Impossible-to-find Apps
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Has no one ever made an FM transmitter app for Android? Every search I have done leads me to either FM receivers or RC transmitters. Is a simple radio transmitter for audio signals so I can wirelessly play stuff through my car speakers in such rare demand?
Am I just blind or using the wrong search terms? Please help me find this app!
Actually, it doesn't even necessarily have to be FM. AM would be fine, too.
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No aux input or Bluetooth on your car stereo? Those are the usual ways to do it if you don't have fancy usb connections or android auto.
It might also be a hardware issue? There could be transmitters that plug into the 3.5mm Jack?
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Something like this. Actually, that reminds me that I had exactly this type of thing when I was driving a cheap car for commuting
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@djls45 Do most/all phones even have the hardware to transmit on FM radio frequencies?
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@loopback0 said in Impossible-to-find Apps:
Do most/all phones even have the hardware to transmit on FM radio frequencies?
IIRC, I had an old phone (pre smartphone) that had an FM transmitter built-in. I don't think I've seen that mentioned since, so I would guess it's not very common.
Apps only tend to get very indirect access to anything related to radio frequency signals, so even if the hardware were capable of transmitting in those bands (which are somewhat apart from the common communications bands), you'd need the OS to somehow expose that functionality to the app.
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@cvi said in Impossible-to-find Apps:
even if the hardware were capable of transmitting in those bands
The hardware probably is capable of it if it can do FM reception, but that capability is locked out of use by the firmware in order to get FCC certification.
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@djls45 said in Impossible-to-find Apps:
Has no one ever made an FM transmitter app for Android? Every search I have done leads me to either FM receivers or RC transmitters. Is a simple radio transmitter for audio signals so I can wirelessly play stuff through my car speakers in such rare demand?
I suspect that doing such a thing will make you a pirate radio station, liable to prosecution under whatever laws in your area require broadcasters in certain frequency bands to be licensed.
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@Gurth said in Impossible-to-find Apps:
@djls45 said in Impossible-to-find Apps:
Has no one ever made an FM transmitter app for Android? Every search I have done leads me to either FM receivers or RC transmitters. Is a simple radio transmitter for audio signals so I can wirelessly play stuff through my car speakers in such rare demand?
I suspect that doing such a thing will make you a pirate radio station, liable to prosecution under whatever laws in your area require broadcasters in certain frequency bands to be licensed.
In Germany, such transmitters are allowed (since 2006) but are strictly limited in their signal strength, yielding a range of roughly 5 meters.
Of course, the Chinese crap often does not care. Some tests have found such devices to blanket an entire city block.
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@Jaloopa said in Impossible-to-find Apps:
No aux input or Bluetooth on your car stereo? Those are the usual ways to do it if you don't have fancy usb connections or android auto.
The system in my auto has just a CD player and a radio. It doesn't have any other sort of input.
It might also be a hardware issue? There could be transmitters that plug into the 3.5mm Jack?
Yeah, there's separate hardware that can do it, but I'm looking for an app that uses the hardware built into the phone.
@loopback0 said in Impossible-to-find Apps:
@djls45 Do most/all phones even have the hardware to transmit on FM radio frequencies?
I would think they do, if they can receive FM radio. It's just reversing the signal flow, which on FM is pretty simple.
@dkf said in Impossible-to-find Apps:
@cvi said in Impossible-to-find Apps:
even if the hardware were capable of transmitting in those bands
The hardware probably is capable of it if it can do FM reception, but that capability is locked out of use by the firmware in order to get FCC certification.
This is probably the case.
@Gurth said in Impossible-to-find Apps:
@djls45 said in Impossible-to-find Apps:
Has no one ever made an FM transmitter app for Android? Every search I have done leads me to either FM receivers or RC transmitters. Is a simple radio transmitter for audio signals so I can wirelessly play stuff through my car speakers in such rare demand?
I suspect that doing such a thing will make you a pirate radio station, liable to prosecution under whatever laws in your area require broadcasters in certain frequency bands to be licensed.
Only if the signal strength is too high. Personal FM transmitters are perfectly fine almost everywhere in the USA. I think the only places where they aren't allowed have a lot of priority communication radio traffic (like airports) so the personal transmitters are banned to avoid interfering with them.
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@djls45 said in Impossible-to-find Apps:
I would think they do, if they can receive FM radio. It's just reversing the signal flow, which on FM is pretty simple.
I'm really interested in this concept. I've never seen any hack-a-day projects turning an arbitrary radio into a transmitter by simply "reversing the signal flow".
