I want to connect the output from the audio jack of an iPhone to an Arduino.
What voltage range can I expect to see on the audio lines from the iPhone? I assume that turning the volume up on the phone will produce a large AC voltage, but how large does it go up to?
I want to make sure that it wont exceed the voltage level that an Arduino can read on its input pins. Will I need to provide any circuitry between the iPhone and the Arduino?
Commercial line out specification is to be able to drive 1 milliwatt to a 600 ohm load. For a sine wave, this means a voltage of 0.77 volts RMS (2.2 volts peak-to-peak) and a current of 1.3 milliamperes RMS (3.6 milliamperes peak-to-peak).
Unfortunately there is a lot of "audiophile" nonsense around headphone amplifiers and headphone impedance. Probably the top 5 results for "headphone impedance" on Google are just wrong. This site contains some useful information (though a lot of it is wrong too).
But anyway if you look at the graphs which I assume are correct, you can see that in the audio frequency range most headphones have a fairly small reactance compared to their resistance. And most headphones have an impedance around 16-32 Ohms with some crazy "audiophile" headphones having higher impedance (e.g. 300 Ohms). He suggests that 5 mW is sufficiently loud for portable headphones. Audiophile headphones will require higher power.
Power is \$P=V^2 / R\$ so \$V = \sqrt{R*P}\$, so high impedance headphones will need a much higher output voltage because they require more power and have a higher impedance. Anyway, for the Sony MDR-EX51 headphones shown on the page linked above you can see that they are fairly close to a simple 17 Ohm resistor. At 5 mW that would mean a voltage of 0.3 V and a current of 16 mA.
An Arduino can supply this fairly easily but I don't think you can just hook it up to PWM since 5V across 17 Ohms gives 300 mA which is well above Arduino's 25 mA limit. A simple solution may be to insert a 4.7 V / 16 mA = 290 Ohm resistor in series with the pin.
I haven't tried any of this - you'll have to experiment!
Check out: http://en.wikipedia.org/wiki/Line_level
The most common nominal level for consumer audio equipment is −10 dBV, ... Expressed in absolute terms, a signal at −10 dBV is equivalent to a sine wave signal with a peak amplitude of approximately 0.447 volts, or any general signal at 0.316 volts root mean square (VRMS). ... There is no absolute maximum, and it depends on the circuit design.
This is however for the "Line out" plug which, apparently, carries a signal at a fixed amplitude and lets the receiving end determine the volume.
In most cases changing the volume setting on the source equipment does not vary the strength of the line out signal.
For a speaker-driving headphone plug I believe things might get more complicated, since that signal is really rather a current signal (used to drive the coil of a speaker).
In contrast to line level, there are ... those used to drive headphones and loudspeakers. The strength of the various signals does not necessarily correlate with the output voltage of a device; it also depends on the source's output impedance which determines the amount of current available to drive different loads.
I guess your best bet might be to look at the wave with an oscilloscope, which should have a high-impedance input like the Arduino's analog input (ADC).
(I'm no expert, take with a grain of salt and feel free to edit)
Edit: The Wikipedia article I used as a source has been edited a lot since I originally posted this answer. Among other changes, the qouted pieces above have been removed/changed. Therefore I'm striking most of this answer out and recommend referring to the Wikipedia article linked at the top.
This is in addition to PkP's answer.
While "line level" audio is typically 1 mW into 600 Ω, and this comes out to 1.1 V p for a sine, audio is far from a sine. Even if the spec is adhered to and you only get 775 mV RMS on average, the peaks can be considerably higher than 1.1 V. It is generally good to accept and handle without distortion peaks up to ± 5 V at least.
There is no hard-and-fast rule for headphone jacks; be it a laptop, MP3 player or a regular stereo system.
I would say that a typical headphone output adheres to Line Level specifications, although for headphones they become more of a guideline than a stringent set of figures.
As you have already discovered, different devices have different output levels.
The power that can be provided by your PC is, for example, X milliwatts. As the PC power supply can give up 12V to the soundcard, the XmW could well be generated with an emphasis on the voltage rather than the current. Some top end motherboards (the latest Asus ROG boards, for example) boast a headphone-jack output of over 2V rms.
A portable MP3 player may only have a 3.7V lithium battery. Its output power could be the same XmW as the PC, but at a lower voltage therefore higher current - without some boost convertors it would be impossible to match the voltage of the aforementioned high-end motherboard.
A fundamental difference between a 'headphone output' and a 'line out' is that the latter isn't designed to power a low impedance load. I tend to assume that the input impedance of a generic audio device to be 50kOhms; if it's ever critical to know then it's typically stated by the device manufacturer. Headphones or earphones can be as low as 32 Ohms, meaning that plugging headphones into a Line Out socket could result in both poor volume and poor quality. There is not generally the same problem with connecting a line-level device to a headphone output unless you consider a dedicated headphone amplifier; an audiophile might argue that the output would become unbalanced.
Thus there is no correct answer. Perhaps start with 1.4V RMS as a maximum and then increase or decrease as you work through your prototype.
The arduino would need a higher voltage.
Use an non inverting OP amp on the line which should bring the voltage to about 2ish Volts, something which is better for the arduino.
:)
http://www.instructables.com/id/Arduino-Audio-Input/step3/Non-Inverting-Amplifier/
