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.
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 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.
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.