[HN Gopher] Turn Raspberry Pi's GPIO into an FM Transmitter (2012)
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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:...
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