Rewinding Live Radio

August 20, 2018 by Brian Benchoff 35 Comments

Even though it’s now a forgotten afterthought in the history of broadcasting technology, we often forget how innovative the TiVo was. All this set-top box did was connect a hard drive to a cable box, but the power was incredible: you could pause live TV. You could record shows. You could rewind TV. It was an incredible capability, that no one had ever seen before. Of course, between Amazon and Netflix and YouTube, no one watches TV anymore, and all those platforms have a pause button, but the TiVO was awesome.

There is one bit of broadcasting that still exists. Radio. For his Hackaday Prize entry, [MagicWolfi] is bringing the set-top box to radio. He’s invented the Radio Rewind Button, and it does exactly what you would expect: it rewinds live radio a few minutes.

To have a pause or rewind button on a TV or radio, the only real requirement is a bunch of memory. The TiVO did this with a hard drive, and [MagicWolfi] is doing this with 256 MB of SDRAM. That means he needs to access a ton of RAM, and for that he’s turning to the Digilent ARTY S7 board. Yes, it’s an FPGA, but actually a fairly simple solution to the problem.

The rest of the circuit is an FM receiver chip and an I2S audio codec on an Arduino-shaped daughterboard. The main controller for this project is a big red button that will simply rewind the audio stream a few minutes. There’s no telling exactly how long [MagicWolfi] will be able to rewind the audio stream, but 256 MB is a ton in the audio world.

The Engineering Of An Ultrasonic Phased Array

August 19, 2018 by Jenny List 19 Comments

Ultrasonic phased arrays are one of the wonders of the moment, with videos of small items being levitated by them shared far and wide. We’ve all seen them and some of us have even wondered about building them, but what about the practical considerations? Just how would you drive a large array of ultrasonic transducers, and how would you maintain a consistent phase relationship between their outputs? It’s a problem [Niklas Fauth] has been grappling with over the three iterations so far of his ultrasonic phased array project, and you can follow his progress on the latest build.

The arrays themselves are a 16 by 16 grid of cheap ultrasonic transducers on a PCB, fed by HV583 high-voltage shift registers. These chips have proven to be particularly problematic, their drivers having a relatively high internal resistance which leaves them prone to overheating.

An interesting solution to a problem comes from the transducers having a polarity, but because it doesn’t matter in their usual application, that polarity not being marked. He’s overcome this by using the STM32 he has managing power alongside his BeagleBone to listen through a sensor as the ‘Bone supplies each transducer in turn with a known phase. An internal map can then be created, such that the appropriate phase can be applied on a transducer-by-transducer basis.

It’s the fascination with the subject that we find appealing, this is version three and version two worked. Most of us would make one and call it a day. It’s something we’ve seen before from [Niklas], after all this is someone who plays with turbomolecular pumps for fun. Meanwhile if you would like to learn more about ultrasonic arrays and acoustic levitation, it was the subject of one of this year’s Hackaday Belgrade talks.

The VU Meter And How It Got That Way

August 9, 2018 by Dan Maloney 44 Comments

Given its appearance in one form or another in all but the cheapest audio gear produced in the last 70 years or so, you’d be forgiven for thinking that the ubiquitous VU meter is just one of those electronic add-ons that’s more a result of marketing than engineering. After all, the seemingly arbitrary scale and the vague “volume units” label makes it seem like something a manufacturer would slap on a device just to make it look good. And while that no doubt happens, it turns out that the concept of a VU meter and its execution has some serious engineering behind that belies the really simple question it seeks to answer: How loud is this audio signal?

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Shell Script Synthesizer Knocks Your SoX Off

August 2, 2018 by Sven Gregori 5 Comments

Sound eXchange, or SoX, the “Swiss Army knife of audio manipulation” has been around for as long as the Linux kernel, and in case you’re not familiar with it, is a command line tool to play, record, edit, generate, and process audio files. [porkostomus] was especially interested about the generating part, and wrote a little shell script that utilizes SoX’s built-in synthesizer to compose 8-bit style music.

The script comes with a simple yet straightforward user interface to record the lead and bass parts into a text file, and play them back later on. Notes from C2 to C5 are currently supported, and are mapped to the keyboard in a two-row piano layout. The output file format itself is just a plain text listing of the played note, wave form, and note length. This lets you easily edit the song or even generate it from an alternative source, for example MIDI. Also note that there are no initial audio files required here, SoX will generate them as needed.

