One of the most obvious ways to get analog signals into an iPhone or 2nd generation iPod touch is through the headset connector. Several options exist for getting acoustic or electric signals into the headset input, which are discussed below. Any of these options will work with the iPhone, iPhone 3G, or iPod touch 2G. The original iPod touch does not have a headset connector with a mic input channel, so it is left out of this discussion.
When making a decision about what to use the headset input for, or what to connect to it, you may want to take a look at the frequency response measurements of the various iPhone OS devices.
Acoustic Signals
Acquiring acoustic signals requires some type of microphone. Several off-the-shelf options exist for attaching a microphone to the headset jack, as listed here:
- Apple iPhone headset–included with every new iPhone
- Apple earphones with remote and mic (MB770G)
- Apple in-ear headphones with remote and mic (MA850)
- SwitchEasy ThumbTacks microphone
- USBFever mini microphone
Standard iPhone headsets can be used for basic sound level estimates with SignalScope Pro, SoundMeter or dB, without any further calibration. Using any other microphone (besides the iPhone’s built-in microphone) will require calibration before meaningful sound level measurements can be made. (dB does not support microphone calibration.)
Original iPhone users will be glad to know that the SwitchEasy ThumbTacks microphone will fit into the iPhone’s recessed headset jack. This is not true of the USBFever mic, however, so an adapter cable will be required for that mic. Another benefit that the original iPhone gains from using these mics is that the full audio sample rate will be available rather than being limited to 8 kHz, as it is with the built-in microphone.
Connecting either of the SwitchEasy and USBFever microphones will preclude the use of headphones, unless some special adapter cable is used (I’m not currently aware of an off-the-shelf solution).
Frequency response measurements of these microphones can be found here.
Electric Signals
Acquiring signals from some other source can be a little tricky for the following reasons:
- The headset microphone input is very sensitive (it expects a low-level microphone signal).
- A bias voltage is present on the headset input to power electret condenser microphone capsules (used by the afore-mentioned microphone accessories).
- The headset input expects to see a particular load in order to signal the OS that an external microphone is present.
Of the three issues, the third one is perhaps the most difficult. To be sure the iPhone OS will select your input signal, you can place a suitable resistor in parallel with your input. One user reported that a 3.3 kOhm resistor dropped the bias voltage from 2.7 to 1.9 VDC. When connecting the headphone output directly to the headset input for some basic frequency response measurements, I have had good success with a 670 Ohm resistor. I have also had success connecting external measurement microphones and accelerometers, using a constant-current power supply, without using an additional resistor.
The best adapter cable I have found for connecting to the headset input is a standard A/V cable, which has a four-conductor mini-plug on one end (for connecting to the iPhone) and three RCA plugs on the other end. To work with the original iPhone, the A/V adapter cable needs to have some of the plastic carved off around the mini-plug, or another adapter cable is required to fit the iPhone’s recessed headset jack.
Sometimes, connecting external signals to the iPhone’s headset jack is the most convenient, portable solution. However, working with dock connector input devices allows for up to two input signals without the complicating issues of the headset input.
It should also be noted that the iPhone 3G rolls off the low frequency response of it’s headset input below 100 Hz.
Hi Ben,
This is a super help – thanks so much for all your work with this. I wanted to ask a related question. I’m shooting video/audio for a music festival next week and will be doing interviews as well as on-site shots of concerts. So as to be as portable and unencumbered as possible, I’m going to try to do all the video and audio work on the iphone 3gs.
If I pick up a decent all-around dynamic omni mic – say an Audiotechnica ATR-50 – and plug it into the iphone via a coupler to the A/V 4-conductor mini-plug you mentioned, will this record ok? Or will I need to put something (a line attenuator?) in between mic and iphone to modify the input signal such that it will record. problem of course is that ill be on location and so wont really have ability to have extra equipment along with me – was hoping to do this with just the iphone and an external mic.
what do you think? possible at all? would appreciate any thoughts on the situation, as am feeling a bit lost in the little writing that’s been done on the audio recording capabilities of the new 3gs.
thanks in advance for any advice…
best
Roger
Roger,
A “decent” dynamic mic may still not be as good as the electret condenser mic in the headset that came with your iPhone (and the ATR50 is not an omnidirectional microphone).
Also, with the iPhone 3GS, unless you use the dock connector for audio input, the low frequencies will be significantly attenuated below 200 Hz. Take a look at the the frequency response plots in related blog posts.
Ben
Hey Ben,
Thanks again for your follow-up advice. I see what you mean about a dynamic mic not being as good as the electric condenser in the iphone, but I’m hoping to get an external more to shove in ppls faces to interview while I’m filming video on the iphone. Otherwise, I would probably just use the internal mic, whose quality is actually quite good.
I went down to J&R Music World in New York this morning to test out some options. I tried out the the ATR50 and had an interesting discovery. The mic plugged into the iphone via the A/V four-conductor miniplug did actually record sound at a decent level, *but* the recording was accompanied by a strange regular metronomic clicking sound. This sounded like some sort of power loop. I’ve uploaded a copy of the recording here – www.norums.com/Memo.m4a.
Any idea what this is and/or if there is any way to get rid of it? Or do you think I am fighting a losing battle?
