HDMI is compatible with DVI-I and DVI-D. DVI (Digital Visual Interface) uses an identical electrical video transmission to HDMI. This means that HDMI-to-DVI cables and adapters and DVI-to-HDMI cables and adapters, both provide video compatibility. HDMI and DVI both have 19 pins, but DVI-I and DVI-D transmit only a digital video signal where HDMI transmits a digital video and digital audio signal. As they both use the same digital signal, no conversion is needed when connecting HDMI and DVI, and when using a HDMI to DVI cable or adaptor or vice-versa there will be no loss of data quality.
If connecting a PC with DVI out or a DVI switch to a TV or monitor with HDMI in, a DVI-to-HDMI adaptor or cable may be used. As DVI transfers only video, audio needs to be separately connected. The aux audio can be sourced from the PC via either a digital or analogue audio out depending on the type of PC or Graphics card being used.
When connecting a TV with HDMI out, only one HDMI cable is needed, but if the cable box has DVI out, a DVI-to-HDMI cable is used to connect the video, and the audio connected using left and right analogue stereo connectors.
Traditional cables such as VGA and composite video will transmit analogue video signal only, which is not great for LCD screens. Many current devices will still accept VGA but work better with DVI or HDMI.