[LAU] high-pass filter on Android audio output

[Len Ovens]

On Thu, September 6, 2012 9:02 am, Charles Henry wrote:
> 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.

Are you sure that is not just the normal (ugly) result of digital
clipping? You may wish to try a SW limiter in the chain somewhere or
something that limits slew towards clipping in software. That is limit the
max output to less than 0db so that some sw has a chance to "round the
corners" to keep the frequency within 20khz or so.

-- 
Len Ovens
www.OvenWerks.net