10 ms·

Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

123 points by tambourine_man 4y ago

Spivak 4y ago
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 https://www.adafruit.com/product/1958 and am very happy with it.
- Joel_Mckay 4y ago
  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 4y ago
    > 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.
    - 13of40 4y ago
      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.
    - Joel_Mckay 4y ago
      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/0385546238 https://www.amazon.ca/When-McKinsey-Comes-Town-Consulting/dp... Have a wonderful day, and have faith most are lazier than they are evil =)
    - BenjiWiebe 4y ago
      I haven't done it but I've heard you can use a PO box for your license.
      - wrycoder 4y ago
        You can in the USA. Mine does.
  - Spivak 4y ago
    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 4y ago
      If I recall, license free 88.7MHz under 15mW transmitters for FM audio were very common in some zones.
      - fest 4y ago
        I believe the EU limit is 50nW (at least it is in the EU country I live).
    - Johnythree 4y ago
      > 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.
tpmx 4y ago
Note that the video is from 2012. The page itself was last updated in 2015.
deleted 4y ago
[deleted]
pvg 4y ago
87 comment HN thread in 2013: https://news.ycombinator.com/item?id=5103031 https://news.ycombinator.com/item?id=5103031
BrandoElFollito 4y ago
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".
- deleted 4y ago
  [deleted]
- pvg 4y ago
  A special car These have a surprisingly long history, e.g. https://en.wikipedia.org/wiki/Direction_finding#/media/File:British_Post_Office_interference_finding_truck_1927.jpg https://en.wikipedia.org/wiki/Direction_finding#/media/File:...
- avian 4y ago
  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.
- Johnythree 4y ago
  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.
  - anthk 4y ago
    I know the reverse of your story. And OLD TV from a grandma jamming an entire town: https://www.popularmechanics.com/science/a34229436/old-tv-breaks-internet/ https://www.popularmechanics.com/science/a34229436/old-tv-br...
squarefoot 4y ago
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.
- Aloha 4y ago
  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 4y ago
    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.
    - anthk 4y ago
      UHF and VHF? with an RPI?
      - Johnythree 4y ago
        Any square-wave signal (eg PWM) will have spurs going up to UHF and possibly higher.
- stefan_ 4y ago
  It's a GPIO with the corresponding mA drive, it can not "seriously jam" anything. The hyperbole is ludicrous.
andrewstuart 4y ago
If you want to build fully legal applications that transmit FM you can use the Si4713. https://learn.adafruit.com/adafruit-si4713-fm-radio-transmitter-with-rds-rdbs-support 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 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/ https://radiofidelity.com/best-fm-transmitter-for-your-car/ Bitluni made a fun (illegal) AM radio transmitter with an esp32 https://youtu.be/lRXHd3HNzEo https://youtu.be/lRXHd3HNzEo
- anthk 4y ago
  I would like an USB FM transmitter which shows up as an USB audio card. You set the sound card output at your OS settings and everything it's broadcasted over FM. I know some of them exist for bluetooth audio, but not for FM.
  - rahimnathwani 4y ago
    It would be cool to have this in one unit, but it's also easy to put together by combining two readily-available devices: - a USB sound card with 3.5mm jack, and - an FM transmitter module (which can usually be powered with 3v-12v, so should be able to work with 5v USB power
  - aussiegeek 4y ago
    This is an interesting challenge, would be a great homebrew project. (If I wasn't already neck deep in interesting challenges I would consider picking this one up)
- rahimnathwani 4y ago
  The 'Roav by Anker Bluetooth FM transmitter' is amazing. So much better than the generic ones, and well worth the extra money. It provides a good signal, is easy to adjust, and works well for phone calls (not just music).
  - 4y ago
pifm_guy 4y ago
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 4y ago
  > "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 4y ago
    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?
    - Johnythree 4y ago
      I'm talking about the broad band of spectral components (both close in and further out) which are generated by the synthesizer. These are a separate issue to any sidebands generated by the modulation process. see https://en.wikipedia.org/wiki/Phase_noise https://en.wikipedia.org/wiki/Phase_noise It's surprisingly difficult to generate a clean carrier which doesn't have a broad band of spectral components, but this is essential for the design of effective Receivers and Transmitters. These sidebands are generated by noise in the oscillator, spurs caused by mixing products, jitter in the synthesiser, and non-linearities in the modulation process. The bottom line is to look at the output with a high-quality spectrum analyser and compare the result with the spectral mask which is published by the FCC.
natas 4y ago
this is completely illegal.
- cpsns 4y ago
  So? It's a proof of concept. Tinker away, laws be damned.
  - Johnythree 4y ago
    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 4y ago
      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.
    - thebruce87m 4y ago
      How do I get a job as a Radio Inspector? Sounds like I can just drive around taking my sweet time and a judge will guarantee I get paid.
  - oplaadpunt 4y ago
    >> 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.
Animats 4y ago
If you're generating RF with homebrew stuff, get a spectrum analyzer.[1] They're much cheaper than they used to be. Then you can find out if you're blithering all over the RF spectrum, and how badly. It's better to find it out yourself before someone from a cell phone company shows up, spectrum analyzer in hand. Cellular providers call this "spectrum clearing". There are now automated systems to help with this.[2] [1] https://www.digikey.com/en/products/detail/seeed-technology-co.,-ltd/113991034/15976342 https://www.digikey.com/en/products/detail/seeed-technology-... [2] https://www.viavisolutions.com/en-us/products/interferenceadvisor https://www.viavisolutions.com/en-us/products/interferencead...
- dylan604 4y ago
  Better a visit from a cell phone company employee than one from the FCC. One can present you with a fine, the other can just ask you stop.
funstuff007 4y ago
This guy is making arbitrary waveforms via the R Pi PIO. Probably easily extended to an FM transmitter. https://www.youtube.com/watch?v=Yui6NjrU23c https://www.youtube.com/watch?v=Yui6NjrU23c