To my engineering mind this sounds positively hilarious...
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@Tsaukpaetra said in Impossible-to-find Apps:
@djls45 said in Impossible-to-find Apps:
I would think they do, if they can receive FM radio. It's just reversing the signal flow, which on FM is pretty simple.
I'm really interested in this concept. I've never seen any hack-a-day projects turning an arbitrary radio into a transmitter by simply "reversing the signal flow".
To my engineering mind this sounds positively hilarious...
I have relatives that converted a radio to a transmitter as kids, and played some music and talked a bit through it. Waaaay back when radios didn't have transistors though.
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@Carnage said in Impossible-to-find Apps:
Waaaay back when radios didn't have transistors though.
Yeah. It just feels like those guys that made a "microwave" that cools things instead of hearing them up. All you gotta do is run the microwave in reverse!
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@Tsaukpaetra It's actually fairly easy, even with a transistor radio. I have a kid's-level electronics kit that includes ~100 projects, and two of them are an FM receiver and short-range FM transmitter. The only difference was reversing a few connections. It was kinda fun to find local stations and to also transmit sound to a "regular" consumer radio set.
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@djls45 With AM you even could have a radio receiver without any additional power source - that was one of my physics labs experiments at university. It was really funny to tune into a longwave radio station with just a simple circuit and a pair of headphones. It was not very loud, of course, but you could understand it if someone was speaking.
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@Tsaukpaetra said in Impossible-to-find Apps:
@Carnage said in Impossible-to-find Apps:
Waaaay back when radios didn't have transistors though.
Yeah. It just feels like those guys that made a "microwave" that cools things instead of hearing them up.
Well, that's actually a thing:
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@Rhywden That discovery does assume that the matter is already close to absolute zero.
@Tsaukpaetra might be thinking of this invention which the media branded as "a reverse microwave" / "a microwave, but for cooling": it's an appliance which can cool a bottle of liquid in about the time a microwave would heat things up. However, it doesn't use any electromagnetic radiation, instead it just immerses the bottle in cold water and manages to swirl the liquid in the bottle so that the extra heat can be efficiently exchanged through the glass or plastic bottle (that last thing is actually really novel, as a simple ice bucket needs more time to passively cool the contents of the bottle).
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@JBert said in Impossible-to-find Apps:
@Rhywden That discovery does assume that the matter is already close to absolute zero.
Spoilsport. Next time you can bring your own Bose-Einstein-Condensate!
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@Rhywden said in Impossible-to-find Apps:
@JBert said in Impossible-to-find Apps:
@Rhywden That discovery does assume that the matter is already close to absolute zero.
Spoilsport. Next time you can bring your own Bose-Einstein-Condensate!
Well, I'm pretty sure I've got none in my fridge at the moment so you'll be waiting quite a while.
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@Rhywden said in Impossible-to-find Apps:
@djls45 With AM you even could have a radio receiver without any additional power source - that was one of my physics labs experiments at university. It was really funny to tune into a longwave radio station with just a simple circuit and a pair of headphones. It was not very loud, of course, but you could understand it if someone was speaking.
When I was grade school(?)-age, my dad and I built a simple diode receiver. (I think he designed it — he was a EE, too — and helped me build it, although I don't recall that he did a very good job of explaining how it worked.) Yes, it would pick up AM broadcast. However, there was a powerful AM transmitter a couple of miles a way, and that was the ONLY station it would pick.
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Speaking of radio transmitters, here's a story from my dad's school days
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@Rhywden said in Impossible-to-find Apps:
Microwave-Induced Cooling of a Superconducting Qubit
That doesn't exactly help much with my dinner.
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@dkf said in Impossible-to-find Apps:
@cvi said in Impossible-to-find Apps:
even if the hardware were capable of transmitting in those bands
The hardware probably is capable of it if it can do FM reception, but that capability is locked out of use by the firmware in order to get FCC certification.
And if it were possible to transmit with it, it would almost certainly be garbage and fail every type of cert imaginable (except for Hamsexy Certification, of course)
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@djls45 said in Impossible-to-find Apps:
I would think they do, if they can receive FM radio. It's just reversing the signal flow, which on FM is pretty simple.
I doubt that, to be honest. Practically because a FM receiver these days is basically a discrete IC that doesn't do anything else than be a receiver. There's no reversing the signal flow or anything, you wire it up the correct way and that's it (unless you like smelling the magic smoke). You can find some on e.g. Sparkfun (~$10), though if you get just the IC, you might get it cheaper (<€1, though I didn't read the datasheet carefully).
