HDMI vs DVI vs VGA

[Scott Sullivan]

On 09/28/2013 01:09 PM, Evan Leibovitch wrote:
> On 27 September 2013 16:11, James Knott <james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org
> <mailto:james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org>> wrote:
>
>     William Park wrote:
>      > Right here, you know it's scam.  How can you not support "digital
>      > signal" when going from HDMI (digital) to VGA (analog)?
>
>     DVI supports analog as well as digital.
>
> Does the Pi DVI output both? This gets to the core of the cheap
> adapter's suitability to task.

Just a breakdown on a nuances here.

VGA DVI-A DVI-I DVI-D HDMI
\___1___/       \____3___/
          \__2__/

1) Analog  - DVI-A is VGA on a DVI connector
2) Both    - Analog and Digital on the same connector, different Pins.
3) Digital - DVI-D and HDMI

http://en.wikipedia.org/wiki/Digital_Visual_Interface#Connector

Of note here is that the HDMI spec is electrically (current and 
voltages) compatible with the DVI, but the actual protocols are 
different. The choice was made that DVI-D would be included in the HDMI 
spec to allow greater compatibility. These means that all HDMI devices 
will actually completely change their signalling to be a DVI-D output 
when they detect a DVI-D device connected.

http://en.wikipedia.org/wiki/HDMI#Compatibility_with_DVI

DVI-I/A -> VGA = Passive adapter, signalling is analog VGA

HDMI -> DVI-D  = Passive adapter, digital signalling with a spec 
mandated personality change.

HMDI / DVI-D -> VGA / DVI-A = Active Adapter required.

-- 
Scott Sullivan
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