HiJacking the iPhone's Headset Port

An anonymous reader writes "Researchers at the University of Michigan describe how to hijack the iPhone's headset port to power peripherals, establish bi-directional communications with them, and interface various sensors, all without jailbreaking your iPhone or having to pay thousands to access to the Apple Dock Connector. This makes it possible for students, hackers, and DIYers to extend the phone's functionality to the physical world. The team is giving away 20 HiJack modules/programmers to enable new apps."

Comments

Question:

[Pojut]

Could you rig up something similar to Android's "back" button with this? The lack of a back button and the lack of the notification pull-down are the major things that would ever prevent me from getting an iPhone...

Re:Question:

[tlhIngan]

Could you rig up something similar to Android's "back" button with this? The lack of a back button and the lack of the notification pull-down are the major things that would ever prevent me from getting an iPhone...

But then you'll need apps to actually support it.

Then again, as someone who used iOS, I find the extra back and menu buttons kinda annoying because I'm never remembering to use those buttons - they're just out of the way things - why can't the devs put the damn option on the screen like it is in iOS... and why can't they have the left-pointing pentagon to return me back the way I came.

Annoying either way - but both are valid UI design methodologies. Apple's way is more discoverable because it isn't hiding, Android's way is more efficient because it doesn't waste valuable screen space drawing back buttons and menu selections always on screen. Sort of how in Windows and Linux stuff is often hidden in right-click menus, while in OS X, the design philophy is to not do that (i.e., put the right-click options somewhere else as well).

Re:Question:

[Kadagan AU]

So you want to have a 1 cubic inch device hanging out of your headphone jack with a "back" button on it? Seems like it would be a pain in the ass, and make it more difficult to store your phone in your pocket or holster or wherever you like to carry it.. Or keeping the device in your pocket and only inserting it when you need to hit "back" is also tedious.

Re:Question:

[jeffmeden]

Apple's way is more discoverable because it isn't hiding, Android's way is more efficient because it doesn't waste valuable screen space drawing back buttons and menu selections always on screen.

Discoverable? Both are valid, I agree, but trying to say that a pentagon facing left is "more intuitive" than an arrow is kind of silly. Even before I started using an Android phone I was constantly annoyed by the iPod Touch UI and the need to find/press the little back choice in the app instead of using the "one button", which given the lack of multitasking (until recently) had the nice side effect of completely closing the app.

Re:Question:

[Grizzley9]

"Back button"

In all my use on the iPhone, I'm not sure this has ever been needed? I'm not even sure where you would use a back button besides in Safari and it already has one.

Re:Question:

[Belial6]

I get that you may prefer the iPhone, but how could you possibly think that a back button that is always there because it is a physical button that is always in your field, is "hiding" is beyond me. It also amazes me that using an arrow that is a curvy line with a point at the end would be so much harder for you to use than an arrow that is a pentagon. They are both arrows, and they both take you back the way you came....

Re:Question:

[s73v3r]

The back button on Android isn't consistent in what it does. It varies from App to App, and can switch you between apps. A back button on an iPhone app is going to be consistent in what it does.

Re:Question:

[iluvcapra]

If you put a back button on the phone, the developers are going to use it. iPhones don't have back buttons, thus you never notice one is missing because the devs accomplish the effect through different ways.

Re:Question:

[Belial6]

When it switches you between apps, it is following your trail back the way it came. On the iPhone, the button isn't even consistently there. As far as consistency goes, Androids back button sweeps the floor with iPhone's.

Re:Question:

[Graff]

The lack of a back button and the lack of the notification pull-down are the major things that would ever prevent me from getting an iPhone...

You could always just use a 3rd party web browser, some of them have back buttons and other features not found in Mobile Safari.

As for a notification pull-down menu, submit the idea to the folks at Apple. Believe it or not they do take suggestions and over the years they have made many modifications based on those suggestions. It certainly can't hurt.

Re:Question:

[Graff]

Oh, and I probably misconstrued what you were asking for. Are you looking for a physical button that would activate a "back" feature? I'm not so sure that would be necessary since it's just as simple to have a software button on the touchscreen. I just looked and Mobile Safari does indeed have a software back button.

