HPD(Hot Plug Detection)，在HDMI的一對聯接中，爲熱插拔的實現而設計的。簡單地說，當發送端接入接受端時，接受端會迴應HPD信號給發送端，進而發送端會啓動DDC通道，而讀取接受端EDID的信息，然後進行HDCP的交互，如果雙方認證成功，則視頻、音頻正常工作，否則聯接失敗，不同系統會有不同的處理。
例如，如果EDID信息不支持HDMI，則發送端只發送視頻信息，而沒有音頻信息，這時候的HDMI就只相當於DVI了；如果HDCP認證不成功，有的系統會出現雪花屏幕和噪音，有的系統會由高分辨率（1080I、720P）降低爲低分辨率（480I、480P）而輸出，這樣一來，HDTV就不再是HD了，而變成了普通的SD。
An Important element to proper interpretation of EDID is "Hot Plugging". The following presents a recommendation for achieving consistent results during a Hot Plugging event.
DVI 1.0 define a HPD signal function that indicates to the host whether a monitor is connected. HPD is designed to be powered by the DDC +5V coming from the host, and to be independaent of whether the monitor is powered or not. In this way, a host device can detect the monitor and read its characteristics from EDID without the monitor being powered. On a PC, this feature allows the system to load the correct display configuration without delaying the boot process.
In short, in this context, HPD serves as an indication that the EDID is available to be read, however HPD may also have alternative uses. It does no imply any other state of readiness. The relevant definitions from the DVI 1.0 specification are:
a. HPD - Signal is driven by monitor to enable the system to identify the presence of a monitor;
b. The monitor is required to provide a voltage of greater than +2.4V on the HPD pin of the connector only when the EDID data structure is available to be read by the host.
Implementation Notes: As an example for hot plug support, a simple monitor implementation of HPD support could be a pull up resistor to the EDID power supply. After HPD goes active, the host is only expected to read EDID and determine that a valid display mode is available and supported.
Note --- Whenever the EDID information in a device changes for any reason(e.g. if the EDID was updated, or is capable of dynamically changing its information content), the receiving device pulses HPD low for at least 100ms. This recommendation follows from the HDCP repeater implementation requirement that HDCP repeater pulse HPD low for at least 100ms to indicate the connection of new device or disconnection of an existing one.
這裏只是概要的談談，後續將會對其聯繫EDID和HDCP詳述。
鏈接：http://blog.csdn.net/linweig/article/details/5518858