Is there any way to modify the frequency response of the line in microphone through software? I ask because I would like to record a low-frequency signal (~10 Hz) on the iPhone, and the way things stand currently, the signal would be completely canceled. I am aware of hardware solutions to overcome this, like modulation, but was wondering if there were any software solutions.
Thanks!
The iPhone does not have a line in; I assume you mean the mic on the headset.
No.
I suspect any filtering is done in hardware, and if not, I highly doubt that that you can easily access it in software. If you think that there's some software filtering going on, you can file a bug report (or "enhancement request"/whatever) and see if there's any demand for it, but I suspect the phone gives you mostly raw audio.
Additionally, the characteristics of the mic itself probably make it bad for recording a 10 Hz signal.
the way things stand currently, the signal would be completely canceled
Filtered maybe; not cancelled. But how do you know for sure?
Try passing it thorugh a decent low-pass filter (fifth-order Butterworth at 20 Hz?). You'll probably get something...
If there is an analog DC blocking filter in front of the ADC, there's nothing that can be done about it in software (except amplify the noise in that frequency band and measure if any of the attenuated signal remained above the noise).
Under 20 Hz, modulation of a much higher audio frequency carrier is probably your best bet. It doesn't even need to be a nice linear modulation circuit if you can characterize it and use the inverse of that characterization in the software demodulator.