It seems to me that since you are already interacting with the program on the screen a hardware button designed just to go "back" wouldn't be that useful.

Re:Question:

[PReDiToR]

http://www.appbrain.com/app/button-savior-(root)/com.smart.swkey

Shame you have to root to use it, but as I have rooted (typical geek with Android, rooted and perma-rooted custom Froyo ROM) it works great.
Up and down, left or right, the trigger pixels are mostly undetectable in daily use and there are a few themes included.
I can't see my < £100 phone's buttons lasting very long so this is a great option.


PS If the developer reads this; saviour. That is all.

Re:Question:

[s73v3r]

With a press of the Android back button, you don't know if it is going to take you back a webpage, if it is going to switch an app, or if it is going to switch to another screen on the same app. When it is there, the back button on the iPhone will always do the same thing: Take you back a screen in the app.

Re:Question:

[MichaelSmith]

So in iOS when you open a web page from your RSS reader, how to you then go back to RSS?

Re:Question:

[zn0k]

The vast majority of RSS readers simply include a web browser. It takes a minimum amount of code to make use of the built in control that lets you do essentially everything the built in stand alone web browser does. The same is true for email forwarding.
If you do exit the app, you have to either use the task switcher or re-open the RSS app from the home screen.

Re:Question:

[MichaelSmith]

Sure but for me the android shell is better suited to integration between applications, while iOS encourages monolithic apps.

Re:Question:

[t2t10]

Apple's way is more discoverable because it isn't hiding,

Are you kidding? There are three big buttons on Android devices, always in the same place, consistently doing the same thing; they aren't "hiding".

iOS apps are totally inconsistent in how you invoke menus, navigate back, search, and even invoke settings.

both are valid UI design methodologies

Apple's design methodology is that Jobs hates buttons, it's as simple as that.

Re:Question:

[Hognoxious]

Seems like it would be a pain in the ass, and make it more difficult to store your phone in your pocket or holster

Purse. That's the word you're looking for.

Re:Congrats

[Stavr0]

There's already xGPS / GPSSerial. (but you need to be jailbreaked)

How does Square do it?

[CWCheese]

Seems like the Square credit card swipe dongle is doing something similar. Does anyone know if the method is same as Hijack?

Re:How does Square do it?

[bennomatic]

I was thinking the same thing. The case in the pictures in TFM even looks like the Square dongle.

Re:How does Square do it?

[Belial6]

Yep, the "Researchers" have discovered that it is possible to do something that has been in commercial use since last year.

Re:How does Square do it?

[Jake73]

Probably. This is called a modem.

Re:How does Square do it?

[a_sdh]

It's actually pretty different -- square is a simpler, passive device that essentially uses the mic and a tape head (remember those!) to read out the magnetic stripe on the card. This is actually much more sophisticated, since it harvests enough power from the audio signal to power simple sensors and a microcontroller, as well as giving you two-way communication. Also, it's not iphone-specific -- there are billions of cheap devices that you can now target with the same hardware; from this point of view the advantages are similar to the square.

Not new...

[Anonymous Coward]

The credit card reader Square does this and it has existed for about a year.

This is a modem

[Anonymous Coward]

These guys have rediscovered the modem, circa 1982.

I've done something similar using a Bluetooth headset. Open up the headset and remove the microphone. Use a PWM output of an embedded microprocessor to read the sensor and drive the microphone input. Send data to the host with FSK modulation.

With the Bluetooth headset, your sensor is wireless as well.

Re:This is a modem

[fuzzyfuzzyfungus]

Hey, at least this time they are working around an entirely arbitrary restriction(since the dock connector has a perfectly good logic-level serial port, among other things), rather than advancing the state of the art in data transmission over the legacy copper infrastructure.

Surely that, um, makes it better?

Re:This is a modem

[Announcer]

I think their idea of driving one of the audio channels at 22Khz, and converting that to DC for (a tiny amount of) power, is darned clever! This is "Hacking" in it's purest form! You are working completely within the parameters of the existing hardware, and yet doing stuff the original designers never intended. Bravo for creativity and inventiveness!

To borrow from Mr Spock; "Fascinating."

Re:This is a modem

[kcitren]

It's not just a modem. The interesting thing is the part about providing power to the devices.

