HREF, VREF, and Connections

When a monitor or other display device is displaying an image, it typically scans down the screen, creating an image from left to right, top to bottom. (Sometimes, the device makes two passes down the screen to create a single image; this type of display is called an interlaced display.) The video stream contains signals that instruct the display device when a new line or new screen is to be drawn.

The terms HREF and VREF, also known as hsync and vsync, are the signals within the video stream that tell a display device what to do and when to do it. The HREF signals that a new line is to be drawn and the VREF signals a new screen.

For instance, imagine you are working with a video signal intended for the world's smallest monitor. The monitor only has 4 scan lines. (This is not at all realistic, of course, but it is simple.) On an oscilloscope, the HREF and VREF signals would look somewhat like the following illustration.

In the preceding illustration, both HREF and VREF signals are active high, meaning that they are considered active when in a heightened state (when the waves go up). There is no standard for these signals. In some cases, places where the waves go down (low states) might signal an active HREF or VREF, or sometimes one will be active high and the other active low. Although the preceding illustration is only an imaginary example, note that there are lots of HREF signals for each VREF. This is because for each new screen, there are several scan lines. Of course, in a real video signal for a real broadcast, you would see hundreds of HREFs for a single VREF.

HREF signals, VREF signals, and video data are carried across physical data lines from the decoder to the video port. In many cases, a number of lines are reserved for video data, and others are dedicated to carrying HREF and VREF signals. However, there is no standard for how these data lines are used.

A connection is a protocol that a video port or decoder uses to define how it uses these data lines. Video ports and video decoders will support a variety of connections. DirectDraw video-port extensions use globally-unique identifiers (GUIDs) to identify each type of connection. You can query for the connections that the video port supports by calling the IDDVideoPortContainer::GetVideoPortConnectInfo method. You create a DirectDrawVideoPort object that supports a given connection by calling the IDDVideoPortContainer::CreateVideoPort method.

Keep in mind that the video decoder is outside the scope of DirectDraw video-port extensions, and exposes its supported connections through an interface of its own. By enumerating the connections that the video-port supports and comparing the results with the connections supported by the decoder, you can negotiate a common connection (or language) that both components understand.

 Last updated on Thursday, April 08, 2004

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