[HN Gopher] Turn Raspberry Pi's GPIO into an FM Transmitter (2012) ___________________________________________________________________ Turn Raspberry Pi's GPIO into an FM Transmitter (2012) Author : tambourine_man Score : 78 points Date : 2022-11-12 13:36 UTC (9 hours ago) (HTM) web link (www.icrobotics.co.uk) (TXT) w3m dump (www.icrobotics.co.uk) | tpmx wrote: | Note that the video is from 2012. The page itself was last | updated in 2015. | andrewstuart wrote: | If you want to build fully legal applications that transmit FM | you can use the Si4713. | | https://learn.adafruit.com/adafruit-si4713-fm-radio-transmit... | | John Park did a short video on it | https://www.youtube.com/watch?v=PKP6Z-cEbFQ | | You can also buy a fully legal MP3 FM transmitter. | https://radiofidelity.com/best-fm-transmitter-for-your-car/ | | Bitluni made a fun (illegal) AM radio transmitter with an esp32 | https://youtu.be/lRXHd3HNzEo | squarefoot wrote: | This is so bad, and dangerous. It outputs pwm from an unfiltered | digital output, therefore generating a hell of harmonics on a | wide set of frequencies, not just the one it's set to operate on. | It can seriously jam not only broadcast FM transmission but also | other services. Beware that airband is very close; any jamming on | those frequencies can turn in huge fines if not landing the | perpetrator straight to jail. Want to test it? Fine, but never | ever connect an antenna to it. | stefan_ wrote: | It's a GPIO with the corresponding mA drive, it can not | "seriously jam" anything. The hyperbole is ludicrous. | Johnythree wrote: | Radio Amateurs routinely communicate around the world with | just a few milli-watts. And even a few micro-watts is | sufficient to communicate across town. | | Many years ago the record was "1000 miles per milliwatt" | using CW. | userbinator wrote: | They do that with very carefully optimised equipment. | Aloha wrote: | It'd have to sit down and do some math, but I'm kinda skeptical | that an FM Broadcast band harmonic would land on the air and | right next to it. | Johnythree wrote: | It's not so much "harmonics of the FM band", but the wide | band of spurs either side of the transmission. | | The problem is that the unfiltered output of this device | contains a wide range of spurs and harmonics which will | radiate right across the VHF and UHF bands. | | Plus the antenna (a few feet of wire) will have multiple | resonances across the same range. | | It will radiate hundreds of signals, and the odds of them | interfering with a nearby essential service will be very | high. And note that the essential service receiver is capable | of hearing signals in the micro-watt range and is routinely | trying to receive them. | mwint wrote: | In real life, what would happen to a legitimate clueless | hobbyist who tries this (with no ill intent or knowledge of | what's happening) and gets caught? I'm assuming the real | response is closer to "hey, don't do that, read this PDF" than | it is to "straight to jail"? | tb_technical wrote: | Basically, the FCC can slap you with an expensive fine when | the local HAMs rat you out. | esskay wrote: | I know nothing about how this all works, how do people know | who did it / where it came from? I presume its possible to | get a rough location but in a built up area I'd imagine its | pretty much impossible to find the person that did it | unless it's left running right? | Johnythree wrote: | If you are blocking an essential services channel (eg | Aircraft, Police, Ambulance, military), the authorities | can get a triangulation (within meters) on you in just a | few minutes. | tb_technical wrote: | If you're interested, Hams do an activity called a | "Foxhunt" where they're trying to triangulate the | position of a broadcasted signal. It's sort of like | wargaming, but for Hams. | | This, in conjunction with whoever owns the plot of land | the offending device happens to live on, determines who | is to blame when someone is broadcasting illegally. | | Here's a link describing the activity, in case you're | interested: https://hamradioplanet.com/what-is-a-fox- | hunt-in-ham-radio/ | robotnikman wrote: | You can use radio triangulation to determine where the | signal source is while it is transmitting | connicpu wrote: | A 20cm antenna driven by a raspberry pi's current limited | GPIOs won't be powerful enough to get picked up by anything | more than a few dozen meters away, especially not powerful | enough to get in the way of actual signals being | transmitted by a more powerful transmitter | Johnythree wrote: | A 20cm antenna will be a 1/4 wave at 375 MHZ (and a half | wave at 188Mhz, etc), and thus will be a very efficient | radiator at many different frequencies. | | Even a tiny signal can be enough to