Re:This is a modem

[pz]

I think their idea of driving one of the audio channels at 22Khz, and converting that to DC for (a tiny amount of) power, is darned clever! This is "Hacking" in it's purest form! You are working completely within the parameters of the existing hardware, and yet doing stuff the original designers never intended. Bravo for creativity and inventiveness!

To borrow from Mr Spock; "Fascinating."

Clever, yes, but pretty standard stuff. There are lots of ICs that are so-called 2-wire: one wire is ground, and the other is power *and* bi-directional communication. Mostly the idea is that when sending a command to the remote IC, you end up toggling the data line enough times that it can provide crude power on the receiving side by rectification and capacitive filtering of the incoming signal in addition to digital decoding of the command. The power typically lasts just long enough for the remote IC to send the appropriate reply.

But then, to bring it a couple of generations back, old-style whisker AM radios work the same way: the power to drive the ear piece is actually coming from the radio signal.

Re:This is a modem

[Anonymous Coward]

But then, to bring it a couple of generations back, old-style whisker AM radios work the same way: the power to drive the ear piece is actually coming from the radio signal.

I see, that's very clever! Maybe we could use this hack to send an audio signal to drive some kind of ear piece or small speaker.

Re:This is a modem

[matfud]

Sort of true. Most of those systems maintain an inactive high on the data/power line though as the devices can store an internal charge from that along with drawing energy from the signal when it is high. which brings me to ask why they are using 22KHz to generate the power. Is there something in the Audio hardware that causes a series of FF written to that channel to cause the output to deviate from its max value (I can imagine a number of hypothetical reasons) but most audio hardware I have used (very low level hardware) I could persuade it to produce an almost DC output for the correct software inputs. perhaps current drain on it after the analog filters???

There are two channels and I presume they are using one for power and the other for signaling to the device. resonse comes back on audio in Both channels can be used for power with minor changes with modulation on one providing outbound signals.

Matt

Re:This is a modem

[Miamicanes]

They're probably generating a 22KHz sine wave and running it through a transformer to boost the voltage from ~0.7 volts (more or less what you'd get if you drove the output DAC at the highest power output it could sustain with a non-fully-charged battery without clipping) to 3.3 or 5.0 volts. Yeah, you can do the same thing with about $10 worth of active components, but if you can easily generate a real sine wave to start with, you can basically replace the solid-state charge pump with a transformer, two diodes, and a capacitor.

As far as parasitic power goes (a fancy way of saying, "Drawing power from the data bus"), one way I know of is to drive your data line at two voltages... say, 6v or less for 0, 9v or higher for 1. At the other end, use a linear voltage regulator to convert it back down to 5.0v for power, and connect it through a resistor to an optoisolator whose other end is sampled by a GPIO pin. In real life, it's a little more complicated, because the capacitors you need would affect the signal, but I'm pretty sure the general idea would work (at least, for slow data rates).

My favorite back-channel approach to overloading power lines for data (in scenarios where you aren't trying to cover distances where voltage drops are an issue) is to add a diode bridge, and sample one output leg of it with an i/o pin. For example, suppose you have a microcontroller that has two pins: one for 5v DC, and one for ground. Now, suppose you need a way to convey a single bit of information to that chip in situations that really only matter during development or debugging. By sensing the output of one of the diode bridge's leg, you can tell whether the user connected the power normally or backwards, and act accordingly.

My own experiments have been Android-related (exploring the headphone jack as a gamepad input), but I came up with two obvious configurations for using a jack with ground, mic (input), left audio, and right audio:

* SPI (.7v logic): left audio = clock; right audio = master out, slave in (phone is master, obviously); mic = master in, slave out. Ground=ground. ~8-14 months from now, my Epic4G is going to be my Bioloid's new backpack-mounted controller :-)

* UART. left audio = .7v sine wave for power, right audio = TxD, mic = RxD, ground = ground. Obviously we're talking about nonstandard voltage levels and not RS232.

