[Arm-netbook] HDMI Player Stick as DRM-free Merch Alternative

[Richard Wilbur]

> On Jul 22, 2018, at 16:48, Jean Flamelle <eaterjolly at gmail.com> wrote:
> 
> The device being airgapped so that the only input is power, should
> socially suggest that one can't be tampered with or forged (i.e. by
> extracting a signing key).

One fly in the ointment is that, at least according to my understanding, power on the HDMI is part of VESA (Video Electronics Standards Association) DDC (Display Data Channel) support.  As such the power is supplied by the video signal source which is the I2C bus master on the DDC bus.  This applies to all of the incarnations of VESA DDC on VGA, DVI, and HDMI.

Thus, in order for us to be able to use the HDMI power pin as an input, we need to be connecting that pin to some other HDMI signal source (computer, DVD player, Blu-ray  player, et cetera).  Hence our smallest form factor for HDMI-only connections would be an HDMI(male)-HDMI(female) adapter to plug in between an HDMI source (as above, for power) and an HDMI sink (monitor, television, projector, et cetera).  For convenience, I recommend connecting between the HDMI source and the HDMI cable going to the HDMI sink.

Another option would be to use a USB connection for power.  Not as elegant as the HDMI stick but USB power is relatively ubiquitous for charging mobile devices.  And it doesn't require a separate HDMI source just for power.  In fact, a lot of televisions with HDMI ports also sport USB ports so a USB cable would be sufficient.

Furthermore, if power is truly our only input we'll have a hard time sending out a signal which is compatible with a wide range of displays unless we choose the lowest common resolution/color depth.  We could adapt to the best display mode that the display offers, and that our board can generate, if we connected to the bi-directional VESA DDC bus and read the display's capabilities.

Richard