DVI
Direct Answer
DVI (Digital Visual Interface) comes in five variants -- DVI-D (digital only, 19 or 25 pins), DVI-I (digital + analog, 23 or 29 pins), and DVI-A (analog only, 11 pins) -- with single-link supporting up to 1920x1200 @ 60Hz and dual-link up to 2560x1600 @ 60Hz.
Source: DDWG DVI 1.0 Specification
Specs
| DVI-D Single-Link | 19 pins |
| DVI-D Dual-Link | 25 pins |
| DVI-I Single-Link | 23 pins |
| DVI-I Dual-Link | 29 pins |
| DVI-A | 11 pins |
| Single-Link Max Resolution | 1920x1200 @ 60Hz |
| Dual-Link Max Resolution | 2560x1600 @ 60Hz |
Origin and the Three Variants
DVI was designed by the Digital Display Working Group (DDWG), a consortium including Intel, in April 1998, as the digital successor to the analog VGA connector during the transition from CRT to flat-panel LCD monitors. It exists in three logical types: DVI-D carries a purely digital signal, DVI-A carries a purely analog signal electrically equivalent to VGA, and DVI-I carries both on the same connector for backward compatibility -- each also comes in single-link or dual-link versions, with dual-link adding a second set of data pairs to roughly double available bandwidth.
Why DVI-D and HDMI Are Interchangeable
DVI-D and HDMI share the same underlying electrical specification for their digital video signal (TMDS) and display-data-channel pins, which is why a simple passive DVI-D-to-HDMI adapter reliably carries video with no active conversion hardware needed. The compatibility has real limits, though: only DVI-D works this way, since DVI-I and DVI-A carry analog signals HDMI has no equivalent for; no audio passes through, since HDMI's digital audio packets aren't part of DVI's signal at all; and HDCP copy-protection support isn't guaranteed, since it depends on whether both the source and display support it independently of the adapter.