BTW I just had confirmation from Touchmic and Blue (Mikey) that their microphone products are compatible with the 3GS.
Cheers,
Roger
When connecting external signals to the iPhone’s headset jack, your best bet will generally be to work with devices that were developed under Apple’s Made for iPod program. Anything else can be problematic (and you may not want to risk damaging your iPhone). If the headset mic won’t work for you, another option (though less portable) would be to check out options for dock connector input. The Alesis ProTrack offers XLR inputs and phantom power, but it may not be as portable as you would like.
Sorry, made a mistake. The mic I tested it with this morning to get that clicking sound was the Audio Technica ATR-25 – stereo condenser. The recorded actually didn’t pick up anything fro the ATR-50 – it didn’t even produce enough input to shut off the internal mic…
http://catalog.belkin.com/IWCatProductPage.process?Product_Id=277661 the belkin voice recorder connected to an ipod would work for what you are trying to do. The sound is great and it’s much less expensive. I’m trying to do the same thing, but I want to attach a lapel mic to an iphone 3gs and use the built in voice memo app to record– any ideas on how to do this. I tried a standard 4 connector 3.5 mm av to three RCA cable with an RCA to mono 3.5 adapter and plugged the mic into that with no results. Does the apple av dock cable have any audio in?
++1 again, I would like to do the same as Roger and Isaac.
(sorry its not much of a contribution. Just want to commiserate.)
I’m not exactly versed in audio matters, but as I understand it, recording a signal through the headset input of the iphone 3gs requires a different connector than a standard line-in plug?
Does this mean that a standard unpowered microphone (in particular, this one: http://www.olympus.co.uk/consumer/2587_TP-7_Telephone_Pick-up.htm) will not work when connected to the headset jack of the iphone 3gs? Or perhaps it would work if I use an adaptor cable?
My confusion has reached a new threshold on this topic. Any help would be much appreciated.
The headset input uses a special 4-conductor mini-jack. Your safest option is to use a headset microphone that was developed under the Made for iPod program.
I need a double-sided jack that plugs into the iPhone that has the same 3.5mm plug on BOTH ENDS (with the 3 “stripes”) Any ideas where I might find this?
Try this, Alan: http://www.wiredco.com/3ring-cond-35mm-miniplug-35mm-male-cable-p-348.html
“The headset microphone input is very sensitive (it expects a low-level microphone signal).”
Do you have any specs for this? I’m connecting an externally-powered sensor and want a decent SNR without blowing the input.
“The headset input expects to see a particular load in order to signal the OS that an external microphone is present.”
Have you determined just what this load is?
Thanks!
You can get official specifications by joining the Made for iPod program.
I need to line-in recording using the iphone headphone input, any ways to do that?
The headset jack does not support line-level input. Your input levels will be limited to around 10-15 mV (peak).
@ Isaac, Roger, and Chapito
I used a TRRS breakout cable from http://www.kvconnection.com that takes the iphone headphone jack and breaks it into two TRS plugs. One wired for stereo headphone output and the other wired for mono microphone input. They have the adapter that is the reverse as well so make sure you choose the correct one.
I then connected a microphone that was purchased from http://www.soundprofessionals.com to the microphone input and attached my sennheiser headphones to the headphone output. The mic was an AT831-SP cardiod lapel mic made by audio technica. I believe this mic was made for voice so the frequency response is probably not flat but more tailored for human speech. It was terminated for mono recorders with a TRS plug wired for mono. Works great. Good sound levels in my recording apps (used for voice recording), using the iphone as a phone, and using skype as well. Very loud sounds may overdrive it since there is no built in attenuation. You could add a simple inline attenuator or “volume control” if needed. I have not needed to in the environments I have been using it in. Sound has been excellent!
Great solution so far. I also use it with my new sony m10 which will record sounds below 200hz. Too bad about the iphone 3gs mic input roll off for recording but I am sure it helps with mic input for voice apps.
I am sure other similar quality lapel mics would work as well. I have an AT-943 wired the same way I will try next. It is an omni capsule with a flatter freq response.
Can’t wait till someone comes out with a line in device that allows us to use our own clean preamps and doesn’t roll off the low freqs. Maybe someday.
Hi there, hopefully this isn’t a dead forum.. I’m trying to create an affordable RTA system that I can use for the many different venues that I run sound in. It would be awesome to be able to use a DBX RTA-M real time analysis mic or even the output of my Radioshack Digital SPL meter to go into my 2nd generation iPod Touch. This “Bias Voltage” that you mention.. is that Phantom Power? Do you have any idea about the loads that these sources may present to the touch’s inputs? Do you think that the frequency response would be accurate enough for my line of work? I’m willing to sacrifice some accuracy for the sake of mobility Thanks!
You might want to check out the Alesis ProTrack for phantom power. That would allow you to use your DBX mic (plug it right into the ProTrack).
No.
If you use the ProTrack with your DBX mic, that will probably meet your needs. The ProTrack isn’t the most portable solution, but it’s arguably the best one for phantom power.
I was hoping to setup my JVC digital video camera and take the composite video output and display it on my Iphone screen. Is it possible to plug the video output from the camera into the headphone jack adn display the video on the iPhone? Would it have to go in thru the sync cable? Would it require any additional software?
Let me know if you think it is possible. Thanks
There is no video input on the iPhone.