block the operation | of a nearby police repeater (which is also trying to hear | weak signals). | simfree wrote: | Perhaps public safety shouldn't be entirely beholden to | Motorola rebranding old garbage (ancient TDMA networks | being sold as new P25 compliant hardware for tens of | millions of dollars) that is then sold as new. | | Modern protocols like 5G New Radio that have better | propogation at the fringes of connectivity would serve | life critical applications with improved coverage and | significantly more intelligible voice quality than what | the current public safety networks are capable of doing. | | If you look at the telephone/power poles in your | neighborhood you will see a device with two antennas | hooked into power every few blocks. This is a recieve | only amplifier that has to be used to amplify these weak | signals from handheld public safety radios that use | protocols like TDMA with poor interference resistance and | no error correction. | Johnythree wrote: | It has nothing to do with Motoroa, TDMA, or 5G. | | If the wanted signal is very weak (eg from a distant | aircraft using VHF AM), and there is an interfering | signal on the same channel, the emergency communications | will be distrupted. | kragen wrote: | i don't think this uses pwm | [deleted] | pvg wrote: | 87 comment HN thread in 2013: | | https://news.ycombinator.com/item?id=5103031 | Spivak wrote: | Some notes about this setup. | | 1. The radio will kinda suck. You're probably not doing it wrong. | | 2. The sanest way to get pifm to play a stream (say from modipy) | is a FIFO. gstreamer will be your friend. | | 3. Music will sound like garbage without a high and low pass | filter. | | 3.1 Volume becomes an issue, you'll need to amplify quite a bit | to match the volume of real radio stations so your ears don't get | blown out if it flips stations. | | 4. It's really low power so really really long wire paid | dividends to get whole apartment coverage. | | 5. Tuning will be really finicky since FM kinda "attaches" to a | station and you'll be competing with basically every frequency | already taken by the big towers. | | If you want to take the next step I have | https://www.adafruit.com/product/1958 and am very happy with it. | Joel_Mckay wrote: | Sigh, getting a amateur radio operator license is not | difficult, and teaches people such wonderful concepts like | avoiding interference with noisy transmitters. | | 1. If you are using a modulated square-wave without a filter, | than the harmonics/overtones will be splattering the local | spectrum. | | 2. Tools like KerberosSDR/KrakenSDR make triangulating illegal | broadcasts rather trivial. | | 3. Keep in mind, some places have a $5k fine and 1 year in | jail... if you are illegally broadcasting. | msla wrote: | > Sigh, getting a amateur radio operator license is not | difficult, | | It is if you are trying to run from someone who's liable to | pluck your address out of a public database (like the one the | FCC runs) and come after you, possibly with murderous intent. | It is if the government persists in misgendering you because | hating you is politically fashionable in some circles. I'm | not arguing that it's conceptually difficult, but "That Hobby | Where The Government Publishes Your Home Address" might be | unpopular for reasons beyond the difficulty of understanding | the hobby's conceptual level. | BenjiWiebe wrote: | I haven't done it but I've heard you can use a PO box for | your license. | wrycoder wrote: | You can in the USA. Mine does. | Joel_Mckay wrote: | Sounds like you should move someplace nicer, as many zones | use a checkbox on the application to prevent publishing | such details. ;) | | Read about halfway through this book so far, and It argues | a fairly good case that Mr. Burns was based on a real | person: https://www.amazon.ca/When-McKinsey-Comes-Town- | Consulting/dp... | | Have a wonderful day, and have faith most are lazier than | they are evil =) | 13of40 wrote: | On a related note, I see a lot of amateur radio enthusiasts | who use their call sign as a vanity license plate. If you | do that you should be extra polite in traffic because if | someone types that into Google they get your full name and | address. | Spivak wrote: | That's cool and all, but getting an amateur radio license | would not have allowed me to do what I want legally anyway | which is run a low power music FM station broadcasting to | common radios in my own apartment. I do use a filter but it | wouldn't matter if I didn't, I don't really care that I'm | splattering the spectrum when the broadcast doesn't even make | it through my walls. | Joel_Mckay wrote: | If I recall, license free 88.7MHz under 15mW transmitters | for FM audio were very common in some