iPad Audio Input Options

It turns out that in some ways, getting audio signals into the iPad is similar to getting audio signals into the iPhone 3GS, and in some ways it’s not. Like the iPhone, the iPad includes a built-in microphone as well as support for a headset microphone through its headset jack. Unlike the iPhone, the iPad does not support audio line level input via the 30-pin dock connector, so you won’t be able to use existing iPod mic/line accessories with your iPad.

The iPad’s lack of audio line input support would appear to be a major limitation of the device, particularly for audio test and measurement apps. However, it turns out that the iPad can act as a USB host with support for 16-bit, 48 kHz USB Audio Class compliant devices. All you need to do is plug a compatible USB audio device into the USB port of the iPad Camera Connection Kit and you’re good to go. (I’ll spend more time explaining what “compatible” means in a future post.)

Frequency Response

So how does the frequency response of the built-in microphone and the headset input compare? I measured both and offer comparisons with the iPhone 3GS. It turns out that the two devices have very similar characteristics for their mic inputs.

The iPad's built-in microphone frequency response is nearly identical to that of the iPhone 3GS.

The iPad headset input seems to begin it's low-end rolloff above where the iPhone 3GS does, but again, their responses are similar.

How about an iPhone-ProTrack SLM

Alesis finally got around to making shields for securing an iPhone or iPod touch to the ProTrack stereo iPod recording device. This means that you can insert your iPhone (any model) or iPod touch (2G or later), plug in a phantom-powered measurement microphone, run SoundMeter, and turn the unit upside down for a handheld (albeit somewhat large) sound level meter. Of course, if you use SignalScope Pro, you’ll also have access to 1/3-octave and narrowband spectral analysis.

SignalScope Pro on iPhone with a ProTrack

One downside is that the headphone output is completely blocked for the iPod touch. The shield itself blocks the headphone output of the iPhone, although a drill could solve that problem fairly easily.

iPhone Microphone Frequency Response Comparison

With the advent of sound level meter apps for the iPhone OS (of which SoundMeter was the first) people began to ask, “How flat is the frequency response of the iPhone’s microphone?” Early testing indicated that the built-in microphone of the original iPhone was not a good candidate for sound level measurements, but that the iPhone’s headset microphone enjoyed a fairly flat response. Since then, additional iPhone models have arrived on the scene, each with its own set of weaknesses with respect to microphone frequency response. Additional Apple and third party headset microphones have also been introduced.

At long last, some relevant frequency response measurements are presented here for the benefit of those who would really like to “see” how flat a particular microphone is. These results have implications on the use of certain microphones for making sound level measurements, as well as on the use of these microphones for spectral analysis in which relative amplitudes need to be determined with some degree of accuracy.

The following measurements were made relative to a Type 1 precision microphone in a fairly quiet room. These measurements were not made in an anechoic chamber and although the coherence was very good across the audio band, the measurement error is non-negligible at high frequencies, because of diffraction effects.

Built-in Microphones

Built-in iPhone Microphone Frequency Response

Built-in iPhone Microphone Frequency Response Comparison

As I have often said, “The built-in microphone of the original iPhone is not recommended for sound level measurements.” Now, you can really see what I mean. Interestingly, the built-in microphone of the iPhone 3GS isn’t recommended, either, unless you don’t care about frequency content below 200 Hz. This behavior is consistent with the headset input frequency response of the iPhone 3GS (I suspect that the built-in microphone signal goes through the same high-pass filter that gets applied to the headset input). The iPhone 3G microphone’s response is clearly the best of the bunch, but its low end rolls off by 15 dB or more at 20 Hz. Not surprisingly, none of the iPhone models rivals a lab-grade sound level meter with its built-in microphone, but either of the 3G models can potentially give you a decent ball-park estimate of the current sound level, although the low frequencies will be de-emphasized.

Headset Microphones

The goal, here was not to measure every headset microphone on the market, but to take a look at some of the more common options. These measurements were made of each microphone’s electrical output, so they do not include the response of any iPhone input or output circuitry. The microphones included in these measurements are:

iPhone Headset Microphone Frequency Response Comparison

iPhone Headset Microphone Frequency Response Comparison

In the world of headset microphones (at least those that are presented, here), the official iPhone headset microphone and the SwitchEasy ThumbTacks microphone win the day. The USBFever microphone also exhibits a flat response between 20 Hz and 2 kHz, although its response appears to break up more severely by the time it gets up to 10 kHz. In light of recent headset input frequency response measurements, the best case scenario for inexpensive sound level measurement might be to use the ThumbTacks microphone with the original iPhone.

These results are also interesting, in that they strongly suggest that the newer Apple headsets, which are designed primarily for iPods, shouldn’t be used for sound level measurements, either. Their response certainly seems to follow an apparent trend with Apple’s microphone-related circuitry to de-emphasize low frequencies.

It may be important to keep in mind that the goal, here, is to see what makes sense in terms of using iPhone OS devices as inexpensive, portable sound level and spectrum analysis tools. Obviously, there was never an expectation that the iPhone’s inexpensive microphones would perform in a manner consistent with precision measurement mics that are (justafiably) much more expensive. It is possible to connect such high-end microphones to an iPhone, though (via the dock connector)–more on that, later…

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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