Although I already listed some options for getting line-level audio into an iPhone or iPod touch, that list didn’t include much information that would suggest which option would be best. One important metric that people frequency ask about is frequency response. Well, I finally have some frequency response comparisons available to help answer that question.
These measurements were made of various dock connector devices, attached to an iPhone 3GS. As in other frequency response measurements, the audio was routed through the iPhone, with a little help from SignalScope Pro. This means that each measurement includes the frequency response of the iPhone 3GS headphone output.
The tested devices include:
Line Input Frequency Response Comparison
Based on these measurements, and the unreliable nature of the iTalk Pro, I would recommend the Tunewear Stereo Sound Recorder for iPod as the Line-input device of choice. However, if you need balanced input circuitry, adjustable gain, or phantom power, then the added cost of the Alesis ProTrack may be justified.
As indicated previously, the iVoice Pro may still be the best option for the original iPod touch, depending on your needs.
People often ask about the frequency response of iPhone and iPod touch audio inputs. To shed some light on the issue, I made some frequency response measurements of the iPhone, iPhone 3G, iPhone 3GS, and iPod touch 2G with Electroacoustics Toolbox and an Edirol FA-101 audio interface. These measurements are broken into two groups, one for headset input and one for dock connector input.
Since measurements were made by routing audio through each iPhone OS device (by way of the Audio Play Through function built-in to SignalScope/Pro), all measurements include the frequency response of the headphone output in addition to the response of the selected input. The frequency response of the Edirol FA-101 was removed from the measurement, using a not-yet-released version of the Dual FFT analyzer in the Toolbox.
Headset Input Frequency Response
The original iPhone wins, hands down, for the flattest frequency response of the headset input. It’s too bad that the iPhone 3GS drops the low end more severely than any other iPhone OS device.
Dock Connector Input (Line In)
I used a Tunewear Stereo Sound Recorder for iPod for these measurements. It has one of the flattest frequency responses I have seen in a dock connector device (more on that will be published soon).
Dock Input Frequency Response (using Tunewear Stereo Sound Recorder for iPod)
There is some ripple evident in the line input frequency response (it’s not yet clear whether the ripple exists on the input, the output, or both), but other than that, the frequency response is quite flat. The low frequency 3 dB cutoff appears to be at around 7 Hz and things stay pretty flat beyond 20 kHz.