Admittedly, the command line interface may not be the most convenient way to create music, but nevertheless, it is a way — and that is [porkostomus]’s main mission here. Also, SoX is fun — and versatile, you can apply its audio effects even on images, or decode strange signals sent from a helicopter with it.

A Stereo Tube Amp For Less Than $5

July 30, 2018 by Ben James 21 Comments

Many of us have aspirations of owning a tube amp. Regardless of the debate on whether or not tube audio is nicer to listen to, or even if you can hear the difference at all, they’re gorgeous to look at. However, the price of buying one to find out if it floats your boat is often too high to justify a purchase.

[The Post Apocalyptic Inventor] has built a stereo tube amplifier in the style of the Fallout video games. The idea came when he realised that the TK 125 tape recorder manufactured by Grundig was still using tube audio in the late 60s. What’s more, they frequently sell on eBay for 1-10€ in Germany. [TPAI] was able to salvage the main power amplifier from one of these models, and restore it so that it could be re-purposed and see use once more.

The teardown of the original cassette recorder yields some interesting parts. Firstly, an integrated motor transformer — an induction motor whose stator acts as the magnetic core of the transformer responsible for the tube electronics. There’s also an integrated capacitor which contains three separate electrolytics. The video after the break is well worth a watch (we always find [TPAI]’s videos entertaining).

A new chassis is created out of a steel base plate and aluminium angle, and some neat frames for the motor transformers are made from scrap copper wire bent and soldered together. It looks great, though there’s always the option to use a cake tin instead.

If you’re interested in the design of tube amps, we’ve covered heaps of cool builds: from this low-voltage design to this tiny guitar amp, or even ones using tubes which are flat.

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The Hot And Cold Of Balanced Audio

July 27, 2018 by Dan Maloney 45 Comments

A few summers of my misspent youth found me working at an outdoor concert venue on the local crew. The local crew helps the show’s technicians — don’t call them roadies; they hate that — put up the show. You unpack the trucks, put up the lights, fly the sound system, help run the show, and put it all back in the trucks at the end. It was grueling work, but a lot of fun, and I got to meet people with names like “Mister Dog Vomit.”

One of the things I most remember about the load-in process was running the snakes. The snakes are fat bundles of cables, one for audio and one for lighting, that run from the stage to the consoles out in the house. The bigger the snakes, the bigger the show. It always impressed me that the audio snake, something like 50 yards long, was able to carry all those low-level signals without picking up interference from the AC thrumming through the lighting snake running right alongside it, while my stereo at home would pick up hum from the three-foot long RCA cable between the turntable and the preamp.

I asked one of the audio techs about that during one show, and he held up the end of the snake where all the cables break out into separate connectors. The chunky silver plugs clinked together as he gave his two-word answer before going back to patching in the console: “Balanced audio.”

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Transforming A Bookshelf Speaker Into A Portable Boombox

July 18, 2018 by Steven Dufresne 7 Comments

There’s a lot of fun to be had in modernizing an old boombox but what about turning one of those ubiquitous shelf speakers into a portable boombox, complete with a handle for carrying? That’s what [GreatScott] did when a friend gave him a just such a shelf speaker.

These days you’d very likely use your phone as the audio source so he included a 20 watt stereo class D amplifier which could be disconnected at the throw of a switch if not needed. To power the amplifier he used 16 18650 lithium-ion batteries which were leftover from previous projects. He estimates they should give him around 100 hours of enjoyable tunes. And to make further use of the batteries, he also added a USB charger so that he could charge up his phone from it, something else which is nice to be able to do when on the road.

A battery management system (BMS), an XT60 connector for charging the batteries, his battery level indicator circuit which we talked about before, a new passive audio crossover, and some rather nice work on that case all round out the boombox. Check out his full construction in the video below and make sure to stay until the end when he gives a taste of its awesome sound (you may even swear your desk is vibrating from the bass despite wearing earbuds, like we did).

And on the subject of speaker-to-boombox conversions, here’s one from a few years ago which makes use of a car MP3 player module giving it FM, USB, and SD card support.

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