@dkf said in Impossible-to-find Apps:
The hardware probably is capable of it if it can do FM reception, but that capability is locked out of use by the firmware in order to get FCC certification.
Same as mentioned above. IIRC the circuits for a receiver and transmitter are quite different. There's no reason that you would have one just because you have the other.
Also, there's no reason to have full SDR capabilities in those bands (especially considering you can get the same functionality for a IC that costs a couple of cents and draws next to no power). Happy to be proven wrong, of course.
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@djls45 said in Impossible-to-find Apps:
@Jaloopa said in Impossible-to-find Apps:
No aux input or Bluetooth on your car stereo? Those are the usual ways to do it if you don't have fancy usb connections or android auto.
The system in my auto has just a CD player and a radio. It doesn't have any other sort of input.
Did you check the glovebox? Mine has USB and 3.5mm aux connections located inside the glovebox.
@loopback0 said in Impossible-to-find Apps:
@djls45 Do most/all phones even have the hardware to transmit on FM radio frequencies?
I would think they do, if they can receive FM radio. It's just reversing the signal flow, which on FM is pretty simple.
It's simple IIF the exact transmitter power doesn't matter. With a phone's compact electronics, everything matters.
Another thing to note is that most phones (that I've seen) use the 3.5mm jack as the FM radio antenna. Plug in the headphones if you want a signal. The jack also houses the headphone signals. I wonder how cheap headphones'd interact with an FM transmission, if used as the transmitter antenna.
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@acrow said in Impossible-to-find Apps:
I wonder how cheap headphones'd interact with an FM transmission, if used as the transmitter antenna.
Poorly.
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@cvi said in Impossible-to-find Apps:
IIRC the circuits for a receiver and transmitter are quite different.
Not that different. You have a tunable oscillator to generate the central frequency and then either subtract that from the received signal to get an audio signal, or use the audio signal to modulate the tuning of the oscillator to make a transmittable signal. There's really not that large a difference. The transmission capability is not present mainly for regulatory reasons, and there's little customer demand given that the key use case used to be using a car's audio system to listen to music on your phone, and that's now served by bluetooth (while enabling other capabilities that the FM transmission technique couldn't do).
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@dkf said in Impossible-to-find Apps:
You have a tunable oscillator to generate the central frequency and then either subtract that from the received signal to get an audio signal, or use the audio signal to modulate the tuning of the oscillator to make a transmittable signal.
I was looking at an off-the-shelf FM receiver yesterday, specifically the Si4702, which is the one that Sparkfun uses in one of their products.
Based on my somewhat limited understand of the circuit diagrams, I think one would have to duplicate quite a large part of the circuit to make a transmitter. I.e., all the amplifiers and so on would not only have to change direction, but also work at entirely different signal levels. I have a vague recollection of having to build a circuit that converts the FM signal to an AM signal (IIRC, you multiply the two signals, the tricky part is synching up the central frequency). I have no idea how you would do the reverse, though (which seems more difficult).
Receiver and transmitter might be very similar in theory, but based on the quick inspection, I think the IC would be quite a bit more complex, essentially having to (at least) double the amount of stuff inside (I guess you can keep the resonator?). (And there's the power issue, a receiver can work with very low power levels, much less than what even a very short-range transmitter would need).
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@cvi said in Impossible-to-find Apps:
I think the IC would be quite a bit more complex
That makes almost no difference to the cost of the IC (other than the more complex post-manufacture testing required). The big factors in ICs are usually producing the initial masks — a cost you usually amortize over many chips — and the overall area, which determines how many you make per wafer.
It's the power needed that typically makes
FMradio transmitters large and expensive. If you only want to transmit to a couple of inches away at most, you need hardly any power at all.
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@dkf said in Impossible-to-find Apps:
That makes almost no difference to the cost of the IC (other than the more complex post-manufacture testing required). The big factors in ICs are usually producing the initial masks — a cost you usually amortize over many chips — and the overall area, which determines how many you make per wafer.
That makes sense (I only ever hear of how everybody likes to shave of the last fractions of cents in their circuits). But, regarding the original question, practically, there are ICs out there that are receivers only (and can't transmit), and no amount of software (or "reversing signals") is going to change that. Whether or not those are the ones that end up in a certain phone/device is of course another question.
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@acrow said in Impossible-to-find Apps:
Did you check the glovebox? Mine has USB and 3.5mm aux connections located inside the glovebox.
