[Cryptography] "Great. Now Even Your Headphones Can Spy on You"

[John Gilmore]

> That one of the widely used D/A converters will happily work in the
> reverse direction is ... interesting.

Modern sound chips have a few-wire serial interface to the system,
provide multiple fast and wide D/A and A/D converters, variable sample
sizes and bit rates, put both input and output drivers on most of
their analog I/O pins, and offer an internal mux so that any
particular pin can be connected to any particular converter.  This
flexible interface was spec'd as part of Intel's HD Audio spec:

  https://en.wikipedia.org/wiki/Intel_High_Definition_Audio
  https://www-ssl.intel.com/content/dam/www/public/us/en/documents/product-specifications/high-definition-audio-specification.pdf

This allows software to change the audio ports for 2-channel, 5-channel
or 6-channel sound, for example.

All these factors -- low pin count, flexibility, and high integration
-- were aimed at making these into mass market chips at low generic
prices.  It worked, and they or variants also end up used in many
cellphones.

	John

PS:  The Wired article doesn't link directly to the academic paper
(you had to go via YouTube and arxiv for that), so here's the direct link:

  https://arxiv.org/pdf/1611.07350v1.pdf