Sending Music Long Distance Using A Laser

This isn’t the first time we’ve seen DIYers sending music over a laser beam but the brothers [Armand] and [Victor] are certainly in contention for sending the music the longest distance, 452 meter/1480 feet from their building, over the tops of a few houses, through a treetop and into a friend’s apartment. The received sound quality is pretty amazing too.

In case you’ve never encountered this before, the light of the laser is modulated with a signal directly from the audio source, making it an analog transmission. The laser is a 250mW diode laser bought from eBay. It’s powered through a 5 volt 7805 voltage regulator fed by a 12V battery. The signal from the sound source enters the circuit through a step-up transformer, isolating it so that no DC from the source enters. The laser’s side of the transformer feeds the base of a transistor. They included a switch so that the current from the regulator can either go through the collector and emitter of the transistor that’s controlled by the sound source, giving a strong modulation, or the current can go directly to the laser while modulation is provided through just the transistor’s base and emitter. The schematic for the circuit is given at the end of their video, which you can see after the break.

They receive the beam in their friend’s apartment using solar cells, which then feed a fairly big amplifier and speakers. From the video you can hear the surprisingly high quality sounds that results. So check it out. It also includes a little Benny Hill humor.

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Suddenly, 4G Feels Slow

Researchers at University College London successfully transferred data over an optical transmission system at a rate of 1.125 Tb/s. That’s over ten times as fast as typical commercial optical systems, and thousands of times faster than the standard broadband connection. The study appeared in Scientific Reports and takes advantage of encoding techniques usually seen in wireless systems.

The prototype system uses fifteen channels on different wavelengths. Each channel used 256QAM encoding (the same as you see on cable modems, among other things). A single receiver recovers all of the channels together. The technology isn’t commercially available yet. It is worth noting that the experiment used a transmitter and receiver very close to each other. Future tests will examine how the system performs when there are hundreds or thousands of feet of optical fiber between them.

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Hams Talk Digital

Morse code qualifies as a digital mode, although organic brains are somewhat better at copying it than electronic ones. Ham radio operators that did “phone” (ham-talk for voice) started out with AM modulation. Sometime after World War II, there was widespread adoption of single side band or SSB. SSB takes up less bandwidth and is more reliable than AM modulation. On the digital side, hams turned to different and more sophisticated digital transmission types with computers pushing bandwidth down and reliability up. However, a recent trend has been to encode voice over ham radio–sort of VoIP with radio instead of Ethernet–using an open source program called freedv.

[AA6E] made a very informative video where he carries on a QSO (a conversation) with a distant station using freedv. What makes it interesting, is towards the end when the two stations switch to regular SSB. The difference is dramatic and really points out how even with less bandwidth (roughly 3 kHz for SSB vs 1.25 kHz), the digital mode is superior. The freedv software (available for Windows or Linux) compresses audio to 700-1600 bits per second and spreads it over 16 QPSK signals.

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Transferring Audio to an AVR at 12kbps

Back in the bad ‘ol days of computing, hard drives cost as much as a car, and floppy drives were incredibly expensive. The solution to this data storage problem offered by all the manufacturers was simple – an audio cassette. It’s an elegant solution to a storage problem, and something that has applications today.

[Jari] was working on a wearable message badge with an 8-pin ATTiny. To get data onto this device, he looked at his options and couldn’t find anything good; USB needs two pins and the firmware takes up 1/4 of the Flash, UART isn’t available on every computer, and Bluetooth and WiFi are expensive and complicated. This left using audio to send digital data as the simplest solution.

[Jari] went through a ton of Wikipedia articles to figure out the best modulation scheme for transferring data with audio. What he came up with is very simple: just a square wave that’s changed by turning a pin off and on. When the audio is three samples long without crossing zero, the data is 0. When it’s five samples long without crossing zero, the data is 1. There’s a 17-sample long sync pulse, and with a small circuit that acts as a zero crossing detector, [Jari] had a simple circuit that would transfer data easily and cheaply.

All the code for this extremely cheap modem is available on GitHub.

The Basics of Frequency Modulation

fm-modulation

[brmarcum] takes us back to analog building block basics with his Frequency Modulation and Demodulation tutorial. Frequency Modulation (FM) sounds simple at first, but understanding the electronics behind modulation and  demodulation of an FM signal can be confusing. We’ve covered the basics before, but FM is so tightly associated with broadcast radio that searches often become muddled with references to RF, stereo, antennas, and transmitters.

[brmarcum] hopes to fill that gap with a simple circuit that modulates an audio signal to FM, then demodulates and amplifies it to be played on a small speaker. He used a Digilent Analog Discovery kit in his experiments, but an oscilloscope (an older analog scope would be perfect here) would work for output. Signal generation duties could easily be handled by a 555 circuit at the low end, and a computer sound card at the higher end.

[brmarcum] obviously put some time into his tutorial, but it’s not a tome of FM modulation. He’s broken down the modulation and demodulation circuits into their basic op-amp stages with examples of what the signal should look like on a scope after each stage. That’s the beauty here. By building and testing each section, anyone new to analog can learn how things work. In places where the theory behind what’s going on gets too in-depth for an Instructable, [brmarcum] gives links to Wikipedia.

Retrotechtacular: Fundamentals of AM and FM Radio Communication

How radios send and receive information can seem magical to the uninformed. For some people, this week’s Retrotechtacular video, “Frequency Modulation – Part 1 Basic Principles”, from the US Army Department of Defense 1964 will be a great refresher, and for others it will be their first introduction into the wonderful world of radio communications.

The stated objective is to teach why FM radio communication reduces interference which normally afflicts AM radio communications. Fundamentals of AM and FM is a better description, however, because the first part of the video nicely teaches the principles of AM and FM radio communications. It isn’t until later in the clip that it delves into interference, advantages of FM modulation, and detailed functioning of FM radio. The delivery is slow at times and admittedly long, yet the pace is perfect for a young ham to follow along with plenty of time to soak in the knowledge. If you’re still on the fence about becoming a ham here’s some words or encouragement.

Though the video isn’t aimed at ham radio users it does address core knowledge needed by amateur radio hobbyists. Amateur radio is full of many exciting communication technologies and you should have a clear understanding of AM and FM communication methodologies before getting on Grandpa’s information super highway. Once you have your ham license (aka ticket) you have privileges to create and test amazing ham related hacks, like [Lior] implementing full programmable control of a Baofeng UV5R ham radio using an Arduino.

Join us after the break to watch the video.

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Retrotechtacular: First laser transmitter built 50 years ago

helium-neon-laser-transmitter

Most of the time we feature hokey film footage in our Retrotechtacular series, but we think this hack is as cool today as it was fifty years ago. [Clint] wrote in to tell us about Operation Red Line. It was an experiment performed May 3rd and 4th, 1963, which means the 50th anniversary just passed a few weeks ago. The hack involved sending data (audio in this case) over long distances using a laser. But back then you couldn’t just jump on eBay and order up the parts. The team had to hack together everything for themselves.

They built their own helium-neon laser tube, which is shown on the right. The gentlemen involved were engineers at a company called Electro-Optical System (EOS) by day, and Ham radio enthusiasts by night. With the blessing of their employer they were able to ply their hobby skills using the glass blowing and optical resources from their work to get the laser up and running. With that side of things taken care of they turned to the receiving end. Using a telescope and a photomultipler they were able to pick up the beam of light at a distance of about 119 miles. The pinnacle of their achievement was modulating audio on the transmitter, and demodulating it with the receiver.

[Clint] knows the guys who did this and wrote up a look back at the project on his own blog.