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 https://www.adafruit.com/product/1958 and am very happy with it.

Joel_Mckay 4y ago
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 4y ago
  > 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.
  - 13of40 4y ago
    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.
  - Joel_Mckay 4y ago
    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/0385546238 https://www.amazon.ca/When-McKinsey-Comes-Town-Consulting/dp... Have a wonderful day, and have faith most are lazier than they are evil =)
  - BenjiWiebe 4y ago
    I haven't done it but I've heard you can use a PO box for your license.
    - wrycoder 4y ago
      You can in the USA. Mine does.
- Spivak 4y ago
  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 4y ago
    If I recall, license free 88.7MHz under 15mW transmitters for FM audio were very common in some zones.
    - fest 4y ago
      I believe the EU limit is 50nW (at least it is in the EU country I live).
  - Johnythree 4y ago
    > 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.

tpmx 4y ago

Note that the video is from 2012. The page itself was last updated in 2015.

deleted 4y ago

[deleted]

pvg 4y ago

87 comment HN thread in 2013: https://news.ycombinator.com/item?id=5103031 https://news.ycombinator.com/item?id=5103031

BrandoElFollito 4y ago

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".

deleted 4y ago
[deleted]
pvg 4y ago
A special car These have a surprisingly long history, e.g. https://en.wikipedia.org/wiki/Direction_finding#/media/File:British_Post_Office_interference_finding_truck_1927.jpg https://en.wikipedia.org/wiki/Direction_finding#/media/File:...
avian 4y ago
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.
Johnythree 4y ago
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.
- anthk 4y ago
  I know the reverse of your story. And OLD TV from a grandma jamming an entire town: https://www.popularmechanics.com/science/a34229436/old-tv-breaks-internet/ https://www.popularmechanics.com/science/a34229436/old-tv-br...

squarefoot 4y ago

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.