Other possibilities:

power circuit using left audio sinewave, use right audio to modulate infrared LED and make a cheap remote control.

bitbang NTSC s-video chroma & luma signals via right and left audio connectors... or mono audio + composite video. If a 20MHz 8-bit CPU can pull off color video better than most late-1970s videogames, an Android phone with 1GHz CPU hijacked into single-user Linux mode devoting its full attention to bitbanging the i/o should be able to pull off graphics at least as well as the Atari 2600. OK, pretty useless, but damn it would make a cool video on Youtube ;-)

If the left audio, right audio, and mic pins are really GPIO with data direction registers somewhere, you could hack a decent 3-channel logic analyzer (or 2 channels + trigger). If the phone has a crossbar switch between the USB port and the rest of the phone, you might even be able to hijack the two pins normally used for USB data and get two more inputs. Add a bit more hardware, like a FTDI usb bridge chip with FIFO and bitbang-mode capabilities, and an old Android phone (or iPhone) capable of activating USB-OTG would make a totally kick-ass pocket logic analyzer.

Re:This is a modem

[matfud]

- They're probably generating a 22KHz sine wave and running it through a transformer to boost the voltage from ~0.7 volts
That is a very good point. I was presuming the login would be working at a level compatable with the max out voltage from the audio system. I think a solid state voltage coverter may be more efficent though. Mind you that is a good hack if it is what they are doing (I am not sure if the pics shown where of the 22KHz output or not as they are definately not sine waves.

- one way I know of is to drive your data line at two voltages... say, 6v or less for 0, 9v or higher for 1
I've been playing around with the 1 wire bus for about 12 years. It pulls the bus (single line) high by defualt. everything on it charges. signaling is performed by wibbling it up and down. It is however designed for very low power devices that contain a capacitor to handle the lows on the line.

Either system you propose will work for game pad inputs and if you are really carefull about it you could acually have a real audio pass through to your head phones and still tap power (as things go quiet raise the ambient noise or some thing. A small Cap should enable plenty of button presses so you probably wont even need to do that)

As for grabbing NTSC. Well it can be done but you need to massively oversample to get an image in. I.e read the same frame lots of times sampling different bits or have a frame entered into your system very slowly. If you do not care about quality and the 22KHz/44KHz is enough to catch the flybacks and frame start you may be able to get a blury grey scale which is totally incorrect but may look OK

Matt

Re:This is a modem

[matfud]

SPI
UARAT
1-Wire

all can be done as long as you can satisfy the lowest baud rates they require (ie you can reasonably hit the required baud specs they have) and don't care about signal levels. Sig levels are easy to change but as you posted; not if you are using parasitic power.

That does add an interesting aspect. Power and data on less than 0.7 volts. Much of what I've been playing with (FPGAs) are runinning on 1.2 volts core but most of the surrounding hardware is 2.5v (mostly handled by the fpga) and then outside that 5v volts (legacy). I'm having to step up and down the signals to interface with other systems all the time (easy but always extra hardware).

Re:This is a modem

[pz]

Nearly all audio hardware is AC coupled at interfaces, that is, there are DC blocking capacitors in serial with the output. Writing 0xFF will not produce V_max for an indefinite time but only for the period allowed by the filter characteristics of the final stage. Low-frequency cutoffs are typically in the range of 0.1 to 10 Hz.

The reason DC blocking capacitors are used is, among other things, to limit the output current since many loads are fundamentally resistive at low frequencies, and to limit the heat buildup in voice coils.

Re:This is a modem

[matfud]

cheers,
I think the low freq cutoff for audio is probably the best reason. I think you have answerd my question. I think I was biased as most of my work was on DC coupled DACs. So yes if you drove them to max out they would produce max out but they did have have analog stages after that were designed to be band blocking in the 20 to 0.5 KHz range (to get rid of the AC currents induced by machinery in the analog sections of the devices)

I think that may be the answer I was looking for

Cheers

Matt

Thousands?

[cain]

Thousands to access the dock connector? What does tha mean? You can get one at OverStock.com for 6 and half American dollars.

Or did you mean developers pay thousands by having to buy licenses and Apple machines to develop on? Confusing. Unclear.

Re:Thousands?

[Anonymous Coward]

Thousands to access the dock connector? What does tha mean? You can get one at OverStock.com for 6 and half American dollars.

Or did you mean developers pay thousands by having to buy licenses and Apple machines to develop on? Confusing. Unclear.

