Hacker News Reader

Comment by GauntletWizard | Hacker News Reader

Nice bit of reverse-engineering, and an impressive hack from whoever built the system.

Quite funny - I know someone who uses a similar setup to send GPS Coordinates for a Quadcopter and a Plane via the audio channel of the video link (he flies the Quadcopter/Plane with video goggles) to the ground station. There, an Arduino controls some stepper motors to point directional antennas at the copter/plane.

Project is at https://github.com/KipK/Ghettostation - video at https://www.youtube.com/watch?v=tiO4YCX7hJo - you can hear the modulation in https://www.youtube.com/watch?v=nm0okv8mTsg

Cool! I hadn't seen that project before, although I'd seen a few other telemetry-over-audio solutions.

They usually use simple FSK just like the "real life" helicopter in the article.

Here's an example of a common simple protocol:

http://www.flytron.com/pdf/audio_modem.pdf

Not really a hack. Subcarriers are how they added color to TV, stereo to FM radio, and so on. WWV/WWVH also sends machine-readable time codes at 100Hz like this.

Indeed--it was more of a hack sixty years ago perhaps. Since then, similar tricks have been used for closed captions, Dolby surround (the original kind), Radio Data System (which puts song names on your car dashboard via FM radio), TV Second Audio Program (listen to shows dubbed in a second language, just press the SAP button on your ancient high-end TV!)....

Wow. I always thought SAP stood for Spanish Audio Program. TIL.

>Subcarriers are how they added [...] stereo to FM radio...

Set your time machine for 1958, or follow this link [0] to see how RCA did both FM stereo (throughout the article they call it "multiplex") and 67 KHz "SCA" subcarriers. Admire the awesome 5KW FM transmitter cabinetry and transformer shielding.

[0] http://fmamradios.com/BTE-10B.html

One thing I find particularly interesting about articles from this time period is that Hz is not yet the name for 1/s, and so everything is in "cycles per second".

According to Wikipedia, the transition started in 1960, so we're two years away when this article was published.

Using spare audio capacity like this is actually very common - prior to digital recording, engineers would dedicate one tape channel to recording a sync signal, and in mission-critical applications would even 'stripe' the tape with the sync signal beforehand (many pro recorders can have recording and read heads active simultaneously).

Reply on news.ycombinator.com