[LAU] high-pass filter on Android audio output

[Charles Henry]

Hi all

I know this is not the ideal place to ask a question about
Android--but I hope someone here can help confirm my suspicions.

I wanted to create some circuit tools using the 1/8" stereo output +
line input jack on my android phone (a droid razr).  To start with,
I've got a simple signal generator (built with PdDroidParty)--and what
I found is that there is a high-pass filter on the output.  It is
mostly unsurprising, but what *kind* of high-pass filter is it?

If it was just a RC impedance on the output, I could compensate for
that.  However, it seems likely that there's a high-pass filter in the
software, or somewhere else.

When I drive it to clipping with a combination of sine tone and dc
offset, the noise characteristic is weird.  The pitch of the distorted
tone goes down, and there's a very steep transition between the
undistorted and distorted tone.  The distorted tone is also much
louder.  I'm typically testing with a tone at about 1kHz, but I've
been sweeping through a lot of frequencies to see what happens.  The
symptom is the same whether it's played through the speaker or a set
of headphones.

It's got to be a software filter, DAC hardware, or output amplifier
circuit.  If I could identify that, it would help me figure out
whether this project is feasible or not.

I'd like to make hardware that plugs in to the 1/8" jack to create:
1.  signal generator
2.  RLC meter
... up to ...
linear/non-linear systems identification tools
audio spectrum analyzer
audio oscilloscope

USB hardware is mostly out of my league--and for the effort required,
there's no guarantee of success.  Small steps, first.

Chuck