can iPhone detect digital signal through microphone?

Question

I'm working on a project which needs the iPhone to detect a small set of voltage data (two different values of voltage representing 0 and 1 respectively). My initial thought is to detect it through microphone, but I'm not sure if iPhone would be able to detect these signals since it contains no info of frequency. I searched in iOS Developer resources and on google, but nothing clear about this problem. Can anyone help me with this question? Thanks a lot!

Answer 1

The usual problem when trying to use an audio signal as digital signal input is that it is high-pass-filtered to avoid offsets (long term pressure changes which could destroy the dynamic range of the soundcards Analog-to-Digital converters). This means you will get only the changes of the digital signal (like "clicks"). Actually you will be better of the higher frequency your signal has, but it will be non-trivial to process it well then (this was what ordinary modems did very well).

If you can control the digital signals sent as sound (or to the microphone jack), make sure they are modulated the signal with one or more tones, like a morse transmitter or modem. Essentially you want to use your iPhone as an Aucoustic Coupler. This is possible, but of course a bit computationally expensive considering the quite low bandwidth.

Start by using some dirt simple modulation, like 1200-1800 Hz and make an FFT of it. There's probably some reference implementation for any sound card out there to get started with.

There have been some cool viruses recently that was said to be able to jump air-gaps, they used similar techniques as this one.

If you still really want a DC (or slowly changing digital signal), check out a solution using a separate oscillator that is amplitude-modulated by the incoming signal

Answer 2

as per our discussion, it seems you want to send a digital signal to the iphone. Now there are two main ways to do this.

1. Either sign up to Apples MFi hardware licensing program which allows you to create hardware for the iPhone. MFi program. Or,
2. There is an easy way to do this but it would require the use of the headphone jack. For demonstration and testing purposes you can use the headphone jack and if its a simple on and off signal then you can get good results with the headphone jack overall and might not require to create yourself a piece of hardware. Here's a link to that Grab sensor data and send it through the iPhone headphone jack

In fact, using the headphone approach is not as bad as it may sound, you can receive a nice signal if needed. Anyway, it will suffice for your purposes. Have a look what this guy is doing. I suggest you start with the video demonstration to get an overall idea of this approach.

UPDATE 1 Have a look at this link. People are using the headphone port to detect voltage. The reason this works is because the iPhone jack is a combination earpiece (output only) and microphone (input only) connector. The microphone input is a single wire, with common ground to the earpieces.