Wow. Are you really confused? Yes. It means they have to buy a very expensive license to make a product that uses the Apple dock connector. There isn't any confusion for anybody else, just you.

Re:Thousands?

[s73v3r]

Because the market for iPhone/iPod accessories is much higher than the market for Android accessories.

Re:thousands?

[fluffy99]

"thousands to access to the Apple Dock Connector."

why? is there some kind of testing hardware you have to get to use the dock connector API's?

Scroll back up where several folks already point out the reason. Apple has a patent on the connector and you can't produce and sell that connector without a license from Apple.

Re:Thousands?

[drinkypoo]

No, they have to buy a very expensive license to essentially get permission to use Apple's trademarks, and to say that they are compatible. Which frankly is a bunch of bullshit. Anyway it's clear that they don't have to. Reverse-engineering for the purposes of compatibility is protected by the DMCA. The law explicitly permits you to figure out what is going on in there and do it yourself.

Re:Thousands?

[cybernanga]

You only need to pay for a licence if you want to display the "Made for iPod" / "Made for iPhone" logos.

I have several devices that use the dock connector that don't display the logo's, but they work just fine.

AKA a modem

[mrnick]

Who would have ever thought you could hook a phone up to a modem? OMG! LOL

Re:AKA a modem

[MobileTatsu-NJG]

Who would have ever thought you could hook a phone up to a modem? OMG! LOL

"When it's oversimplified, it sure sounds stupid!"

I wish Slashdot registration required that you pass an Are-You-A-Nerd test.

Re:AKA a modem

[MichaelSmith]

Who would have ever thought you could hook a phone up to a modem? OMG! LOL

"When it's oversimplified, it sure sounds stupid!"

I wish Slashdot registration required that you pass an Are-You-A-Nerd test.

One could create a captcha system which consists of questions like

• What are the color codes for a 2K2 resistor?
• What sequence of keys brings up the AT prompt on a modem
• What are first six key labels on an AZERTY keyboard?

Re:AKA a modem

[MobileTatsu-NJG]

Heh. My ideal test would be something like:

"Click to see Ben Heck's latest portable device."

"Enter desired Username and Password..."

If you click the second link it just takes you to Yahoo.

Re:AKA a modem

[Helen O'Boyle]

That was my first thought. Gee, someone's rediscovered digital/analog conversion... funny how in this industry things that were ubiquitous 20 years ago sometimes pop up as the next new groundbreaking thins 20 years later. (Accessing centralized systems from relatively dumb/low-powered clients, I'm thinking of you, too! ;-)

Bell 202 FSK is nice...

[Muad'Dave]

But couldn't you send a more useful waveform such as a 1-Wire data stream? You could have all sorts of standard sensors/etc on the dev board with an 1-Wire bus.

At 16KHz max clock rate, you probably could just send the 1-Wire waveform directly and square up any audio filter-induced droop in the receiver.

Apple might not like this...

[Chris Huelsbeck]

Won't Apple just reject any app that tries this kind of "mis"-use of the headphone jack? I doubt that they would allow this...

Re:Apple might not like this...

[ScentCone]

There certainly are apps that use it with Apple's blessings. Square is the obvious and rather cool one (works on the iPod, iPad, and some 'droids, too).

Re:Apple might not like this...

[Belial6]

Nope. The Square credit card reader has been out for almost a year, and it uses this method. One of the benefits is that the same hardware works on android. Heck, it would be trivial to make it also work on a PC.

Thousands to access ADC?

[noidentity]

all without jailbreaking your iPhone or having to pay thousands to access to the Apple Dock Connector. This makes it possible for students, hackers, and DIYers to extend the phone's functionality to the physical world

Or if you're a genius hacker, you can buy something with an ADC connector and... cut the cable, strip the wires, and have access to all its functions for a few dollars. Or maybe I'm missing something.

Re:Thousands to access ADC?

[s73v3r]

Not if you have any inclination of marketing it commercially. The thousands mentioned in the article is for the license to be able to develop something for the dock connector.

Slashdot Effect insurance (CORAL link)

[Announcer]

Just in case, here is the CORAL link:

http://eecs.umich.edu.nyud.net/~prabal/projects/hijack/

Will Apple reject apps...

