iPhone Headset Input Options

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:

Standard iPhone headsets can be used for basic sound level estimates with SignalScope ProSoundMeter 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:

  1. The headset microphone input is very sensitive (it expects a low-level microphone signal).
  2. A bias voltage is present on the headset input to power electret condenser microphone capsules (used by the afore-mentioned microphone accessories).
  3. 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.

Dock Connector Audio I/O

Several options exist for getting audio signal into and out of iPhone OS devices via the dock connector. However, not all accessories are compatible with all iPhone OS devices. So, we put together this compatibility chart, based on our own tests with SignalScope/Pro and SignalSuite.

Dock Audio Accessory Compatibility

These devices were chosen for their ability to accept stereo audio input from external sources. Some dock connector devices simply feature built-in microphones, which are of limited use for test and measurement applications. It’s also important to remember that the iPhone OS automatically selects the current route for input audio signals (built-in mic, headset, dock connector, etc).

iPhone iPhone 3G iPod touch iPod touch 2G
Alesis ProTrack In/Out(1,2) In/Out(2) Out(3) In/Out(2)
Belkin TuneTalk Stereo In(1) In In(3,4) In
Griffin iTalk Pro (5) In(5) In(5) In(3,4,5) In(5)
MacAlly iVoice Pro In/Out In/Out In/Out In/Out
Tunewear Stereo Sound Recorder In(1) In In(3,4) In
  1. Even when using the dock connector for input, if the receiver or built-in speaker is the current output device on the original iPhone, the sample rate will be limited to 8 kHz (for input and output). Connecting headphones, or an adapter cable, like a stereo mini-plug to RCA adapter, will cause the headphone output to be selected and push the sample rate back up to 48 kHz.
  2. In SignalScope Pro (or SignalScope) the Alesis ProTrack appears as an input only device, so output signals are not routed to the ProTrack’s headphone out connector. The ProTrack’s headphone output does work with SignalSuite.
  3. Using a dock connector input with the original (1st generation) iPod touch appears to require that something be plugged into the headphone jack, unless the dock connector device also supports audio output (like the Macally iVoice Pro, which, ironically, is one of the few devices that is narrow enough to allow you to simultaneously plug your headphones into the bottom of the iPod).
  4. When using the original iPod touch with standard dock connector input devices, like the TuneTalk or the Tunewear device, a dock extender, like the SendStation device, will be required in order to also plug in headphones. You need to be sure your dock extender supports audio (some do not).
  5. The Griffin iTalk Pro that we tested did not work consistently–sometimes it wouldn’t be selected for input by the device. For now, the iTalk Pro is not recommended.

Frequency response measurements of these devices can be found here.

iPhone OS Audio Routes

Getting audio signals into and out of an iPhone OS device can sometimes be a bit tricky. The information presented below outlines the available means for getting audio signals into and out of each iPhone OS device.

Available Input Routes

Built-in mic Headset input Dock input
iPhone Yes(1) Yes Yes
iPhone 3G Yes Yes Yes
iPod touch No No Yes(2)
iPod touch 2G No Yes Yes
  1. The built-in microphone of the original iPhone appears to be routed through a speech-processing codec, which limits the sample rate to 8 kHz and significantly degrades the frequency response.
  2. Using a dock connector input with the original (1st generation) iPod touch appears to require that something be plugged into the headphone jack, unless the dock connector device also supports audio output (like the Macally iVoice Pro, which, ironically, is one of the few devices that is narrow enough to allow you to plug your headphones into the bottom of the iPod).

Available Output Routes

Receiver Speaker Headphones Dock
iPhone Yes(1) Yes(1) Yes Yes
iPhone 3G Yes Yes Yes Yes
iPod touch No No Yes Yes
iPod touch 2G No Yes Yes Yes
  1. Even when using the dock connector for input, if the receiver or built-in speaker is the current output device on the original iPhone, the sample rate will be limited to 8 kHz (for input and output).

Links to additional information:

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