard to track down on the second hand market. Finding one is a moment of luck for the average engineer then, but [Thomas Scherrer OZ2CPU]’s discovery isn’t quite what he might have hoped for. Nonetheless, his Bang and Olufsen K3 Distortion meter DM1 from 1979 is still an interesting and high quality piece of test equipment, and the video below the break makes for a worthwhile watch.

Bang and Olufsen are best known for high-end design Hi-Fi units, thus it’s a surprise to find that in the past they also manufactured test equipment. This distortion meter isn’t a general purpose one, instead it’s designed to measure tape recorders in particular, and it uses an elegant technique. Instead of injecting a sine wave and removing it from what comes out in order to measure the products of the distortion, it records a 333 Hz sine wave onto a tape, then measures the strength of its 3rd harmonic at 1 kHz as an indication of distortion. It’s a working distortion meter made with clever analogue circuitry for a fraction of the cost of the more conventional models that HP would have sold you at the same time, even if it doesn’t give the same THD figure you might have been looking for.

If distortion interests you, it’s a subject we’ve looked at in the past.

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