[Nix-dev] Screen with HDMI for FOSDEM

[James Haigh]

On 30/01/15 19:16, Domen Kožar wrote:
> Maybe FOSDEM staff can help us out, did you try contacting them?
No. I asked on the Sunday but as expected they didn't. I don't really
think that that should be expected of them anyway. I asked around a
couple of tables over in AW block where the hardware stands were, but
only saw screens with VGA or VGA and DVI input, and 1 of the 2 monitors
on the NixOS stand had both VGA and DVI. Quite predictable really; I
just didn't think about it until sometime during packing.
    I should have bought a DVI adaptor since it is more common on
monitors, and HDMI-to-DVI adaptors are passive and cheap (they are just
a few pounds at my local electronics shop). Not to mention that it'd
mean that I can get an extra HDMI port on my desk monitor which has 1
each of VGA, DVI, and HDMI, which would mean that I can switch between
the BeagleBone Black and the PandaBoard ES, for example, with a touch of
a button.
    It didn't matter so much anyway because, with the NixOS stand, noöne
really sprinted much and I also forgot to bring a blank SD/MicroSD card
(it was on my table at home damnit). I did at least pick up some tips.
Nicolas Pierron explained a solution to a problem that I don't really
understand, but at least I somewhat understand the solution, and while I
was in AW block, one of the stands (PicoTCP I think) that I asked about
a screen had a BeagleBone Black on the table, and one of the people on
that stand gave me some tips on how I could avoid needing a screen if I
don't have a JTAG adaptor (which I don't, but I should probably get one
of those too).
    I'll be more organised next year, and hopefully we could also run a
demonstration presentation from the BeagleBone Black! :-D

On 30/01/15 19:27, Michael Raskin wrote:
>>    In future, I should find some kind of HDMI-to-USB adaptor to somehow
>> allow me to use the screen of one of my devices that has a screen. I
>> actually have 6 HD screens with me but of course none of those devices
>> have an HDMI input.
> Do these exist? Do these exist at remotely sensible price points? I know
> neither of the answers, I would also like such a thing and I hate movie
> industry for their efforts to ensure HDMI is defacto standard for video
> output _and_ a huge pain to capture even from non-movie-playing
> device...
The obvious thing to search for is ‘HDMI to UVC adaptor’, because such a
device would ‘just work’ with anything that supports the standard,
including Android 4.0+ devices when using a MicroUSB OTG host cable.
Some do exist but unfortunately they're marketed as ‘capture cards’ and
priced higher than most screens, which is a throwback to analogue to
digital video conversion devices. Really they should cost around the
same as MHL adaptors.
    USB 2.0 has throughput capacity upto 480Mb/s, effectively ~280Mb/s,
so, uncompressed, it should be able to acheive 640×360×30Hz×24b =
~166Mb/s, 854×480×25Hz×24b = ~246Mb/s, or 1280×720×12Hz×24b = ~265Mb/s.
As I understand, HDMI output devices are only required to support 720p
as their minimum resolution, so the adaptor would have to be able to
either skip frames, downscale, or compress. 720p with frame-dropping
would easily be the simplest and cheapest option, and for viewing user
interface video output of the BeagleBone Black or PandaBoard ES, it
would probably be the most preferred. Even 1920×1080×5Hz×24b = ~249Mb/s
could be preferable in some situations. USB 3.0 has a capacity of 5Gb/s,
effectively ~3.2Gb/s, so can even support the full 1920×1080×60Hz×24b =
~2.99Gb/s without compression. However, USB 3.0 or such throughputs are
not supported by any of my devices. Maybe with a USB 3.0 CardBus card
(my ThinkPads have PCMCIA v5.0+ type II; 32-bit), I should be able to
get upto theoretical 1.056Gb/s, which should support 1280×720×30Hz×24b =
~664Mb/s.
    This could be quite a good open hardware project to be crowdfunded.
Maybe seeing as open hardware seems to be taking a rise, someone my take
this on. In the meantime though, I'll just settle for an HDMI-to-DVI
adaptor.

James.