Aloha 4y ago
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 4y ago
  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.
  - anthk 4y ago
    UHF and VHF? with an RPI?
    - Johnythree 4y ago
      Any square-wave signal (eg PWM) will have spurs going up to UHF and possibly higher.
stefan_ 4y ago
It's a GPIO with the corresponding mA drive, it can not "seriously jam" anything. The hyperbole is ludicrous.

andrewstuart 4y ago

If you want to build fully legal applications that transmit FM you can use the Si4713. https://learn.adafruit.com/adafruit-si4713-fm-radio-transmitter-with-rds-rdbs-support 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 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/ https://radiofidelity.com/best-fm-transmitter-for-your-car/ Bitluni made a fun (illegal) AM radio transmitter with an esp32 https://youtu.be/lRXHd3HNzEo https://youtu.be/lRXHd3HNzEo

anthk 4y ago
I would like an USB FM transmitter which shows up as an USB audio card. You set the sound card output at your OS settings and everything it's broadcasted over FM. I know some of them exist for bluetooth audio, but not for FM.
- rahimnathwani 4y ago
  It would be cool to have this in one unit, but it's also easy to put together by combining two readily-available devices: - a USB sound card with 3.5mm jack, and - an FM transmitter module (which can usually be powered with 3v-12v, so should be able to work with 5v USB power
- aussiegeek 4y ago
  This is an interesting challenge, would be a great homebrew project. (If I wasn't already neck deep in interesting challenges I would consider picking this one up)
rahimnathwani 4y ago
The 'Roav by Anker Bluetooth FM transmitter' is amazing. So much better than the generic ones, and well worth the extra money. It provides a good signal, is easy to adjust, and works well for phone calls (not just music).
- 4y ago

pifm_guy 4y ago

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 4y ago
> "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 4y ago
  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?
  - Johnythree 4y ago
    I'm talking about the broad band of spectral components (both close in and further out) which are generated by the synthesizer. These are a separate issue to any sidebands generated by the modulation process. see https://en.wikipedia.org/wiki/Phase_noise https://en.wikipedia.org/wiki/Phase_noise It's surprisingly difficult to generate a clean carrier which doesn't have a broad band of spectral components, but this is essential for the design of effective Receivers and Transmitters. These sidebands are generated by noise in the oscillator, spurs caused by mixing products, jitter in the synthesiser, and non-linearities in the modulation process. The bottom line is to look at the output with a high-quality spectrum analyser and compare the result with the spectral mask which is published by the FCC.

natas 4y ago

this is completely illegal.

cpsns 4y ago
So? It's a proof of concept. Tinker away, laws be damned.
- Johnythree 4y ago
  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 4y ago
    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.
  - thebruce87m 4y ago
    How do I get a job as a Radio Inspector? Sounds like I can just drive around taking my sweet time and a judge will guarantee I get paid.
- oplaadpunt 4y ago
  >> 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.

Animats 4y ago

If you're generating RF with homebrew stuff, get a spectrum analyzer.[1] They're much cheaper than they used to be. Then you can find out if you're blithering all over the RF spectrum, and how badly. It's better to find it out yourself before someone from a cell phone company shows up, spectrum analyzer in hand. Cellular providers call this "spectrum clearing". There are now automated systems to help with this.[2] [1] https://www.digikey.com/en/products/detail/seeed-technology-co.,-ltd/113991034/15976342 https://www.digikey.com/en/products/detail/seeed-technology-... [2] https://www.viavisolutions.com/en-us/products/interferenceadvisor https://www.viavisolutions.com/en-us/products/interferencead...

dylan604 4y ago
Better a visit from a cell phone company employee than one from the FCC. One can present you with a fine, the other can just ask you stop.

funstuff007 4y ago

This guy is making arbitrary waveforms via the R Pi PIO. Probably easily extended to an FM transmitter. https://www.youtube.com/watch?v=Yui6NjrU23c https://www.youtube.com/watch?v=Yui6NjrU23c