My Subaru's connections are in the recessed area just in front of the gear shifter. Of course, they cheaped out and the USB can't supply enough power to keep the phone charged when I'm running GPS on it. Almost, but not quite. And it definitely can't supply enough power for the dashcam. Luckily, there's a 12v outlet in there too.
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@dcon said in Impossible-to-find Apps:
Of course, they cheaped out and the USB can't supply enough power to keep the phone charged when I'm running GPS on it.
Mine is the same way; the on-board USB barely supplies any power at all. I use it exclusively for my USB stick with music on it, and whenever I want to plug in a phone I use a lighter socket adapter. I used to use one for my dashcam as well but recently hard-wired it into the fuse box to free it up and make it look cleaner
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@djls45 ... how would the phone transmit FM?
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@Captain My thought (apparently wrong, according to earlier posts here) was that it could use either the phone's internal cell radio or the headphone cord that the FM receiver uses.
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@dkf said in Impossible-to-find Apps:
If you only want to transmit to a couple of inches away at most
but, and importantly in this context - you must also transmit over any nearby stations on the same channel, so you cover them in the car receiver. That can require not insignificant amounts of power...
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@djls45 said in Impossible-to-find Apps:
@Captain My thought (apparently wrong, according to earlier posts here) was that it could use either the phone's internal cell radio or the headphone cord that the FM receiver uses.
Yeah, the cell radio will operate at the cell bands (800-900mhz, and higher) and is physically a different circuit than the VHF 87-108mhz band. The headphone cord often functions as the receive antenna (often with receive-only, a wire will work fine), but transmitting requires a much finer-tuned antenna...
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@sloosecannon said in Impossible-to-find Apps:
transmitting requires a much finer-tuned antenna...
Not necessarily, but a mismatched antenna will reflect a significant fraction of the transmitter power back to the transmitter, instead of radiating it to whatever receivers may be out there. The reduced efficiency may or may not be significant in whether the signal is strong enough for a receiver to receive. At sufficiently high power levels, the reflected power may damage transmitter, but I'd guess probably not at the power a cell phone would produce.
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@HardwareGeek said in Impossible-to-find Apps:
@sloosecannon said in Impossible-to-find Apps:
transmitting requires a much finer-tuned antenna...
Not necessarily, but a mismatched antenna will reflect a significant fraction of the transmitter power back to the transmitter, instead of radiating it to whatever receivers may be out there. The reduced efficiency may or may not be significant in whether the signal is strong enough for a receiver to receive. At sufficiently high power levels, the reflected power may damage transmitter, but I'd guess probably not at the power a cell phone would produce.
Well, yes.
award granted.It requires a much finer-tuned antenna to work effectively
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@sloosecannon A random-wire antenna can be effective if you insert an impedance matching network between it and the feed.
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@HardwareGeek said in Impossible-to-find Apps:
@sloosecannon A random-wire antenna can be effective if you insert an impedance matching network between it and the feed.
I've got exactly that setup at home!
Said impedence-matcher, according to reliable sources, can effectively tune a metal fence... If the fence isn't a path to ground at least.
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@sloosecannon said in Impossible-to-find Apps:
@dkf said in Impossible-to-find Apps:
If you only want to transmit to a couple of inches away at most
but, and importantly in this context - you must also transmit over any nearby stations on the same channel, so you cover them in the car receiver. That can require not insignificant amounts of power...
That's why you choose an unused channel (or at least one with as weak a signal as you can find to override) for your personal broadcast.
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@dkf said in Impossible-to-find Apps:
It's the power needed that typically makes FM radio transmitters large and expensive. If you only want to transmit to a couple of inches away at most, you need hardly any power at all.
At 2.4GHz, for example, in a room-sized non-wifi network, a transponder may use less power for transmitting than receiving, since receiving requires constant processing, but transmitting only requires the (very tightly regulated) transmission power.
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@sloosecannon said in Impossible-to-find Apps:
@dkf said in Impossible-to-find Apps:
If you only want to transmit to a couple of inches away at most
but, and importantly in this context - you must also transmit over any nearby stations on the same channel, so you cover them in the car receiver. That can require not insignificant amounts of power...
Years ago, I took a road trip down to the States, and I was using an FM transmitter on my phone because my car at the time had no bluetooth, aux input or anything. I found a station that was pretty much empty at the start of my trip, but occasionally I would drive through a town and for a little while I'd get snippets of whatever was broadcasting coming through in the quiet parts of my audio
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@sloosecannon said in Impossible-to-find Apps:
(often with receive-only, a wire will work fine)
This even works with over-the-air digital TV. For a while I used a paper clip in the coaxial port on my TV to get HD channels
