I ran in a couple of situations in which I noticed my kid was not
using headphones at a safe listening level, so I am a bit worried
about it. Therefore I tried to find something already available in the
market and, besides the feature offered by the iPod for parents
limiting the sound level, I found no general device to passively nor
actively limiting the audio levels automatically for a device. So my
first question is if anyone knows a commercial device I can connect
between the audio output of any gadget and the headphone to limit it
to a value of my interest, regardless of the output level of the
gadget itself.

Assuming such a device doesn't exist, for a DIY project what I did
found was sound "clippers" and "audio limiters" but for a very
different purpose: remove glitches or high spureous from a signal, by
limiting them with two germanium diodes antiparalell or similar
circuit (in some cases with more sophisticated variations of the same
idea), see for instance
http://www.headwize.com/projects/limiter_prj.htm

But those clippers do not exactly avoid a high sound level by
downsizing the audio wave, it just clips the wave amplitud when it is
higher than certain thresold. What I need is really more like a
variable gain amplifier that adjust the input signal and amplifies it
only till certain point, depending on the output level but never
really clips it. Furthermore, an ideal design should take into account
external noise level, to let "safe thresold" to be a bit higher on
noisy environment, but lower on quiet environment.

Probably a DSP could do the job, but may be a little overkill. What
about a programable gain amplifier driven by a PIC with a comparator
that looks for signal level integrating every second or so, and then
adjust the PGA according to that?

TIA
-- 
Ariel Rocholl
Madrid, Spain
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