config.txt - Should I use CEA or DMT for different types of VGA monitors?

Question

When using an HDMI to VGA adapter, converter, or adapter converter (in particular when using this one found on Amazon.com), should I use the setting for CEA (hdmi_group=1) or DMT (hdmi_group=2)? More information about config.txt and CEA and DMT can be found here.

TV screens and monitors I have:

• Dell E178WFPc VGA monitor
• Samsung LN40A550P3FXZA HDTV with HDMI, Composite Video, Component Video Y/Pb/Pr, and VGA (which is what I am trying to use)

• Philips BDL4221V/27 LCD VGA 42" TV Screen - Optimum resolution: 1360 x 768 @ 60Hz, highest number video format: 1080i
  
  I will be using the Tendak HDMI to VGA cable (amazon.com/Tendak-Converter-External-Projector-Supporting/dp/B00P6GSL54) with the Philips VGA TV screen.

• Toshiba HDTV screen with 4 HDMI ports and a VGA port (which is what I am trying to use)

• Another VGA TV screen (may be Samsung and may be High Definition)

I ask this question because I saw at elinux.org/RPi_VerifiedPeripherals under Display Adapters,

and then under HDMI->VGA converter boxes,

and then under HDMI to VGA adapter from amazon - USA,

After "The adapter is very compact and low-power, unlike some of the other adapters. My ViewSonic VA702b monitor required:",

it says "hdmi_group=2 -- indicates VGA".

I am also curious why "hdmi_group=2" indicates VGA.

So if I have an HDTV screen (or just a TV Screen) with VGA, I would think to use hdmi_group=1,
and if I have a computer monitor with VGA, I would think to use hdmi_group=2.
Any help would be appreciated. Thank you.

Easton Werling

Answer 1

You've cited this source documentation, and I think you should treat that as authoritative, vs. the anecdotal blurb on elinux. Technically anyone can add stuff there, which is fine as long as you take the nature of the material into account. That's one anonymous person telling an anecdote about something they bought on Amazon, so:

I am also curious why "hdmi_group=2" indicates VGA.

It doesn't. That's just wrong. Going back to the primary source docs (i.e., stuff the Foundation itself is somewhat accountable for):

hdmi_group

This defines the HDMI output group to be either CEA (Consumer Electronics Association, the standard typically used by TVs) or DMT (Display Monitor Timings, the standard typically used by monitors).

So group 1 is CEA, group 2 is DMT. "VGA" originally referred to a form of adapter but now more generally refers to 640x480 resolution, which as per the documentation could be accomplished with

hdmi_group=1
hdmi_mode=1

Or:

hdmi_group=2
hdmi_mode=4-7

There's also a section there about how to find out what is supported by the display attached to the HDMI port:

• Which values are valid for my monitor?