[mr_majestyk]

...that are proven to use the headset port for accessing peripherals? Given their obsession with control, that seems like exactly the kind of thing they might want to restrict with T&C's.

Re:Will Apple reject apps...

[Belial6]

Nope. It is already in wide use. https://squareup.com/

Re:USB? USB OTG ?

[Curmudgeonlyoldbloke]

Shouldn't Apple be using micro-USB soon anyway? They "voluntarily" agreed along with other major manufacters:

http://channel.hexus.net/content/item.php?item=19086
http://www.newmobile.com/uk/122720/4253/European-Commission:-microUSB-standard-for-charging.html

Uhoh... new iPhone version going to come out soon?

[mysidia]

Now with "headset port" removed, integrated w/ dock port 3.0.

<EG>

Re:Congrats

[Tarlus]

Is it possible to do this with an iPod Touch? I know it's akin to an iPhone and may be similar enough for this hijacking device to function the same way, but TFA doesn't mention iPods at all...

Dock Connector

[Anonymous Coward]

Why doesn't Apple just open up the spec for the dock connector already? It's absurd that practically every stereo, car, etc comes with one nowadays even though it only works with products from one vendor.

Heck they managed it with mini DisplayPort, although I suspect that was because it was they only chance for the connector they already used on their laptops to face wide adoption. If Apple is ever open it's only to benefit themselves and not the industry as a whole which is worrying given their size.ominance.

Re:Dock Connector

[profplump]

It's not that no one knows how to use the dock connector, it's that Apple has a patent on the actual connector itself, you so you can't sell a product that uses the dock connector without paying Apple royalties.

For your own use it's possible to buy a $4 30-pin connector and wire it up to whatever you want:
http://www.allpinouts.org/index.php/Apple_iPod,_iPad_and_iPhone_dock

So I have to assume the summary talking about commercial third-party iPod accessory development rather than personal-use hacking, though they could certainly have been more clear if that's their intent.

USB

[markdavis]

Stupid Android phones with their easy, accessible, standard USB ports and connectors!!! Takes all the fun out of it!

Re:USB

[noidentity]

Hey, it worked for Kinect. If it had come with PC drivers and without the challenge of being "locked up for your own good", it would have been ignored.

Re:Secure

[Graff]

Thank goodness the (closed source) iPhone is more secure than those nasty open source devices.

In what way does this have anything to do with security? They are essentially outputting an audio signal for power and taking in an audio signal for data input. No security was needed or involved, any application can output and input sound via the audio jack.

Seems to me you are making a security issue out of nothing. Either that or you are completely misconstruing the point of the article. Then again the submission did over-sensationalize the original web site, since there is absolutely no mention there of a need to avoid jailbreaking or high costs to use the dock connector. This is just an alternate input/output for the iPhone, one that other people have used before.

Re:USB? USB OTG ?

[Graff]

Shouldn't Apple be using micro-USB soon anyway? They "voluntarily" agreed along with other major manufacters [sic]

Yep, and an adapter is a valid method of providing the micro-USB interface.

From the first page, footnote 2 on the linked PDF:

In case a mobile phone does not have this connector integrated in the device, an Adaptor may be available to ensure compatibility. An "Adaptor" is defined as a device with a Micro-USB receptacle/plug connecting to a specific non Micro-USB connector. An Adaptor can also be a detachable cable.

This means that Apple can continue to use the Dock Connector and just provide a Dock Connector to micro-USB cable or adapter.

Re:Congrats

[Graff]

Is it possible to do this with an iPod Touch?

Yes, it should be possible since the newer iPod Touches use basically the same audio jack as the iPhone.

thousands?

[milkmage]

"thousands to access to the Apple Dock Connector."

why? is there some kind of testing hardware you have to get to use the dock connector API's?

Re:USB? USB OTG ?

[matfud]

Is this something the EU got right? At the moment I think it is. In the long term is it a good idea? Who the fuck knows. But for now (i.e. 10 years ago it is). I like to charge phones anywhere and everywhere. I'm just not shure that it should also be a data port at the same time. Soon you will have to slip a prophylactic onto the other persons micro usb before you try to charge your phone.

that will teach'em!