zones. | fest wrote: | I believe the EU limit is 50nW (at least it is in the EU | country I live). | Johnythree wrote: | > when the broadcast doesn't even make it through my walls. | | But you don't know. If it's radiating a few milliwatts (or | even microwatts), that's sufficient signal to be hear | across town in a sensitive receiver. | | The really sad thing is that the unwanted signals could | well be stronger than the unwanted one due to unknown | resonances in the antenna wire, etc. | natas wrote: | this is completely illegal. | cpsns wrote: | So? It's a proof of concept. | | Tinker away, laws be damned. | Johnythree wrote: | Until an angry Radio Inspector arrives on your doorstep | having spent all day driving around your town with a spectrum | analyzer to track you down. | | When you get to attend court, the judge will award the | standard fines (and possible jail time) but also the cost of | the time spent by the Radio Inspector in traveling to your | town. | cpsns wrote: | Only a fool would run something like this constantly, or | even at all outside of a few minutes to prove it worked. If | you're being sensible you aren't going to have issues. | oplaadpunt wrote: | >> So? It's a proof of concept. | | Mentioning that even this proof of concept is illegal is | useful, as it isn't mentioned in the article. This is so easy | to make that practically anyone could do it, which makes this | very relevant. | pifm_guy wrote: | Author here. | | This code no longer works on new Pi's (it does a lot of low level | very hardware stuff from userspace without a proper kernel | driver). The code was written in a 24 hour Hackathon, so is very | far from maintainable. | | However there are other projects to do the same which do work on | new Pi's. | | There are ways to do a full software defined radio from a pi by | using the I2S hardware, and that can transmit GPS, 433 MHz garage | door openers, and various other things. | | Obviously whatever you transmit you better be using some filters | on the output if you want it to be vaguely legal. | Johnythree wrote: | > "using some filters on the output" | | While it's relatively easy to remove harmonics, it is pretty | much impossible to remove the broad comb of spurious signals | either side of the carrier. | | Which why transmitters are not designed this way in the real | world. | kragen wrote: | Which spurious signals do you mean? I'm assuming you're | talking about stuff well outside the intended sidebands that | result from the modulation. | | If we were talking about AM I would assume you mean the audio | signal isn't bandlimited and so high-frequency audio | components get upconverted by the carrier frequency, but I | don't know what the spectrum of an FM signal looks like. Are | we talking about a broad comb that results in an analogous | way from sudden changes in the frequency? Do they go away if | you vary the frequency of a 50%-duty-cycle square wave | smoothly, like maybe with second- or third-order continuity, | instead of suddenly? | noasaservice wrote: | RPITX is the spiritual successor of your project! | | https://github.com/F5OEO/rpitx | | And it does work with nearly every RPi. | | And unlike only doing FM (97MHz-108MHz), it can emit from an IQ | datastream from 10KHz to 1.5GHz. | BrandoElFollito wrote: | One needs to be careful with such home made devices as they may | clash with the officially assigned frequencies. | | We had in France a similar story where a village was losing 3G/4G | connectivity almost every day around 22:00. It seemed that there | are interferences with something. | | A special car was sent by the national regulation entity, it was | trying to triangulate the source. | | They found a house where the owner bought a jammer on AliExpress | because he wanted his children to sleep and not go on internet. | He thought it was only "for the house". | Johnythree wrote: | In my small town, local TV reception was being completely wiped | out by interference. | | When the Radio Inspector came to check, he found a rechargeable | torch sitting in it's charger bracket, which generated | horrendous interference whenever the batteries reached full. | avian wrote: | Every mobile operator has stories like this. Back in the GSM | times the common theme in Europe was people using illegaly | imported devices made for the US market. The unlicensed 900 MHz | band in the US fell right into the GSM band in Europe. | Apparently the worst offenders were wireless security cameras. | [deleted] | pvg wrote: | _A special car_ | | These have a surprisingly long history, e.g. | | https://en.wikipedia.org/wiki/Direction_finding#/media/File:... ___________________________________________________________________ (page generated 2022-11-12 23:00 UTC)