[t2t10]

Kids, in order to build hot mobile apps, get yourself a piece of locked down hardware from a control freak company. Then, reinvent 50 year old technology to circumvent the restrictions that company put on it. Then, write software in a weird variant of the C language, software that you can install on your own device in development mode but that won't ever make it in the real world because the control freak company won't let you install it.

Wow, great education!

One possible use for this hack

[jseale]

Not sure if this was possible without this hack, but one could record audio through the headset jack as is possible in the Sony PSP's headset jack. An iPhone could easily replace a Tascam/Marantz field cassette recorder. (remember those, from the '70s and '80s?)

Power from 22Khz on audio line

[Announcer]

"PZ" you pointed it out correctly - there is a DC blocking capacitor, so you can't just pull the channel high, it MUST toggle.

Although limiting heat buildup in voice coils is what is happening, that's not quite the right explanation. ;) Without the DC blocking cap, one of the semiconductors in the output (either comp-pair, or "totem-pole") stage would be forcing a (relatively) large current through that voice coil. This would severely unbalance the transistors, resulting not only in severe distortion, but heating as you mentioned.

With a 44.1Khz sampling rate, the output at 22Khz is much closer to a squarewave than anything else. (Nyquist etc.) It will be 50% duty cycle, so it carries the maximum power for conversion. It WOULD be possible to send a datastream through that channel, as well, and cause little power loss by using FSK and then decoding that. (FSK at, say, several discreet frequencies between 22~20Khz, so each shift = several bits.) Keeping the frequency high, and the duty cycle constant, maintains the power supply at a steady level at all times.

This isn't like the crystal set, because in that case, the modulated RF carrier is converted directly to a varying DC level by the detector. That varying DC level drives the crystal earpiece to recreate the sound.

This project is using one of the two audio channels to produce a "constant carrier" for the specific purpose of creating a power supply to run other electronic devices. You want power supplies to be steady.

Re:Power from 22Khz on audio line

[matfud]

Thanks for the response.
That would explain it. Are you involved with the project? Most of the stuff I do does not have audio band pass filters after it. Even when it does I'm normally using probes on the DAC output.The analog stuff after that alwyas confuses me I know how it works, I know how it should work but in reality It does odd things and messes with my head. In this case it is more a case of "I did not remember that"

Matt

Re:Power from 22Khz on audio line

[Announcer]

No, I'm not involved with the project, but I wish I was. ;) I totally "get it", tho! I have been tinkering with electronics since I started connecting flashlight bulbs to batteries with wire, when I was just a little kid.

Yes, analog electronics can, certainly, be fertile ground for MURPHY! That's why what looks fantastic on paper, seldom works right in the real world! There's ALWAYS SOMETHING that's going to need some major tweaking to "get it right".

Re:Power from 22Khz on audio line

[matfud]

I started with electronics in a similar way, light bulbs and wires yes done that. I know all the theory behind analog but more often than not it just does not work out that way. How can a simple band pass filter screw up as it is only a few caps and resistors? I've constrained myself to the digital domain as it is far simpler. Oddly I like analog as there is so much to learn. Even digital work eventually results in analog fiddling if only to rule out the chances that it will affect you.

I'm mostly a software developer who sometimes plays around with hardware in his spare time. An Altera FPGA is the toy of choice at the moment. Geting video in to it is a problem. Image stabilisation in hardware is the goal.

Matt

Re:2-wire IC's

[Announcer]

Yes, your point is valid, of course. Using one of those IC's would be "standard stuff". This project goes outside that... the idea here, was to take advantage of the existing hardware inside the phone, making NO modifications to the hardware of the phone, whatsoever.

This isn't like the crystal set, because in that case, the modulated RF carrier is converted directly to a varying DC level by the detector. That varying DC level drives the crystal earpiece to recreate the sound. You want power supplies to be steady, not varying, which is why the steady 22Khz tone is used in one audio channel.

Re:FPGA "toys"

[Announcer]

That's really cool! Well beyond my current experience... my CPU knowledge is still back in the 80's! ;) I was writing simple ML code for 6502 and Z80 back then. I still dabble with GWBASIC now and then.

I need to "get with the program" at some point, I know.

For now, I continue to tinker with analog electronics, and basic logic ckts. It's fun... that's what matters!